EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: brumbarchris on May 25, 2022, 09:06:51 am
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Hello,
Whereas we do have a fair experience with ceramics, we find ourselves in the situation where we need to place some aluminum electrolytic capacitors on our PCBs, in order to ensure a low impedance of the battery pack we are using.
The battery pack consists of 6 x LR6 alkaline cells. Even if the cells are super-charged to begin with, their individual voltage would not exceed 1.65V, which means that the entire pack would be at 9.9V maximum.
Is it "safe" to use 10V rated electrolytic capacitors?
Sure, I know having margin is better, but here this is a "late" addition to the design and we are quite constrained in terms of volumetric space.
Having browsed some datasheets, I have not seen any temperature derating information, but as I said, our experience with this sort of caps is limited, hence the question.
Regards,
Cristian
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It should be fine. Aluminium electrolytic capacitors inherently have built-in voltage margin (i.e., the dielectric is formed at the factory at higher voltage rating), and leakage current starts to steeply increase only after some 20-30% overvoltage. Contrast this to tantalum capacitors, which have fallacious voltage ratings, and 40% derating is the norm as always explained in the usually separate application note.
For the same reason, alu elcaps used to come with separate working voltage ("WV") and surge/peak voltage ratings in the past. Haven't seen them since 1990's. Nowadays rated voltage is the working voltage.
I always try to derate aluminium elcaps by at least 10-15% as a general principle, but I would not hesitate not to derate it in situation like yours.
1.65V is a decent estimate for initial no-load voltage of an alkaline cell.
Voltage derating helps dissipate heat by increasing the package size, in large ripple current applications, and this way, increase the lifetime. This probably does not apply to your use case.
Go for it.
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alu elcaps used to come with separate working voltage ("WV") and surge/peak voltage ratings in the past. Haven't seen them since 1990's. Nowadays rated voltage is the working voltage.
I have made a new design recently where I selected the filter capacitor from CDE, I was delighted to see on the sleeve of the capacitor that the surge voltage rating was also indicated!
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https://data.energizer.com/pdfs/l91.pdf
1.8V fresh and excitingly low impedance!
the next common step up at 16V seems a bit high, so a good 10V part that specifies transient or surge voltage rating would probably be the best bet (more common in higher voltage/mains parts but available in 10V).
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Check this: https://www.eevblog.com/forum/projects/impact-to-lifespan-of-an-electrolytic-capacitor-run-near-rated-voltage/ (https://www.eevblog.com/forum/projects/impact-to-lifespan-of-an-electrolytic-capacitor-run-near-rated-voltage/)
It’s not a problem, in principle. The only thing you might want tp consider is whether or not you can actually trust the manufacturer’s ratings. Probably just don’t use some garbage bottom tier brand.
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Probably just don’t use some garbage bottom tier brand.
yeah, i think that is the key
and if you're running them close to rated voltage don't also stress them in other ways at the same time. Like temp.
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It should be fine. The battery voltage will soon drop to 9V anyway and presumably it's not going to be used at high temperatures.
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https://data.energizer.com/pdfs/l91.pdf
1.8V fresh and excitingly low impedance!
Very good point about 1.5V lithium chemistry being used. 10.8V would be significantly over 10V rating. I'm still 99% sure it would work in practice without any problems but again this starts to sound like risk-taking.
A decent 16V part might be easier to find than one with surge ratings. Besides, "surge" rating means a short surge, but 10.8V could be applied for days or even weeks if the device sits unused with fresh batteries. So it would be still used outside of specs.
So I'd say it's all about the risk taking mentality of the OP. The risk is practically small, but no one can give a guarantee what will happen when some very unlucky individual inserts fresh lithium cells and applies 11V to the 10V capacitor. Very likely, nothing happens, but can you convince yourself or your managers to trust this educated guess?
The question is, how much capacitance does the OP actually need? What is the purpose of the added capacitor and how were the required C and ESR values calculated?
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In general, I hesitate to recommend operating anything above its rated voltage or power.
How much larger is a 16 V capacitor? Are there no 12 V capacitors any more?
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Ooof - I did not read closely enough - I did not know you wanted to use the capacitor above its rated voltage. Yes, that's a tricky one to recommend. 100% voltage will be OK. 150% voltage - that seems like a stretch, though yes the voltage will no sit there forever.
