Author Topic: Solar BMS a solar charge controller for LiFePO4  (Read 36113 times)

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Offline electrodacus

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The last 5 days on Kickstarter currently at 83% if you where thinking on supporting this project there is not much time left.

Offline ScubaShan

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At the moment I manage my system (8kWh lithium with 1600W solar / 3000W alternator) with Bluesea ACRs, Bluesea RBS, Victron BMV, Victron MPPT controllers, SSRs, junsi cellog8s and some custom software. What would be interesting to me for the next release would be a "lite" version with the BMS functionality (cell voltage logging, cell LV/HV disconnect ) but using external COTS devices for power control. Multiple 100A SSRs ($50) could be used for PWM of charge sources, 500A relays could be used for load/charge circuit disconnection, and external shunts for current monitoring. This could make your box the centre of a very large (or small) system without you having to develop multiple SKUs.

Off topic:
One of the main reasons for wanting MPPT on boats and RVs is the lack of available roof space. Additional panels can be cheaper than an MPPT controller but if you don't have anywhere to put them MPPT comes to the rescue.

Grid tie panels are significantly (20%) more efficient than typical RV panels which means less roof space is required however you can only use these panels with MPPT controllers due to the higher voltage.

Horses for courses, PWM could make sense where you have unlimited space such as an off grid cabin but for those of us living mobile MPPT is king.





 

Offline mtdoc

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Off topic:
One of the main reasons for wanting MPPT on boats and RVs is the lack of available roof space. Additional panels can be cheaper than an MPPT controller but if you don't have anywhere to put them MPPT comes to the rescue.

Grid tie panels are significantly (20%) more efficient than typical RV panels which means less roof space is required however you can only use these panels with MPPT controllers due to the higher voltage.

Horses for courses, PWM could make sense where you have unlimited space such as an off grid cabin but for those of us living mobile MPPT is king.

Excellent points and similar to ones I made earlier.

Electrodacus is close to getting his funding. It's coming down to the wire and I hope he makes it. He has a interesting product that deserves funding.

But IMO he should really rethink his absolutist anti-MPPT stance.
 
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Offline HackedFridgeMagnet

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I think his stance is fine, for his circumstances and for many other people too. Hope he gets his target too.

His hardware wont easily support MPPT so it would be a big leap for him to change.
IIRC he says he will need to use electrolytics and or larger inductors to support MPPT.
This will take him out of his target market.
I am sure he has definitely thought this through.

He is not saying MPPT is bad it just that it wont work with this kind of on-off charger.
Also by not having electrolytics he will get a lifetime matching the solar panels.

One thing though,this statement isn't really fair.
Quote
LiFePO4 protected with Solar BMS can last 20 to 30 years where a typical Lead Acid will only last 4 to 6 years.
I am pretty sure you can easily get 10 years + out of Lead Acid if you look after them.
« Last Edit: April 01, 2015, 09:15:49 am by HackedFridgeMagnet »
 

Offline mtdoc

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He is not saying MPPT is bad it just that it wont work with this kind of on-off charger.

Actually, I think if you look through his postings you'll see he argues that MPPT is never needed now that PV prices are so low.

Regardless, I agree that there is a market for his product as it is now, just that it is a small one.
 

Offline electrodacus

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At the moment I manage my system (8kWh lithium with 1600W solar / 3000W alternator) with Bluesea ACRs, Bluesea RBS, Victron BMV, Victron MPPT controllers, SSRs, junsi cellog8s and some custom software. What would be interesting to me for the next release would be a "lite" version with the BMS functionality (cell voltage logging, cell LV/HV disconnect ) but using external COTS devices for power control. Multiple 100A SSRs ($50) could be used for PWM of charge sources, 500A relays could be used for load/charge circuit disconnection, and external shunts for current monitoring. This could make your box the centre of a very large (or small) system without you having to develop multiple SKUs.

