Lower capacity at lower discharge current

[BYqSXt8Z]

So I'm trying to test some lifepo4 batteries and I've come across a very strange behaviour. I'm hoping someone here may have an explanation for the phenomenon in question.

When I discharge batteries at 0.5c, I get near nominal capacity, and the measured charge Wh is generally pretty close to the discharged Wh. Now there are are thermal losses and I'm suing 2 different devices, so I can forgive the 5% or so difference I see.

Where it becomes puzzling is at 0.025c. Discharge capacity becomes less than half of nominal, but charging on the same device charges up the nominal amount.... What could be causing this? Is it normal?

At first I assumed a faulty cell with a large power loss over time and so a longer test would be more affected, but it seems to be an effect I can repeat. Without a good ir tester I also can't do much to confirm that aspect... Any insight would be helpful.

Thanks!

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[flat tire]

How are you measuring the discharge capacity?

 

[BYqSXt8Z]

Two different ways, the first is with a dedicated dc watt meter, the other is with my b6 charger.

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[amberwolf]

Where it becomes puzzling is at 0.025c. Discharge capacity becomes less than half of nominal, but charging on the same device charges up the nominal amount.... What could be causing this? Is it normal?

Sounds like your discharge wattmeter is not accurate at low currents for some reason, but your charging wattmeter is normal.

Or if charge and discharge are on the same device(s), then the charging is at a higher rate so the meter(s) read normally then, but not at the lower rate.

 

[john61ct]

Are you saying you are relying on mAh totalizers, counting coulombs?

As opposed to using an CC electronic load tester and precise timing to determine discharging capacity?

 

[BYqSXt8Z]

Are you saying you are relying on mAh totalizers, counting coulombs?

As opposed to using an CC electronic load tester and precise timing to determine discharging capacity?

I'm using a load tester for the higher discharge, it's the lower discharge rate (0.025c) that I did with the b6ac in a charge (0.25c) discharge (0.025c) cycle.

However some packs get much closer to their nominal capacity at the 0.025c discharge rate, even if they are still 20-30% off... Still better than 50-60% off.

Do you have a reference device to recommend so I can validate everything?

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[BYqSXt8Z]

Are you saying you are relying on mAh totalizers, counting coulombs?

As opposed to using an CC electronic load tester and precise timing to determine discharging capacity?

Oh wait you mean constant current or coulomb counter?

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[john61ct]

First off, the same constant-current controller device (electronic dummy load) should be used for both rates for accurate comparability.

Make sure the same precise 100% charge point is reached before every test, holding the exact same Absorb termination / voltage until a precisely (accurately) measured endAmps using the same known-good ammeter, ideally IMO until current falls to zero, or a low point like 0.005C.

Discharge at your chosen C-rate, precisely held there - Constant Current - by your dummy electronic load varying watts as V is falling.

Just manually twiddling a dimmer switch on bulbs or something crude like that, is not accurate enough for a test under say the 5-hour rate, 0.2C

Obviously use the same voltage stop point in both tests, too fast a rate will certainly give inaccuracy there, faster the rate lower the capacity tested, I would spec 1C as the fastest.

The standard for large lead banks is the 20-hour rate, 0.05C, just for comparison.

The actual measurement is the number of seconds between 100% and 0%, not using a coulomb-counter directly. Best of course is a load tester that automates the timing and the precision stop-discharge.

And always use constant current, measuring Ah or mAh capacity, never constant power measuring Wh.

Do not let the batteries sit at 0% of course, get them back charging at a slow rate is OK, ASAP after hitting bottom.

Hope that helps.

 

[BYqSXt8Z]

First off, the same constant-current controller device (electronic dummy load) should be used for both rates for accurate comparability.

Make sure the same precise 100% charge point is reached before every test, holding the exact same Absorb termination / voltage until a precisely (accurately) measured endAmps using the same known-good ammeter, ideally IMO until current falls to zero, or a low point like 0.005C.

Discharge at your chosen C-rate, precisely held there - Constant Current - by your dummy electronic load varying watts as V is falling.

Just manually twiddling a dimmer switch on bulbs or something crude like that, is not accurate enough for a test under say the 5-hour rate, 0.2C

Obviously use the same voltage stop point in both tests, too fast a rate will certainly give inaccuracy there, faster the rate lower the capacity tested, I would spec 1C as the fastest.

The standard for large lead banks is the 20-hour rate, 0.05C, just for comparison.

The actual measurement is the number of seconds between 100% and 0%, not using a coulomb-counter directly. Best of course is a load tester that automates the timing and the precision stop-discharge.

And always use constant current, measuring Ah or mAh capacity, never constant power measuring Wh.

Do not let the batteries sit at 0% of course, get them back charging at a slow rate is OK, ASAP after hitting bottom.

Hope that helps.

Thanks, I was going with constant load (resistors)

Do you have a device with constant watts you can recommend?

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[john61ct]

Here's a start, but of course googling the right keywords will yield a lot more

http://www.skoolie.net/forums/showthread.php?p=342306

 

[john61ct]

Thanks, I was going with constant load (resistors)

Do you have a device with constant watts you can recommend?

And again, its **current** amps you are holding constant, Ah the units you are measuring.

Power, watts and resistance are what **varies** as the voltage is not constant.

 

[BYqSXt8Z]

Here's a start, but of course googling the right keywords will yield a lot more

http://www.skoolie.net/forums/showthread.php?p=342306

Huge thanks, and noted for the CC! You've been great help... Time for more troubleshooting!

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[john61ct]

Other useful threads

https://endless-sphere.com/forums/viewtopic.php?t=98407

https://www.rcgroups.com/forums/showthread.php?2406395-An-effective-variable-load-automatic-LiPo-discharger/page32#post40969831

https://www.rcgroups.com/forums/showthread.php?1107072-How-to-build-a-constant-current-discharger

 

[BYqSXt8Z]

Other useful threads

https://endless-sphere.com/forums/viewtopic.php?t=98407

https://www.rcgroups.com/forums/showthread.php?2406395-An-effective-variable-load-automatic-LiPo-discharger/page32#post40969831

https://www.rcgroups.com/forums/showthread.php?1107072-How-to-build-a-constant-current-discharger

Would the charge wh or ah be more reliable than the discharge on a b6ac ?

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[john61ct]

I dunno, they don't have a good rep for accuracy in general.

And dunno what you mean by "wh or ah"?

Again discharge capacity testing is done with constant current, units are Ah or mAh.

Wh should not come into it at all.