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Is not the charging voltage higher than cell voltage? What is the maximum voltage that will be applied during the charging process?
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Ooof - I did not read closely enough - I did not know you wanted to use the capacitor above its rated voltage. Yes, that's a tricky one to recommend. 100% voltage will be OK. 150% voltage - that seems like a stretch, though yes the voltage will no sit there forever.
The original assumption was at 99% rated voltage. However, OP ignored possibility of other chemistries (than alkaline) in AA form factor, and with lithium cells, this could mean ~110% of rated voltage, and at lower impedance than alkaline cells (which drop quickly under any load).
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Is not the charging voltage higher than cell voltage? What is the maximum voltage that will be applied during the charging process?
I think it's fairly safe to assume no one attempts to charge AA cells when inserted in the product. There are special alkaline chargers (yes, these can be more-or-less charged a few times even if forbidden) that could output over 1.65V, but the cells would be removed from the product and inserted into the charger.
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Ooof - I did not read closely enough - I did not know you wanted to use the capacitor above its rated voltage. Yes, that's a tricky one to recommend. 100% voltage will be OK. 150% voltage - that seems like a stretch, though yes the voltage will no sit there forever.
The original assumption was at 99% rated voltage. However, OP ignored possibility of other chemistries (than alkaline) in AA form factor, and with lithium cells, this could mean ~110% of rated voltage, and at lower impedance than alkaline cells (which drop quickly under any load).
Oh. I apparently am not awake. For some reason I saw 10V and interpreted it as 10x cells @ 1.65V each = 16.5V. Literally made that up in my head.
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I wouldn't. Use 16v rated, shouldn't be much more thicker or taller.
You may also want to consider if it's feasible to have a cutout in the pcb and have that electrolytic lay flat partially inside that cutout. You could probably fit it around the battery compartment - with round batteries you could have some wasted space.
Any particular reason you need nearly 10v or 6 AA batteries ?
Maybe you could make your product more "green" by using a lithium battery and a charger chip (so that you'd charge your product from usb or something) and use a boost dc-dc converter circuit or voltage doubler to boost 3.7v..4.2v to your higher voltage, if something needs it.
With AA batteries, you'd gonna have people use rechargeable batteries which will go down to 1v each, so I assume your device already needs to work with as little as 7v.
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Is it "safe" to use 10V rated electrolytic capacitors?
I agree with the comments that a 10V capacitor across 6 AA/LR6 cells is unlikely to fail even if primary lithium cells are used. The way they are manufactured, it would be quite a challenge to make them so that the failure rate at 10V was effectively zero but the failure rate at 10.8V was significant.
However, I have two other thoughts. The first is that even if the capacitors don't actually fail, might this lead someone else to assert that the design is faulty? Is your device subject to regulatory compliance, approval, etc? The second is that given your need to reduce the high frequency source impedance of the battery pack, are you first looking at the actual ESR of the capacitors? I don't know what size or type of capacitor you are looking at, but for example if it were an SMT polymer cap, you might find that a 6.3 or 8.2µF 16V cap actually has the same or lower ESR than a 10µF 10V unit. Or they might at least be comparable.
So, how much capacitance are we talking about here? Are you needing a lower source impedance @ 100kHz or do you need extra current to start a motor? :)
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… aaand once again, engineering is all about trade offs :-)
At risk of sounding arrogant, maybe these questions will be helpful?
- what’s the consequence of a 10V part going poof? Is there enough room for a vent to open, or would it be jammed in so much that the thing builds up pressure until it bursts?
- is the nature of the application such that the occasional random failure is worse than losing battery life (Eg safety equipment)?
- what’s the consequence of using a 16V part with less capacitance / more ESR? Would it reduce the battery life a little? Or a maybe a lot?
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I haven't gone through data sheets, but E = 1⁄2CV2, so a 16V capacitor is going to store 2.56 times as much energy as a 10V one. Of course this might not mean it will be 2.56 times the volume or mass.
I've seen 10V capacitors in plenty of devices operated from a 9V battery or 6 AA cells and not known it to be a problem.
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Hello again, and thank you for all your replies.