You do not need (in fact is not recommended) that you use PWM for Lithium charging. The PWM part is to keep battery at constant voltage. For LiFePO4 you should only use one stage charging just the bulk part as soon as any cells get to your set limit say 3.55V the charging should be completely stop and only start recharging again if voltage drop below say 3.4V or so.
Lithium cells (all of them not just LiFePO4) will degrade while keeps at high voltage the less time they spend there the better.
The new SBMS will have multiple IO pins some designated for LV/HV disconnect so you can use external SSR if you want on top of that there will also be 24bit ADC inputs so you can measure the current on those external loads or chargers to be able to calculate battery SOC. 
Many other automations will be possible like measuring other external sensors or levels and taking actions based on they value or based on SOC values. 
 

Off topic:
One of the main reasons for wanting MPPT on boats and RVs is the lack of available roof space. Additional panels can be cheaper than an MPPT controller but if you don't have anywhere to put them MPPT comes to the rescue.

Grid tie panels are significantly (20%) more efficient than typical RV panels which means less roof space is required however you can only use these panels with MPPT controllers due to the higher voltage.

Horses for courses, PWM could make sense where you have unlimited space such as an off grid cabin but for those of us living mobile MPPT is king.

I see that there are more comments about MPPT here so for those that want to see in details my argument here is a link to my recent youtube video made specifically to explain this but there are some other obsolete technologies in there related to solar PV panels price


In short additional PV panels cost less than MPPT (for 8 cell LiFePO4 the 60 cells panels will work at max power point so MPPT is completely useless)
If space is a concern is usually on boats and RV where you can use LiFePO4 to save weight and use 60 cell panels to work at the max power point.
Also a 2% more efficient solar PV panel will do the same as the average 15% MPPT while costing less and lasting over 25 years.
But a very important reason that alone will make MPPT obsolete is that in Off Grid your energy use factor us usually well under 80% since battery is finite no matter how large.
So if you have you batteries charged before noon like I do all that an MPPT will do is have them charged 20 or 30 minutes earlier and that is completely useless since all the available energy available after that remains unused. 

Offline electrodacus

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One thing though,this statement isn't really fair.
Quote
LiFePO4 protected with Solar BMS can last 20 to 30 years where a typical Lead Acid will only last 4 to 6 years.
I am pretty sure you can easily get 10 years + out of Lead Acid if you look after them.

Most people I talk to are replacing they battery after 5 to 6 years at most. But probably at least in theory you can get 10 years out of high quality well cared Lead Acid but that is still not the important point. You will need a realy large capacity battery so that you can only use the top 10% to 20% in order to last that long 10 years are 3500 cycles (that sort of use is also extremely inefficient since the charge efficiency of Lead Acid in top 20% is around 50% ). Also look at the fact that I mentioned "typical" as in what is valid for most installations. 
Then you also need a generator and charge the batteries after a few days of cloud when you where not able to fully charge them else sulfation will occur.
I'm referring to offgrid energy storage not UPS type applications where you can probably get 10 years with very few cycles most of the time kept full.

Offline HackedFridgeMagnet

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I was referring to offgrid energy storage too.
I do know of maybe 200 installations in outback northern Australia where the batteries have never been replaced. Some of these have been around for over 10 years. Batteries are Sonneschein.
But yes they are only using 20% of installed capacity, and do have sophisticated charge controllers, although no generators.
The other thing is they get mild winters, but they do have to be careful about temperature.
« Last Edit: April 01, 2015, 01:45:15 pm by HackedFridgeMagnet »
 

Offline electrodacus

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I was referring to offgrid energy storage too.
I do know of maybe 200 installations in outback northern Australia where the batteries have never been replaced. Some of these have been around for over 10 years. Batteries are Sonneschein.
But yes they are only using 20% of installed capacity, and do have sophisticated charge controllers, although no generators.
The other thing is they get mild winters, but they do have to be careful about temperature.

