Author Topic: EEVblog #393 - LiPo Battery Discharge Testing  (Read 28468 times)

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Offline EEVblog

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EEVblog #393 - LiPo Battery Discharge Testing
« on: December 02, 2012, 02:36:03 am »


Dave.
 

Offline TriodeTiger

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #1 on: December 02, 2012, 02:37:54 am »
Wowy, I was literally thinking about testing a few cells I've had with loads for the purpose of creating resistive heaters for the cold winter, and this appeared, literally seconds before I checked!

*curls up like a kitten to watch*
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Offline LEECH666

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #2 on: December 02, 2012, 03:44:47 am »
I was literally about post questions in the beginner forum on how to characterize a Lipo, or well a Lipo powered USB power bank. Not sure if the claimed 12000 mAh are realistic. Especially since I found a little instruction manual paper card that says 4000-14000 mAh, but it doesn't say if this is model dependent or just a vague guess on the capacity.


Anyway, excellent video and timing Dave!
 

Offline baoshi

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #3 on: December 02, 2012, 04:12:12 am »
Nice video Dave! Exactly something I'm waiting for.

Do you need to change the charging circuit to cater for the larger capacity battery?


Bob

PS: Notice the unpopulated footprint onboard. FT230XS is now stocking in farnell UK

PS2: The Turnigy Accucell 6 charger seems to be a good candidate for your next tear down  ;D
« Last Edit: December 03, 2012, 09:31:41 am by baoshi »
 

Offline EEVblog

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #4 on: December 02, 2012, 04:59:04 am »
Do you need to change the charging circuit to cater for the larger capacity battery?

No, it just takes longer.

Dave.
 

Offline Psi

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #5 on: December 02, 2012, 08:10:35 am »
Nice video :)

There is an issue you may not be aware of, RC batteries, like the ones from hobby king, are a slightly different LiPo design to the kind you might get in a laptop or other device. They do not have any protection at all and will puff up/explode if shorted.

But their main difference is they take advantage of a battery characteristic which is normally undesirable (but dont matter with RC planes) in order to get their really high capacity.
High power RC LiPo batteries do not like to be fully charged for very long, it will shorten their live considerably. When you use one of these batteries for flying your RC plane you charge it up to 100% and then fly your plane in the next few days. When you get home all the batteries get put onto the charger in "storage mode" which will either charge or discharge them to a set voltage safe for storage.

I don't know just how significant this effect/damage is but its something that should be checked out as this kind of high discharge RC battery may not be very suitable for a usb powersupply where you want to keep it fully charged in your bag.

Here's a quote from http://www.rchelicopterfun.com/rc-lipo-batteries.html
Quote
You must store them charged, but not fully charged either – that will also degrade the cell matrix. Basically, the speed at which a LiPo pack ages (during storage) is based on both storage temperature and state of charge. You are likely ok to store a fully charged RC LiPo battery at room temperature for up to 4 days without doing much damage. Never store a LiPo in a hot car fully charged for an extended time, that will certainly cause damage as I explained earlier, but it worth repeating.

For optimum battery life, store your RC LiPo batteries at room temperature and at about 40-60% charged. That equates to around 3.85 volts per cell (open terminal resting voltage). The actual storage range is likely a little broader than this (I have heard some say numbers as high as 20-80% is fine, but since computerized chargers set the storage charge at 50% (3.85 volts per cell) that's what I recommend and what I follow myself.
« Last Edit: December 02, 2012, 09:03:36 am by Psi »
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Offline T4P

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #6 on: December 02, 2012, 08:32:26 am »
Yeah, trying to protect a battery capable of putting out 100A is not particularly easy with limited board space ^-^
The most powerful batteries are usually 26650 with possibly fake plate ratings @ 5000mAH (more likely 4000mAH) at 1C ratings for 26650 with a maximum of 2C for 18650 batteries so 900lm SSC-P7/XML-T6 blindlights are usually stretching the very cheap 18650s with only much less than rated faceplate probably about 1AH
 

Offline IanB

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #7 on: December 02, 2012, 08:46:56 am »
I have to say I was slightly surprised when the capacity measurement came out bang on spec like that. Some low cost batteries are notorious for having vastly inflated capacity numbers on the label. Thumbs up for the vendor in this case  :-+
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Offline EEVblog

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #8 on: December 02, 2012, 08:48:37 am »
Yeah, trying to protect a battery capable of putting out 100A is not particularly easy with limited board space ^-^

If you don't need the 100A, not at all!
A 5A polyswitch or something in series will do nicely.
But of course once the battery is installed and connected to the correct charging circuitry, it doesn't need protection. As a) the terminals aren't going to spontaneously short out, and b) the charger circuitry isn't going to try and mysteriously overcharge charge.

Maybe I could just leave the power on all the time to prevent full charge ;D

Dave.
 

Offline EEVblog

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #9 on: December 02, 2012, 08:50:13 am »
I have to say I was slightly surprised when the capacity measurement came out bang on spec like that. Some low cost batteries are notorious for having vastly inflated capacity numbers on the label. Thumbs up for the vendor in this case  :-+

Yes, Hobby King have a big rep, and a review comments system. So if a product doesn't meet spec, everyone would soon know about it!

Dave.
 

Offline Psi

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #10 on: December 02, 2012, 08:56:05 am »
Yeah, trying to protect a battery capable of putting out 100A is not particularly easy with limited board space ^-^
A 5A polyswitch or something in series will do nicely.

I think a typical polyswitch is rated for 50-100A maximum input and you'd get far more than that if the battery did get shorted, probably 1kA or so.