What we have here is an ''update'' of an existing product, which also in the past used 6x alkaline cells. Requirement is that it continues to do so, therefore upgrade to other battery types is excluded. We are aware of the risk that lithium based cells might be used in the same battery slot, but doing so would run against the user manual of the final product, therefore running the warranty void. Surely, user manual or not, there will be people doing that and maybe busting the caps; moreover, it would be difficult to prove they have done so, but this is a risk we are willing to take.
Regarding the value of the caps, we are looking at some 1000uF...2200uF... tentatively. Sure ESR is a concern and we will try out some different values, before comitting to an update.
Again, thank you all, you helped me decide to go ahead with using 10V rated capacitors.
Regards,
Cristian.
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What function does the 1000uF...2200uF have and how did you figure out you need this amount of capacitance?
Damping for hot plugging wire inductance transients? Much less is needed, say 100uF. Providing peak current for communications? You can probably calculate quite exact value for the capacitance and ESR needed, given maximum allowable voltage drop and magnitude + length of the current peak. It can be way below than way over 1000uF.
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Regarding the value of the caps, we are looking at some 1000uF...2200uF... tentatively. Sure ESR is a concern and we will try out some different values, before comitting to an update.
:o
That must be an interesting issue...
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ESR tends to go down and reliability tends to go up, with increasing voltage rating. Maybe a 16V part will be effective even with a lower capacitance value. Don't know the product, but remember that cap performance falls off with falling temperature.
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For every 10 degree Centigrade decrease in operating temperature, the capacitor life is extended by a factor of two.
Capacitor life ratings generally are specified at their maximum rated temperature.
This life-temperature dependence actually impacts how you should derate the voltage on the capacitor.
In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage.
Always check with the manufacturer since all capacitors may vary.
http://www.interfacebus.com/How_to_Derate_Capacitor.html (http://www.interfacebus.com/How_to_Derate_Capacitor.html)
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This life-temperature dependence actually impacts how you should derate the voltage on the capacitor.
In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage.
I don't believe either of these statements are true..
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This life-temperature dependence actually impacts how you should derate the voltage on the capacitor.
In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage.
I don't believe either of these statements are true..
You are certainly entitled to an opinion. I would suggest you take it up with Cornell Dubilier and ask them to change their capacitor usage guide.
from https://www.cde.com/resources/technical-papers/AEappGuide.pdf (https://www.cde.com/resources/technical-papers/AEappGuide.pdf)
Estimating Lifetime for Capacitors without an Online
Calculator
We offer online calculators for many of our capacitor series such
as our screw-terminal, snapmount and flatpack capacitors. For
our capacitor series without a calculator, you will find on its
datasheet a “load life rating” with an ambient test temperature
and duration at the rated ripple current, which is tabulated for
each capacitor within that series. To estimate the minimum lifetime, you may select a capacitor whose tabulated ripple current
is at least equal to your application’s ripple current, and apply
the “doubles every 10 °C” rule between the load life test ambient temperature and your application’s ambient temperature. If
your DC voltage is derated, then you may multiply the lifetime
by the voltage multiplier Mv given in equation (6). This will be an
estimate of the minimum expected life
...
Operating Lifetime Model
Onset of wear-out is determined mainly by the capacitor’s average operating temperature. Operating voltage has some effect.
For capacitors operating at moderate temperatures the operating life doubles for each 10 °C that operating temperature is
reduced. Our online lifetime calculators available at
http://www.cde.com/technical-support/life-temperature-calculators (http://www.cde.com/technical-support/life-temperature-calculators)
our best estimates of the useful lifetime for many of our more
popular capacitor series. The expected operating lifetime is approximately
Lop = Mv × Lb × 2((Tm – Tc)/10[°C]) (5)
The above equation is the simple, classic “doubles every 10 °C”
rule used for many decades, and Mv is a DC voltage derating
multiplier equal to
Mv = 4.3-3.3 VA / VR (6)
Thus for example when the DC voltage is derated by 10%, this
multiplier is equal to Mv = 1.33. As in the failure rate equation
discussed in the previous section, VA is the applied DC voltage,
VR is the rated DC voltage, Tc is the core temperature, Tm is the
maximum rated core temperature and Lb is the base lifetime.