I'm guessing they used Gel batteries.
This is the spec that I found http://www.sonnenschein.org/PDF%20files/GelHandbookPart1.pdf
Best one here the A600 series has quoted 1600 cycles based on IEC 60896-2 standard and this document is from 2003 so probably one of this batteries was used if is 10 years old already.
Here is part two of that document
http://www.sonnenschein.org/PDF%20files/GelHandbookPart2.pdf
From there page 27
"
Discharge conditions acc. to IEC 896
2: 20° C, discharge for 3 h at a current of I = 2.0 * I10
This is equivalent to a depth of discharge (DOD) of 60% C10
The possible numbers of cycles depends on different parameters, i.e.
sufficient re-charging, depth of discharge (DOD) and temperature.
"
It seems test are done at 20C and there is a high dependence with themperature of those particular Gel bateryes from page 35
"
15 years at 20° C becomes reduced to
7.5 years at 30° C
"
High temp do affect Lithium cells also.
As example storage degradation over 10 years on Sony LiFePO4 is about 3% of initial capacity at 23C and 10% at 40C if I remember correctly the numbers.

Comparing even this probably quite expensive gel with 7000 cycles at 70% DOD 0.3C of a Winston LiFePO4 there is still a huge difference.
Then LiFePO4 does not care about being fully charged in fact it prefers not to and as long as it is kept in the voltage limits and that is done automatically by the BMS
And I seen tests done at 10% DOD on LiFePO4 with 20k to 80k cycles.

Ideally in this times when solar PV is so inexpensive is to have larger PV array and smaller battery. That is easily possible with LiFePO4 do to higher charge discharge rates possible and better deep cycle life.
And as I mentioned the Digital MPPT here that will have a function to divert more or less of the large 9kW PV array used for heating to the Solar BMS for battery charging.
Ex my daily power consumption is peak 4kWh/day down to 0.8kWh / day in low power mode day (when more than a few days with clouds are expected).
Monthly average is 80kWh so just under 3kWh/day average.
Now the current 720W PV array can produce 3.5kWh in the best sunny day in the short day of winter in December or January and the worst case scenario extremely cloudy winter day 0.3kWh same months.
With the large 9kWh array extrapolating from this I will produce about 3.6kWh/day in the worst cloudy day so I can charge my battery using that large array as if it was a full sunny day so I only need capacity storage over night (no more need for 4 to 5 days autonomy or low power mode) so infinite autonomy no matter the weather condition.
All the electricity I use inside the house in any form will still end up as heat so no loss there.
The Digital MPPT will have multiple PV inputs so it will divert more or less of them to the Solar BMS for battery charging based on the amount of sun so that charge rate is maintained below 0.3C.
It is incredible how many things change do to low cost PV panels and the cost will probably continue to drop even is maybe not at the same rate as in the last few years. 
« Last Edit: April 01, 2015, 03:00:49 pm by electrodacus »
 

Offline HackedFridgeMagnet

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I don't know if they use Gel or Sealed or Unsealed Pb. I could find out but as you say it isn't that important. I was assuming Sealed Lead Acid.
I checked my email I have slightly misquoted the battery life.
Quote
Some of these have been around for over 10 years.
I Should have said
Quote
...systems are up to 10 years old and haven’t changed a battery bank yet

But don't get me wrong, you have sold me on the idea of LiFePO4 and I have even suggested it to the people who installed all the Sonnensheins.
 

Offline electrodacus

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I don't know if they use Gel or Sealed or Unsealed Pb. I could find out but as you say it isn't that important. I was assuming Sealed Lead Acid.
I checked my email I have slightly misquoted the battery life.
Quote
Some of these have been around for over 10 years.
I Should have said
Quote
...systems are up to 10 years old and haven’t changed a battery bank yet

But don't get me wrong, you have sold me on the idea of LiFePO4 and I have even suggested it to the people who installed all the Sonnensheins.

In OffGrid most people use flooded that require quite a bit more maintenance and if that is not done correctly they will last quite a bit less than specified. Gel are probably the type that will last the most so if they have 10 years already is probably a type of Gel battery.
Even between those Gel battery's there is a huge difference in cycle life and expected life and I'm sure there is also a difference in price to reflect that.
Gel is also used usually with very low charge discharge rates else irreversible damage will occur.
This are probably installed in some remote communication towers or something like this where load is almost constant and relatively small compared to battery capacity.
I have no interest in promoting any particular type of battery but my preference is clear that is why I chose to install LifePO4 on my house.
I see this almost like a capacitor or capacitor all you need to take care of is voltage limits low and high. As long as you stay in those limits they will last as specified and you will be able to estimate how much life they have left based on the amount of degradation. I'm realy curios to do this degradation test on my battery in about one months after two years of use. My prediction is 4% or less but I have if that will be the case. This GBS are not the greatest LiFePO4 available the 100Ah cells have about 1.8mohm internal DC resistance where mot others for same size batteries have about half and Winston even less at around 0.5mohm this seems to be a good indication of battery quality.
I do not think I will need a new battery any time soon but if I do I will probably go with Winston this time but I will be more sure after I do the capacity test next month.