Although i guess the PCB tracks to the polyswitch will limited the current somewhat. So it might be fine.
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Offline EEVblog

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #11 on: December 02, 2012, 09:02:56 am »
I think a typical polyswitch is rated for 50-100A maximum input and you'd get far more than that if the battery did get shorted, probably 1kA or so.
Although i guess the PCB tracks to the polyswitch will limited the current somewhat. So it might be fine.

That's the tick isn't it. How is the battery going to get magically (solidly) shorted when it's in the case.
The polyswitch would be almost token.

Dave.
 

Offline EEVblog

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #12 on: December 02, 2012, 09:20:12 am »
I don't know just how significant this effect/damage is but its something that should be checked out as this kind of high discharge RC battery may not be very suitable for a usb powersupply where you want to keep it fully charged in your bag.

The micro can sense the battery voltage and also switch the unit off. So it's possible for the firmware to go into a "storage" mode if you say, try and switch it off when the voltage is above 4.xV or something.

Dave.
 

Offline ejeffrey

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #13 on: December 02, 2012, 09:35:50 am »
I would expect that the cold resistance of the polyswitch would limit the current to considerably less than 1000 amp.  Even if it doesn't, the battery won't care a bit about the brief overload while the switch heats up.

In this case, a polyswitch wouldn't be so much to protect the battery as to protect the PCB tracks/wiring from the battery.
 

Offline Psi

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #14 on: December 02, 2012, 09:53:04 am »
I don't know just how significant this effect/damage is but its something that should be checked out as this kind of high discharge RC battery may not be very suitable for a usb powersupply where you want to keep it fully charged in your bag.

The micro can sense the battery voltage and also switch the unit off. So it's possible for the firmware to go into a "storage" mode if you say, try and switch it off when the voltage is above 4.xV or something.

Dave.

Yeah, it would need to have a built in dummy load though.

When plugged into a usb port it would charge to 100% then start a counter which would begin an automatic discharge to 3.8V after 4 days.

« Last Edit: December 02, 2012, 09:54:40 am by Psi »
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Offline EEVblog

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #15 on: December 02, 2012, 09:56:17 am »
Yeah, it would need to have a built in dummy load though.
When plugged into a usb port it would charge to 100% then start a counter which would begin an automatic discharge to 3.8V after 4 days.

Yes, it assumes of course that the USB wouldn't be connected permanently.

Dave.
 

Offline EEVblog

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #16 on: December 02, 2012, 10:32:20 am »
Well, it seems that many places recommend storing these high C batteries at much reduced capacity, but I have yet to find any actual definitive data on it.
In which case that kinda sucks, and it's probably better to go with a regular low-c type, like an extended capacity mobile phone battery or something. But of course fakes in that market are rife.
It would have to be universally available too.
Suggestions?

Dave.
 

Offline SeanB

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #17 on: December 02, 2012, 10:44:32 am »
Then just charge it to 4.05V and accept the battery will never be fully charged, and compromise on life slightly. If charging on USB that would be the likely case, as it will take 4 days to fully charge the 5AH cell from a standard high power USB current of 500mA. It would probably be good to have an external 12-19V input to a separate charger chip, which will enable it to be charged from a laptop PSU easily, and will satisfy most users. Charge time on that would be under 2 hours, and it would be usable while charging ( though at the expense of higher output noise though) without killing USB power delivery, allowing use on a unpowered hub ( common laptop limited ports).
 

Offline Psi

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #18 on: December 02, 2012, 11:01:34 am »
yeah, i agree. just have a default max charge volts that's lower than spec.

People can hack the firmware to use 100% charge if they want, or make it a menu option.
« Last Edit: December 02, 2012, 11:07:20 am by Psi »
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Offline EEVblog

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #19 on: December 02, 2012, 11:09:28 am »
yeah, i agree. just have a default max charge volts that's lower than spec.
People can hack the firmware to use 100% charge if they want, or make it a menu option.

The micro does not control the charge rate, it's a Microchip charger IC.

Dave.
 

Offline Psi

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #20 on: December 02, 2012, 11:12:01 am »
ah. i figured the charge IC would have SPI and be programmable for the charge specs of the battery used.
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Offline ecat

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #21 on: December 02, 2012, 11:14:45 am »
Well, it seems that many places recommend storing these high C batteries at much reduced capacity, but I have yet to find any actual definitive data on it.

See the table on page 1
http://ozarkmountainbarnstormers.com/pdf%20files/Lipo%20Storage%20Tips.pdf

It's something I want to know too.

Given the above and the cost of the batteries and this application, is even a potential 20% reduction in capacity over the 1st year a significant concern?
 

Offline firewalker

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #22 on: December 02, 2012, 11:17:12 am »
Are those batteries bundled with a protection circuit of some kind (temperature e.t.c.). Can someone access the the data?

Are batteries for famous tablets/readers available and suitable for your needs?

Alexander.
Become a realist, stay a dreamer.

 

Offline Psi

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #23 on: December 02, 2012, 11:21:43 am »
Are those batteries bundled with a protection circuit of some kind (temperature e.t.c.). Can someone access the the data?

Are batteries for famous tablets/readers available and suitable for your needs?

Alexander.

na, RC batteries have no protection. It's almost impossible to have protection when you have legitimate loads of up to 200A.
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Offline EEVblog

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Re: EEVblog #393 - LiPo Battery Discharge Testing
« Reply #24 on: December 02, 2012, 11:22:56 am »
Then just charge it to 4.05V and accept the battery will never be fully charged, and compromise on life slightly. If charging on USB that would be the likely case, as it will take 4 days to fully charge the 5AH cell from a standard high power USB current of 500mA.

How do you figure 4 days?
at 1/10th C it takes only 10 hours to get to 85% capacity in the constant current mode.

Dave.
 


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