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Well, in practice, 80% sounds about right. Not for any reason than that's how most people seem to pick 'em. I think the temperature rule is aimed more at the higher temperatures. Above I talked about falling temperatures. That was more about equipment running in a cold environment, or powering things up right off the UPS truck in the winter. Not a good thing to do in a marginal design case.
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As per my post above, it is up to the engineer to apply the suggestions of the capacitor manufacturer.
The linked article lists the manufacturers estimate cold effect as well as derating for cap aging and a link to that manufacturer's derating calculators.
It could be that if a designer were cheap, they might only estimate the cap failure to be the length of whatever warranty they are offering. IF they offer no warranty, they could just as well not derate as suggested by the cap manufacturer and let their device fail relatively sooner than later.
Not a good way to make repeat customers, that is for sure.
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Well, in practice, 80% sounds about right. Not for any reason than that's how most people seem to pick 'em. I think the temperature rule is aimed more at the higher temperatures. Above I talked about falling temperatures. That was more about equipment running in a cold environment, or powering things up right off the UPS truck in the winter. Not a good thing to do in a marginal design case.
The US Navy requires derating Aluminum electrolytic caps by 70% as of 2007.
https://www.navsea.navy.mil/Home/Warfare-Centers/NSWC-Crane/Resources/SD-18/Products/Capacitors/Derating/ (https://www.navsea.navy.mil/Home/Warfare-Centers/NSWC-Crane/Resources/SD-18/Products/Capacitors/Derating/)
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IF they offer no warranty, they could just as well not derate as suggested by the cap manufacturer and let their device fail relatively sooner than later.
Not a good way to make repeat customers, that is for sure.
And not an option in the EU, where you have to provide a minimum warranty.
This life-temperature dependence actually impacts how you should derate the voltage on the capacitor.
In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage.
I don't believe either of these statements are true..
You are certainly entitled to an opinion. I would suggest you take it up with Cornell Dubilier and ask them to change their capacitor usage guide.
from https://www.cde.com/resources/technical-papers/AEappGuide.pdf (https://www.cde.com/resources/technical-papers/AEappGuide.pdf)
Estimating Lifetime for Capacitors without an Online Calculator
We offer online calculators for many of our capacitor series such as our screw-terminal, snapmount and flatpack capacitors. For our capacitor series without a calculator, you will find on its datasheet a “load life rating” with an ambient test temperature and duration at the rated ripple current, which is tabulated for each capacitor within that series. To estimate the minimum lifetime, you may select a capacitor whose tabulated ripple current is at least equal to your application’s ripple current, and apply the “doubles every 10 °C” rule between the load life test ambient temperature and your application’s ambient temperature. If your DC voltage is derated, then you may multiply the lifetime by the voltage multiplier Mv given in equation (6). This will be an estimate of the minimum expected life
...
Operating Lifetime Model
Onset of wear-out is determined mainly by the capacitor’s average operating temperature. Operating voltage has some effect. For capacitors operating at moderate temperatures the operating life doubles for each 10 °C that operating temperature is reduced. Our online lifetime calculators available at http://www.cde.com/technical-support/life-temperature-calculators (http://www.cde.com/technical-support/life-temperature-calculators) our best estimates of the useful lifetime for many of our more popular capacitor series. The expected operating lifetime is approximately
Lop = Mv × Lb × 2((Tm – Tc)/10[°C]) (5)
The above equation is the simple, classic “doubles every 10 °C” rule used for many decades, and Mv is a DC voltage deratingmultiplier equal to
Mv = 4.3-3.3 VA / VR (6)
Thus for example when the DC voltage is derated by 10%, this multiplier is equal to Mv = 1.33. As in the failure rate equation discussed in the previous section, VA is the applied DC voltage,
VR is the rated DC voltage, Tc is the core temperature, Tm is the maximum rated core temperature and Lb is the base lifetime.
Thanks for posting that. It also specifies the life at the rated ripple current, so the original poster can expect it to last for much longer, as the ripple current will be much lower, probably zero most of the time.
Well, in practice, 80% sounds about right. Not for any reason than that's how most people seem to pick 'em. I think the temperature rule is aimed more at the higher temperatures. Above I talked about falling temperatures. That was more about equipment running in a cold environment, or powering things up right off the UPS truck in the winter. Not a good thing to do in a marginal design case.
