charge up to 8300 mAh, but get 6400 mAh in capacity test

wattwatt

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I'm capacity testing a Headway LiFePO4 38120 cell - charging puts 8300 mAh in, but capacity testing/discharging it only gives me6400 mAh (cut off is 2.5 volts). I've ran this test multiple times and get essentially the same resulting mAh's even after tryingdifferent charging current (12 A, 6 A, 1 A and trickling in 100 mA), different discharging current (800 mA and400 mA) and even discharging immediately after charging. Any idea why I'm not getting 1900 mAh more when discharging?
 

daromer

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What tester? Is the cell warm?

If you have a proper tester like an icharger that do the test properly.. This will show you that you have losses during charge in heat as example. Either due to high ir or selfdischarging or something else.
 

Redpacket

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Like daromer is suggesting:
- the charger may not be logging the AHrs in/out correctly (most likely this)
- if the cells are getting warm (big temp rise above ambient, they should barely get any warmer)
- you're relying on the ends of the curve areas too much (set low = 2.9V, high = 3.45V)
- might have thin cables charger to battery or bad cell contacts (too much resistance = losses...)?

For the first point, you could measure the currents & voltages when charging/discharging & compare to what the charger says.
For last point, measure the voltage drop across the wire when charging/discharging from charger output terminal to the cell end metal.
 

Korishan

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Considering you have very consistent results, I would suggest you have bad connection somewhere. A Self Discharge cell would not necessarily discharge the same each time.
Or the timer function on the charger is messed up somehow.

Please inform us of the charger you're using and how it's hooked up to the cell
 

daromer

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A self discharging cell doesnt randomly change over time. It Will stay the same or gets worse. Not going back
 

Crimp Daddy

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If all of your equipment and cables are good, and the charger is reporting power in/out accurately... then you have a cell that is in poor health.

Usually when you see this, the cell is accepting power and burning it off as heat as it accepts the power, which is ultimately no longer there when it comes time to discharge.

You need to do more testing to answer these questions for yourself... hopefully you have more cells so you can establish a baseline:

What is the internal resistance
Temperature during charge/discharge
Power in/out compared to other cells from the same batch
Try another charger to see if they agree

Its for time like this I consider "one is none", I have multiple multi-meters, chargers, and other important equipment so I can at least cross check my own results.
 

wattwatt

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daromer said:
What tester? Is the cell warm?

If you have a proper tester like an icharger that do the test properly.. This will show you that you have losses during charge in heat as example. Either due to high ir or selfdischarging or something else.

The cell never gets warm during charge or discharge.

The charger is a HTRC branded hobby charger and the discharger/capacity tester is a ZB2L3. I'm going to discharge/capacity test now with the HTRC hobby charger (has a discharger too)instead of switching to the ZB2L3(I forget why I was doing thisin the first place). Just realizedI also have a ISDT hobby charger (has a discharger too) and a Kunkin DC electronic load so I'll try them too.

I have known good cells for that lot, so I'll try them too.
 

daromer

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So you test with 2 different devices. Thats a big no no unless you have them callibrated and with exact samealgorithm and voltage limits.
Charge and discharge with same brand and type and same limits.
 

Crimp Daddy

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I always charge/discharge with the same piece of hardware if possible.

Sounds like you have enough gear to test this more than one way. Please report back when you do.
 

Redpacket

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So if the cell is cool, it's unlikley to the the cell.
Sounds 98% like it's the charger/dischargers giving you different readings.
One (or both!) units ie the charger &/or the discharger are pretty wildly out with the readings, you've gota 25-30% difference.
You need a decent multimeter to verify what's going on.
At least initially, check the current in/out of the cell & compare to what the charger/discharger says while it's charging/discharging.
 
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