Offline mtdoc

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With routine maintenance and proper charging, people routinely get 10+ years from standard flooded LA T105 deep cycle batteries from a reputable brand (e.g. Trojan).  Of course if you chronically undercharge  your batteries or discharge them below 50% SOC, lifespan is less. Many people undersize their solar array /battery bank size. Many try to use arrays with too low string voltages (e.g. expecting a "24 volt" grid tie panel to reliably charge a 24V battery bank). These things lead to shorter lifespan.  Higher temperature exposure also shortens lifespan.

For traction type flooded LA batteries (eg forklift type) 20 year lifespan is not unheard of.  For example one company that markets these specifically for solar installations - the HUP Solar 1 warranties their batteries for 10 years.

Another advantage of traction batteries are ability to tolerate deeper discharge. Disadvantages include lower charge efficiency and higher maintenance needs.

Flooded LA batteries have been used for decades in solar installations so these are well established facts.

Gel batteries are a very poor choice for solar installations - so are not used very much. There are several reasons for this but a big one is that they are very easily ruined by a single mishap of overcharging - so they typically have a short lifespan in real world solar installations.

AGMs can be a good choice. They generally have a longer float life, higher charge efficiency and require little maintenance so are a good choice for battery back up systems. The do not tolerate as many deep discharges/lifespan as FLAs.

LiFeP04s have several advantages - but the biggest are high charge efficiency, the ability to discharge deeper and more cycles over lifespan.  In theory they should have a longer lifespan than most FLAs  IF cared for properly.   But at this point they have not been proven to last more than 10 years in solar installations -they just haven't been widely available that long.  We all know that real world lifespan of anything involving electricity does not always match theoretical lifespan.

For proven lifespan alone - nothing beats Nickel Iron batteries. But they have several disadvantages as well.
« Last Edit: April 02, 2015, 06:34:54 am by mtdoc »
 

Offline electrodacus

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With routine maintenance and proper charging, people routinely get 10+ years from standard flooded LA T105 deep cycle batteries from a reputable brand (e.g. Trojan).  Of course if you chronically undercharge  your batteries or discharge them below 50% SOC, lifespan is less. Many people undersize their solar array /battery bank size. Many try to use arrays with too low string voltages (e.g. expecting a "24 volt" grid tie panel to reliably charge a 24V battery bank). These things lead to shorter lifespan.  Higher temperature exposure also shortens lifespan.

For traction type flooded LA batteries (eg forklift type) 20 year lifespan is not unheard of.  For example one company that markets these specifically for solar installations - the HUP Solar 1 warranties their batteries for 10 years.

Another advantage of traction batteries are ability to tolerate deeper discharge. Disadvantages include lower charge efficiency and higher maintenance needs.

Flooded LA batteries have been used for decades in solar installations so these are well established facts.

Gel batteries are a very poor choice for solar installations - so are not used very much. There are several reasons for this but a big one is that they are very easily ruined by a single mishap of overcharging - so they typically have a short lifespan in real world solar installations.

AGMs can be a good choice. They generally have a longer float life, higher charge efficiency and require little maintenance so are a good choice for battery back up systems. The do not tolerate as many deep discharges/lifespan as FLAs.

LiFeP04s have several advantages - but the biggest are high charge efficiency, the ability to discharge deeper and more cycles over lifespan.  In theory they should have a longer lifespan than most FLAs  IF cared for properly.   But at this point they have not been proven to last more than 10 years in solar installations -they just haven't been widely available that long.  We all know that real world lifespan of anything involving electricity does not always match theoretical lifespan.