The US Navy requires derating Aluminum electrolytic caps by 70% as of 2007.
https://www.navsea.navy.mil/Home/Warfare-Centers/NSWC-Crane/Resources/SD-18/Products/Capacitors/Derating/ (https://www.navsea.navy.mil/Home/Warfare-Centers/NSWC-Crane/Resources/SD-18/Products/Capacitors/Derating/)
Military has much more stringent requirements, compared to commercial/consumer electronics, so that's probably irrelevant.
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You are certainly entitled to an opinion. I would suggest you take it up with Cornell Dubilier and ask them to change their capacitor usage guide.
You demonstrate well the problem of "authority argument".
While everything they say is true, it has a context, and the problem is, you don't seem to understand it.
Whether the application is of high ripple current or low ripple current type totally changes everything. Because the Cornell Dubilier text does not emphasize this directly, you as a designer must be experienced and knowledgeable enough to see it. "core temperature" being a parameter is a red flag!
In high ripple current rating application, the "hour rating" dominates, and all the classical derating formulae apply.
In low ripple current rating application, the aging is more similar to "shelf storage" aging. Except that certain amount of bias voltage will actually help maintain the capacitor. At some point, too high voltage starts doing damage instead. But it's totally different mechanism! It's not related to heating, like the hourly ratings in SMPS applications.
To me, it seems OP's case is a nearly zero ripple current one, although I would like to have a confirmation of that.
So instead of appealing to authority, you need to build understanding, to be able to give correct advice.
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I apologize if you take offence to my statements.
Perhaps some meaning is lost in translation. I am not a scientist that researches and designs capacitors. I freely admit I may be wrong. I like so many other people only follow what we have been instructed by manufacturers.
I stated my understanding of the general rules of derating capacitors used by a number of sources and was told that it was not to be believed. No reason given.
I then quoted 'A' manufacturers directives on derating. If that is bad, I am again sorry.
I work in a field where things change. Rules that were true before change. The internet exacerbates this problem by retaining information that is used for refence effectively forever. When old material on the internet has wrong information, it makes people like me think and say the wrong things.
If I am not permitted to use references to illustrate my understanding of general principals of implementation without being accused of "an argument from authority" because I cited my sources, I don't know what to say.
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No need to apologize, you are the only one taking offense.
You can go on and blah blah :blah: until cows come home, but that does not address the actual issue that the text you linked to is actually about thermal aging of capacitors due to ripple current; the usual lifetime/derating consideration mostly relevant for SMPS design. It's not the best resource for OP's case which probably deals with small enough ripple current that it does not cause capacitor aging.
So the aging mechanisms OP will see are different.
Sadly, I don't have a good source of information off-hand. Shelf-life aging and low-ripple aging under DC bias are much, much less discussed in appnotes, because expected lifetime will normally be very long, and manufacturers don't want to give guarantees that long.
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blah blah? OK, have a nice life!
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Sadly, I don't have a good source of information off-hand. Shelf-life aging and low-ripple aging under DC bias are much, much less discussed in appnotes, because expected lifetime will normally be very long, and manufacturers don't want to give guarantees that long.
It would take a long time to accumulate that information as there probably isn't a good way to do accelerated testing and manage to correctly mix in all of the factors. Even moderate ripple and temperature applications like linear PSUs with very large capacitors where the capacitor temp might be 30C and the ripple a miniscule percentage of the rating result in units that still work after hundreds of thousands of hours. So after 30 years of testing they can tell you what the spec is was...
I the OPs case the capacitor will typically see 6 to 9V, so unless there is something unusual going on I don't see a problem. However, I have to wonder if something unusual is going on.
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I apologize if you take offence to my statements.
Perhaps some meaning is lost in translation. I am not a scientist that researches and designs capacitors. I freely admit I may be wrong. I like so many other people only follow what we have been instructed by manufacturers.
I stated my understanding of the general rules of derating capacitors used by a number of sources and was told that it was not to be believed. No reason given.
I then quoted 'A' manufacturers directives on derating. If that is bad, I am again sorry.
I work in a field where things change. Rules that were true before change. The internet exacerbates this problem by retaining information that is used for refence effectively forever. When old material on the internet has wrong information, it makes people like me think and say the wrong things.