For proven lifespan alone - nothing beats Nickel Iron batteries. But they have several disadvantages as well.

I agree with most of your comments.
Solar OffGrid is relatively bad for any type of Lead Acid battery since is intermittent and all type of Lead Acid hate to stay discharged for long period of times (Gel are the most immune to this and in some particular applications they are a great choice and can last a long time).
LiFePO4 are much more suited for Solar OffGrid since they like to stay in a partially state of charge.
LiFePO4 also dose not require any maintenance and as long as is protected by a proper BMS can not be damaged by user.
Thus LiFePO4 will actually survive much longer in a typical offgrid solar installation and get a much better return on investment.
I know what I say has less value since it seems that I have an interest in this with the Kickstarter campaign for the Solar BMS :)
I hope people will do their own research.
Sony and Bosch have a nice LiFePO4 complete solution but they mostly ofter that for grid tie where market is much larger.
Sony for example offers up to 20 years warranty on their complete solution and their battery can do 6000 cycles of 100% DOD before getting at 80% of original capacity.
They also made a degradation test in storage and extrapolated the data to 10 years and in 10 years the loss in capacity was 3% at 23C and 10% at 40 or 45C.

Lead Acid battery is normally quote as end of life at 60% of original capacity and from there degradation is steep nothing usable.
LiFePO4 is quoted as end of life at 80% of original capacity and degradation continues to be almost linear so if you want to go to 60% it will probably last 2x or more since degradation seems to slow down similar to what you see on solar PV panels.
I also find LiFePO4 much safer than any other battery and quite green no Cobalt as on the other Lithium batteries it also uses quite a bit less Lithium than those for the same capacity.

Offline mtdoc

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Lead Acid battery is normally quote as end of life at 60% of original capacity and from there degradation is steep nothing usable.
LiFePO4 is quoted as end of life at 80% of original capacity and degradation continues to be almost linear so if you want to go to 60% it will probably last 2x or more since degradation seems to slow down similar to what you see on solar PV panels.

I'm not sure where your are getting this from but it has nothing to do with real world uses IME.  No one off grid would keep their batteries down to 60% unless they severely overestimated their power needs when purchasing the original pack (usually it's the opposite - people underestimate their power needs).

Solar 1's 10 yr warranty is for 80% of original capacity (prorated after 7 yrs) as is other lead acid battery warranties I have seen.  Any examples of warranties on FePO4 battery packs? I'm sure there must be some - I just haven't seen any yet.
 

Offline electrodacus

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I'm not sure where your are getting this from but it has nothing to do with real world uses IME.  No one off grid would keep their batteries down to 60% unless they severely overestimated their power needs when purchasing the original pack (usually it's the opposite - people underestimate their power needs).

Solar 1's 10 yr warranty is for 80% of original capacity (prorated after 7 yrs) as is other lead acid battery warranties I have seen.  Any examples of warranties on FePO4 battery packs? I'm sure there must be some - I just haven't seen any yet.

Cycle tests on all Lead Acid batteries I seen are for 60% original capacity so the cycle numbers you see in their spec is for that.
Lead Acid also has quite a steep drop once it starts to do so.
Here is a graph but just google search images on "Lead Acid cycle life" and you will see all graph down to 60% original capacity as the end of useful life

here is another example

And I remember checking the standard tests and how they are done and I found the same thing.
It makes not much difference anyway since from 80% to 60% there are very few cycles left so when you noticed that (hard to notice if you are using just 20% DOD usually) you will need to act fast in replacing them.
On the other side the cycle life for LiFePO4 is almost a linear degradation so you have years to replace your battery maybe even a decade if you find a way to reduce consumption with say 10% so that you can use it down to 70% of original capacity.
Here is the Sony LiFePO4 cycle life
« Last Edit: April 02, 2015, 11:39:35 am by electrodacus »
 

Offline mtdoc

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Nice graphs but not really relevant to the question of real world lifespan as I discussed in prior post.

I really hope that LiFePO4 live up to their theoretical lifespan in real world RE system use. My current 5 yr old AGM (backup) battery bank likely has 8-10 years left in it (they're rated for 15 yr float life).  I'd like to be able to replace it when the time comes with a battery bank that will last 30 yrs! 