If I am not permitted to use references to illustrate my understanding of general principals of implementation without being accused of "an argument from authority" because I cited my sources, I don't know what to say.
There is a cool presentation that I need to dig out somewhere, sorry for the blunt "I don't believe that's true" without further explanation. But the gist of it is, most manufacturers' aging calculators don't include the voltage stress in the equation whatsoever, it's not even a variable. That tells us that the voltage stress on a elcap is at the very least a minor stressor, if not totally negligible compared to thermal stress. Most literature has well established that it is temperature alone (no matter the cause, although of course ripple current is the major one, but could be anything), is the major stressor that affects aging, not the DC bias. Different manufacturers have secret sauce data and don't seem to even agree about whether voltage stress is a major factor or not between themselves.
You happened to chose one of the manufacturers that does indeed include it (C-D), but that is not the whole story and IMO misleading. It's misleading because it might lead people into believing that reducing DC bias will extend lifetime, which isn't true in 99% of cases in the field, but reducing the temps absolutely, and dramatically would.
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E.g. from Nippon Chemi-Con which has been already posted somewhere:
Where a capacitor is used at lower than the rated voltage, the lifetime may not be adversely affected, which means that the effect of the applying voltage is negligibly small, while the effect of the ambient temperature and heat generation due to ripple current is significant.
However, for capacitors of larger size and higher rated voltage contain a larger volume of electrolyte, difference in applying voltages can affect degradation of the oxide layer, other than the diffusion of electrolyte. Therefore, for screw mount terminal type capacitors with the rated voltage of 350Vdc or higher, the lifetime estimation includes the effect of applying a lower voltage than the rated voltage (derating voltage).
I'm not arguing that blanket de-rating a cap by 80% will hurt anything, instead I'm claiming that it:
- Is not the whole story, and doesn't explain the why
- Is sub-optimal for no good reason
- Is not necessary in most cases with minimal attention. In cases where it is needed, that "minimal attention" will catch them.
- Distracts from the real problems which are often overlooked and what kill products in the field, namely thermal effects (whether due to ripple or external heating)
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There is a cool presentation that I need to dig out somewhere, sorry for the blunt "I don't believe that's true" without further explanation. But the gist of it is, most manufacturers' aging calculators don't include the voltage stress in the equation whatsoever, it's not even a variable. That tells us that the voltage stress on a elcap is at the very least a minor stressor, if not totally negligible compared to thermal stress.
The problem is also that it's hard to say if voltage is a direct physical stressor, or an indirect one. By indirect, I mean: when you consider heating, if you pick two capacitors with the otherwise same ratings, the larger voltage version comes in larger package, with more surface area, so it cools better (assuming they still have same ESR). This is the indirect reason why voltage derating is sometimes suggested to remedy thermal aging. I think that ripple current rating should be the primary parameter to derate, though, in such thermal lifetime application. I do both, a 15V switcher with 1A of ripple current gets a 25V (as 20V isn't a typical size) cap with 2A ripple current rating.
OP's application is close to "DC biased shelf life" case, though. Voltage and ambient temperature are the two factors (ripple current isn't since it's near zero; and core temperature simply becomes equal to ambient temperature).
It's well known that when the voltage rating is exceeded by a lot - maybe say 30-50% - DC leakage current starts to increase steeply, resulting in heating, thermal runaway, and destruction. Who knows if there is some certain magical overvoltage % which causes slow damage. In any case, applying rated voltage may actually lengthen the life by keeping the passivation layer formed, and as the capacitors were originally formed at a significantly over the rated voltage (like 30-40% IIRC), 10% overvoltage likely does not do any damage when it is free of transients.
But it's hard to find reliable data. I guess OP's case is fine if they print "don't use lithium cells" in the manual. Those who do will be unlikely to have any problems.
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OP's application is close to "DC biased shelf life" case, though. Voltage and ambient temperature are the two factors (ripple current isn't since it's near zero; and core temperature simply becomes equal to ambient temperature).
We're assuming this, but without confirmation. Suppose whatever is going on with their device results in an occasional 15V spike from inductive kick of some sort?
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I don't see the need for the argument. I found that application note interesting. Yes, as I said, it's not completely relevant to this application, but it's still useful information nonetheless.