Still would like to see some LiFePO4 battery warranties...

 

Offline electrodacus

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Nice graphs but not really relevant to the question of real world lifespan as I discussed in prior post.

I really hope that LiFePO4 live up to their theoretical lifespan in real world RE system use. My current 5 yr old AGM (backup) battery bank likely has 8-10 years left in it (they're rated for 15 yr float life).  I'd like to be able to replace it when the time comes with a battery bank that will last 30 yrs! 

Still would like to see some LiFePO4 battery warranties...

I'm mostly talking about OffGrid applications and not backup. In backup you barely use the battery so is more about the storage life span.
Sony offers 20 years warranty but only for the complete system including the BMS.
The real advantage of LiFePO4 is when is used quite hard even in the case of Sony battery.
6000 cycles of 100% DOD is about 20 years of 100% DOD every day and that is why they target the Grid tie solar applications where they can use the battery to store energy when is inexpensive and sell it or use it when is expensive.
Also LiFePO4 will not like to stay in float so you will probably need to keep it charged at 80% or so most of the time and only charge back to around 80% when you need to discharge it.
Is just a totally different type of battery in therms of use scenario and is mostly great in OffGrid solar where you do quite a lot of cycling.
If you look at my 7 day energy production consumption graph you will see that there are days where I use more than battery capacity from the battery in the way that I charge and discharge the battery a few times in the same day maybe just 20 to 30% DOD but 3 or 4 times a day.
I will say for just backup type application an AGM battery is a good choice (at least that is my guess I did not think about that to much).
I think is not fair to think about more than 20 years of life with LiFePO4 maybe is possible but in 20 years I expect a lot to have changed in battery technology. LiFePO4 is quite new about 2002 for the first commercial one if I remember correctly.

Offline mtdoc

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I'm mostly talking about OffGrid applications and not backup.

So was I.  I only brought up my current system to point out that I am hoping LiFePO4s live up to the hype!

I have experience with off grid systems and that is what I was referring to previously in my posts (go back and look).

Again - off grid users who properly care for their lead acid batteries often get 10+ (or 20+ in the case of traction batteries) out of them.

Quote
Sony offers 20 years warranty but only for the complete system including the BMS.

Cool. Link?

Quote
LiFePO4 is quite new about 2002 for the first commercial one if I remember correctly.

Yes - that was my earlier point. It's too soon to know how long large LiFePO4 battery banks will last in the field when used in actual off grid PV systems. Time will tell.
 

Offline electrodacus

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So was I.  I only brought up my current system to point out that I am hoping LiFePO4s live up to the hype!
I have experience with off grid systems and that is what I was referring to previously in my posts (go back and look).
Again - off grid users who properly care for their lead acid batteries often get 10+ (or 20+ in the case of traction batteries) out of them.

You need a battery a few times larger in capacity so that you can only discharge the top 10 to 20% to achieve long battery life with Lead Acid that is not always possible in s solar application here winter is about 4 to 5 months and there are groups of 3 to 4 days where you can not say where the sun is and that Lead Acid battery will get quite a bit deeper discharge unless you have a separate gasoline generator to charge it back up and not keep it for days at low charge levels.
It also need venting and again in this cold climate I live in that will have the battery at almost ambient temperature to to this external venting requirement.
It seem to me that was just impossible to use a Lead Acid offgrid here in Canada that is why looked at other possibilities.
I do not have a noisy and dirty gasoline generator as most do with Lead Acid and use that quite a bit in winter. 
   
Cool. Link?
http://download.solarshop.net/english/uploads/FS-UK-Sony-Storage-system-data-sheet-10-08-2012.pdf

What I consider a benchmark when selecting a battery is the cost of storing a unit of energy.
This are the sort of rough calculations that I do.
And I trust LiFePO4 way more since is not possible to damage by keeping it discharged because you have no sun or not doing proper maintenance in the case of flooded types.
Keep in mind the L16RE-2V is the newest "smart carbon" battery available based on spec hugely superior to the previous generation not sure I buy that but I respect the spec based on the new spec for this battery it makes no difference to cycle life if you discharge the battery 20% or 100%

« Last Edit: April 03, 2015, 04:00:07 am by electrodacus »
 

Offline ScubaShan

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I see that there are more comments about MPPT here so for those that want to see in details my argument here is a link to my recent youtube video made specifically to explain this but there are some other obsolete technologies in there related to solar PV panels price



You make a few assumptions and mistakes in this video otherwise well presented video.
Your Solar BMS chart shows 27v @ 65c panel temp however at 65c the panel Vmp will be 26v which is below the battery charging voltage so you wouldn't be getting much charge into the battery.