OP's application is close to "DC biased shelf life" case, though. Voltage and ambient temperature are the two factors (ripple current isn't since it's near zero; and core temperature simply becomes equal to ambient temperature).
We're assuming this, but without confirmation. Suppose whatever is going on with their device results in an occasional 15V spike from inductive kick of some sort?
400V capacitors on switch mode power supplies routinely cope with those sorts of spikes.
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We're assuming this, but without confirmation. Suppose whatever is going on with their device results in an occasional 15V spike from inductive kick of some sort?
That is unlikely because large electrolytic capacitors by their very nature damp inductive peaks very well - lot of capacitance, high enough ESR. For this reason, just slapping a standard elcap is the classical silver bullet to tame large MLCC banks and their hotplug voltage transients.
If the application is just some sort of AA battery holder with short wires into the electrolytic cap, I don't think any significant voltage transient is possible, unless the user connects some external power supply into the battery holder - which would definitely void the warranty.
BTW, OP, is there reverse polarity protection? If not, adding one would be a good measure. Maybe just use a boringly simple series diode for the job, and it also sheds some half of a volt off the voltage, solving the voltage rating issue as well.
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It's misleading because it might lead people into believing that reducing DC bias will extend lifetime, which isn't true in 99% of cases in the field, but reducing the temps absolutely, and dramatically would.
There is going to be a relationship between lifetime and both DC bias and temperature, but it isn't going to be a simple one that applies at all values. For example, I don't think lowering the temperature from 1000C to 970C will increase the capacitor life 8X, nor will raising it from -25C to -15C reduce it by half. It's all very complex and all the rules are just gross oversimplifications that make it possible to make some decisions.
I would point out that failure rate and lifetime, while related to an extent, can also be looked at as different issues. Derating addresses more than simple models, it also addresses manufacturing variances in both the component and the device in which it is installed, as well as abuse, user error and other random or unknown issues.
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A long time ago, I posted a question about voltage derating of electrolytic capacitors.
Basically, in my youth I learned that one should not operate an electrolytic "far below" its voltage rating, as well as not above it.
I have sought a more quantitative statement ever since.
None of the replies to my question was quantitative.
In vacuum tube days, the power supply filter capacitors were rated at a few hundred volts, and the cathode resistor bypass capacitors were rated at around 10 V, and they were not confused.
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Long ago, in the '60s, Sprague was very free with technical data, unlike today where everything is a state secret. Technology has changed, though the general trends are probably still true. I have several of the old Sprague papers on my site, all of which are worth reading, but you might look at page 16 of this one- http://www.conradhoffman.com/papers_lib/Sprague_62_7.pdf (http://www.conradhoffman.com/papers_lib/Sprague_62_7.pdf) One of the other papers talks about running caps at low voltages and says there's no issue right down to zero volts, or at least not enough to worry about.
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It's misleading because it might lead people into believing that reducing DC bias will extend lifetime, which isn't true in 99% of cases in the field, but reducing the temps absolutely, and dramatically would.
There is going to be a relationship between lifetime and both DC bias and temperature, but it isn't going to be a simple one that applies at all values. For example, I don't think lowering the temperature from 1000C to 970C will increase the capacitor life 8X, nor will raising it from -25C to -15C reduce it by half. It's all very complex and all the rules are just gross oversimplifications that make it possible to make some decisions.
I would point out that failure rate and lifetime, while related to an extent, can also be looked at as different issues. Derating addresses more than simple models, it also addresses manufacturing variances in both the component and the device in which it is installed, as well as abuse, user error and other random or unknown issues.
A I stated in my orignal post "Always check with the manufacturer since all capacitors may vary."
I was taught the adage that real engineers do the math, psychics and contractors make guesses. Rules of thumb are fine for making estimates, but they are no substitute for the math.
If an engineer designs an automobile device for average temperatures in Florida, it will be invalid if the owner moves to Alaska and takes their automobile with them. The only real way I know to approach that issue is to design not only for the worst-case scenario, but to also add my own personal unknow error factor.
If a bridge has over engineered specs nobody notices, if it is under engineered people could die.
Without knowing the application or the criticality of any device it is easy to say an 8v spec is no different than a 9v spec, but it all depends on how many lives and how much money are in play. The rules of play are set by the manufacturers of the component, they may not always be followed to the letter, but they should always be considered.
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Long ago, in the '60s, Sprague was very free with technical data, unlike today where everything is a state secret. Technology has changed, though the general trends are probably still true. I have several of the old Sprague papers on my site, all of which are worth reading, but you might look at page 16 of this one- http://www.conradhoffman.com/papers_lib/Sprague_62_7.pdf (http://www.conradhoffman.com/papers_lib/Sprague_62_7.pdf) One of the other papers talks about running caps at low voltages and says there's no issue right down to zero volts, or at least not enough to worry about.
Thanks for the reference. The curves on p. 16 show a monotonic decrease in failure rate with respect to voltage reduction that are truncated below 50% of rated voltage, apparently because of too few failures in the tests.
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Long ago, in the '60s, Sprague was very free with technical data, unlike today where everything is a state secret. Technology has changed, though the general trends are probably still true. I have several of the old Sprague papers on my site, all of which are worth reading, but you might look at page 16 of this one- http://www.conradhoffman.com/papers_lib/Sprague_62_7.pdf (http://www.conradhoffman.com/papers_lib/Sprague_62_7.pdf) One of the other papers talks about running caps at low voltages and says there's no issue right down to zero volts, or at least not enough to worry about.
or https://www.conradhoffman.com/papers_lib/Sprague_62_4.pdf (https://www.conradhoffman.com/papers_lib/Sprague_62_4.pdf)
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It is a shame that work like that has not been converted to an internet friendly form.
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I was taught the adage that real engineers do the math, psychics and contractors make guesses. Rules of thumb are fine for making estimates, but they are no substitute for the math.
Well math doesn't do you much good unless you have accurate data and models. A manufacturers specification doesn't give you the underlying information and the way those specs are determined can vary. At least a well-considered rule of thumb can add in some empirical data--the wisdom of experience.
If a bridge has over engineered specs nobody notices, if it is under engineered people could die.
Whether the bridge collapses or not is a matter of the actual materials and construction used, not their listed specifications. Headline or advertised specifications are especially unhelpful and misleading in many cases. I suppose modern expectations would be that specs would be exaggerated best-case maximums, but not always. How much weight can a 1-ton pickup truck haul?
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I was taught the adage that real engineers do the math, psychics and contractors make guesses. Rules of thumb are fine for making estimates, but they are no substitute for the math.
Well math doesn't do you much good unless you have accurate data and models. A manufacturers specification doesn't give you the underlying information and the way those specs are determined can vary. At least a well-considered rule of thumb can add in some empirical data--the wisdom of experience.
If a bridge has over engineered specs nobody notices, if it is under engineered people could die.
Whether the bridge collapses or not is a matter of the actual materials and construction used, not their listed specifications. Headline or advertised specifications are especially unhelpful and misleading in many cases. I suppose modern expectations would be that specs would be exaggerated best-case maximums, but not always. How much weight can a 1-ton pickup truck haul?
I don't disagree, models are math.
How much weight can a 1-ton pickup truck haul? What kind tone? A Tonne? Short Ton? Long Ton? Metric Ton? Ton Shortweight? Ton Longweight? They are all different specifications. With options or without? With driver or not?
I can guess from your words and context what you mean, but to find out I need to either check the manufactures specification and use the math to answer or destructively test some number of pickup trucks and math to find a statistical answer.
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The sequence to this thread focuses on the broader problem and solution:
https://www.eevblog.com/forum/projects/alkaline-battery-characteristi-effect-on-product/msg4204183/#msg4204183 (https://www.eevblog.com/forum/projects/alkaline-battery-characteristi-effect-on-product/msg4204183/#msg4204183)
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i just did a test and teardown video of a lab dc powersupply, sold in high volume,
it contain 35V large size caps, and they charge them to 42V DC all day long,
the unit is 20 years old, and nothing bad happened from this..
but it is a VERY bad design style, it reveal they take specifications light,.
some handle this fine, others not at all,,
it is recommended to perform design DERATING,
that means you always run at a voltage that is, the wanted margin, UNDER the max specified voltage,
this way you are sure to create good quality designs,