In warmer climates like Australia where temps regularly see 45c we get panel temps in excess of 80c so with that panel we'd be looking at a Vmp of just 24v, which would be pretty useless for charging a 24v battery.

Boats and RVs usually operate across various climates, here in oz for example it can be 45c in one location and 0c a few hours drive away. A higher string voltage + MPPT makes more sense because of the wide range of operating environments encountered while travelling.

As I said, horses for courses and I'm sure PWM or on/off charging is working fine for your cabin system but it would be foolish to write off MPPT for all users.
 

Offline mtdoc

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Cool. Link?
http://download.solarshop.net/english/uploads/FS-UK-Sony-Storage-system-data-sheet-10-08-2012.pdf
Thanks for the link but still no warranty info. It's fine and good to claim 20 yr life but without a warranty or a proven track record it's all just marketing hype.

Quote
And I trust LiFePO4 way more since is not possible to damage by keeping it discharged because you have no sun or not doing proper maintenance in the case of flooded types.

Not true. This is something I do have first hand experience with. i have used LiFeP04 batteries in eBikes for several years and you can if fact damage cells irreversibly by failing to keep them charged. I did it once with a $500 LiFePO4 ebike pack! ::)

Again - I am a fan of LiFePO4 and suspect they will eventually replace LA as the dominant chemistry for RE systems. I'm just trying to separate the reality from the hype.
 

Offline mtdoc

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You make a few assumptions and mistakes in this video otherwise well presented video.
Your Solar BMS chart shows 27v @ 65c panel temp however at 65c the panel Vmp will be 26v which is below the battery charging voltage so you wouldn't be getting much charge into the battery.

In warmer climates like Australia where temps regularly see 45c we get panel temps in excess of 80c so with that panel we'd be looking at a Vmp of just 24v, which would be pretty useless for charging a 24v battery.


Yes - I was making the same point earlier in this thread (see post #5). You need an STC VMP of 32+V to reliably charge a 24V battery bank. Warm temps, any shading, etc prevent the standard, lowest cost "grid tie"  "24V" panels from being used in single panel strings for 24V nominal systems or 2 panel strings for 48V nominal systems.  I suppose with LiFePO4 you could get away with it by using non traditional bank sizing [e.g. 7s or 14s arrangement for  "24V" or "48V" nominal systems)
 

Offline electrodacus

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    • electrodacus
You make a few assumptions and mistakes in this video otherwise well presented video.
Your Solar BMS chart shows 27v @ 65c panel temp however at 65c the panel Vmp will be 26v which is below the battery charging voltage so you wouldn't be getting much charge into the battery.

In warmer climates like Australia where temps regularly see 45c we get panel temps in excess of 80c so with that panel we'd be looking at a Vmp of just 24v, which would be pretty useless for charging a 24v battery.

Wrong about the way PV panels work. Yes Vmp may be lower say 26V but if your battery is 27V the current will drop a bit and you will not be anymore at the max power point but still quite close.
So even if cells are at 80C and battery at 27V you will still see a charge current.
Not to mention that usually when is sunny at peak in the afternoon is when you have those cells temperature and is also when you have the most excess power anyway.
Here is a graph from a 250W (60 cells) but is low resolution sorry is what I found on a quick google search
If you look closely even at 30V and 65C you still get around 4A LiFePO4 is basically charged at 27V and probably with no load as soon as you connect a significant load the voltage will drop and you get the max power point current. I'm offgrid with this sort of set-up and even if here is just below 40C panels still get relatively hot in some summer days. From what I remember 38C is about the record hot days here.


Quote from: mtdoc
Not true. This is something I do have first hand experience with. i have used LiFeP04 batteries in eBikes for several years and you can if fact damage cells irreversibly by failing to keep them charged. I did it once with a $500 LiFePO4 ebike pack!

You probably had that improper BMS used on eBikes that uses that 5 pin IC designed for LiCoO2 and not LiFePO4 with 2V and 3.9V limits well outside the LiFePO4 limits of 2.8V and 3.6V
If you live a LiFePO4 for months at 20% SOC there will be absolutely no degradation. My SBMS limits are set at 3V and 3.55V so say I'm not home for months the panes from so reason fail and the battery discharge slowly to 3V when the SBMS will disconnect the load there is enough charge in the battery 10% at least at this point to keep the battery above the critical limits for quite some time.
With my 100Ah cells 10% represent 10Ah with your smaller bike cells it will have been much less. There is a self discharge of around 2% probably per months so I cold not live them there from more than 4 to 5 months in theory then there is the SBMS as a load even if is just 3mA for SBMS4080 it will still add 2.16Ah/month or about the same as self discharge so that will cut the possible survival to about 2 months from 3V or around 10% SOC .
So is very easy to see how your smaller ebike battery failed. As long as you keep the battery always above 2.8V (2.5V is acceptable under high load) and below 3.6V they will last as specified.

Offline mtdoc

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You make a few assumptions and mistakes in this video otherwise well presented video.
Your Solar BMS chart shows 27v @ 65c panel temp however at 65c the panel Vmp will be 26v which is below the battery charging voltage so you wouldn't be getting much charge into the battery.

In warmer climates like Australia where temps regularly see 45c we get panel temps in excess of 80c so with that panel we'd be looking at a Vmp of just 24v, which would be pretty useless for charging a 24v battery.

Wrong about the way PV panels work.
Nope, he's not wrong.

Quote
Yes Vmp may be lower say 26V but if your battery is 27V the current will drop a bit and you will not be anymore at the max power point but still quite close.
So even if cells are at 80C and battery at 27V you will still see a charge current.
Not to mention that usually when is sunny at peak in the afternoon is when you have those cells temperature and is also when you have the most excess power anyway.
It's not just about the power. PV panels act as current sources. But batteries have minimum voltages that they require to charge correctly.

Quote
You probably had that improper BMS used on eBikes that uses that 5 pin IC designed for LiCoO2 and not LiFePO4 with 2V and 3.9V limits well outside the LiFePO4 limits of 2.8V and 3.6V
  Nope. That wasn't it.


Quote
As long as you keep the battery always above 2.8V (2.5V is acceptable under high load) and below 3.6V they will last as specified.
Yep, that's the issue. If you don't have your battery connected to a charging source and voltage drops too low (due to slow spontaneous discharge) a BMS can't prevent that.  So LiFePO4, just like LA can be ruined if not kept sufficiently charged - which was my point.
« Last Edit: April 03, 2015, 04:47:21 am by mtdoc »
 

Offline electrodacus

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    • electrodacus
Nope, he's not wrong.
It's not just about the power. PV panels act as current sources. But batteries have minimum voltages that they require to charge correctly.

Look at the IV curve especially the 65C one even if your battery is 30V you still get 4A. Do not forget that 60 cells PV panels is exactly what I use and even if my location does not get as warm as other places is still warm enough to be relevant.
The max power point voltage is all that it is an optimum point on a curve. If that is 26V at 65C you still get current at 27V a bit less and not optimum but you do.

Yep, that's the issue. If you don't have your battery connected to a charging source and voltage drops too low (due to slow spontaneous discharge) a BMS can't prevent that.  So LiFePO4, just like LA can be ruined if not kept sufficiently charged - which was my point.

What does slow spontaneous discharge means ?
A BMS protected battery (of course proper BMS) will have no problem as explained in the earlier example. You need to consider the battery self discharge rate and the BMS power consumption the you can know exactly how long will it last before it needs recharge after charger is lost and load is disconnected. The two months for my system seems more than reasonable since I will never be away from the system for more than two months. And the chance of PV array to fail is extremely low.


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