LiFePO4 capacity test question?

snbrooker

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Mar 20, 2022
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I am trying to test my 200Ah LiFePO4 battery cells for capacity and I bought a small load tester to use, thankyou for the videos…
(https://www.makerhawk.com/products/...intelligent-discharge-resistance-power-tester)

First I charged my battery to 3.65v. After it sat for a couple hours the voltage dropped to 3.35v and I hooked up my load tester. When I try to pull 20amps my single cell battery voltage drops to the 2.6v cutoff almost immediately. So I dropped discharge amperage to 8.5A and the voltage drops to 2.9V and more or less hovers there.

I am now up to 145Ah in the test and hoping it gets close to the 200Ah mark. But why wont it allow more amps....?
  1. Is this battery damaged because it wont let me pull 20A off it?
  2. What is the correct discharge amperage should I use for this test?
  3. Is it better to hook up the BMS and test these as a 4s bank at 12v?


Thanks for your guidance.

Shane
 

acdoctor

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Are you connected to the cell with ring terminal wire ends and 12 awg minimum wire? If so the cell isn’t looking good. When you stop discharge what does the cell voltage rebound to?
 

RCinFLA

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If cell voltage dropped to 3.35v after couple of hours of rest time it is not fully charged. It is about 85-90% state of charge (if your voltmeter reading is accurate to a couple of mV resolution and cell is around 25 degs C temp).

You have to allow charge current to taper off at absorb voltage to low level to ensure full charge.

"low level" of taper off current depends on your bulk charge current rate. It is less than 20% of your bulk current charge rate or 5% of AH C(A) current, whichever is a lower amount.
 
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RCinFLA

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Make sure your voltage measurement is at the battery terminals directly and not including cables voltage drop.

To load a battery at 20 amps with a load tester you need a load tester that has remote voltage sensing (four wire) so voltage reading can be made directly at cell terminals. DL24P tester has four wire remote voltage sensing. Many load tester will saturate at 1.5v to 2.5v at their input so with too much voltage drop in cables to cells the tester will drop below point where it maintains controlled load current.

If your tester does not have remote voltage sensing use at least #8 gauge wires as short as possible to reduce their voltage drop.

Two foot long pair (pos & neg leads) of #8 wire will have 0.05 volt drop in cables at 20 amps. Also don't use alligator clips.
 
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Samcat

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May 31, 2022
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This battery cell testers have no separate leads to check for voltage under load. If you using same leads for discharging and voltage measurement it will not be correct Voltage you should see. Other work around with wanna be cell battery tester you have posted is babysitting with voltmeter that is calibrated and you use this as reference. Also since this is not dual leads battery cell tester your Ah ratings on the instrument will be out of true Ah ratings . So your final Ah ratings should rough estimates.
Good luck
 

Samcat

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May 31, 2022
Messages
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If cell voltage dropped to 3.35v after couple of hours of rest time it is not fully charged. It is about 85-90% state of charge (if your voltmeter reading is accurate to a couple of mV resolution and cell is around 25 degs C temp).

You have to allow charge current to taper off at absorb voltage to low level to ensure full charge.

"low level" of taper off current depends on your bulk charge current rate. It is less than 20% of your bulk current charge rate or 5% of AH C(A) current, whichever is a lower amount.
I will agree with you on your statement and add at 3.650 V fully saturated cell will after 24 hours be in the 3.370- 3.400 V this is natural phenomena with this chemistry and if you monitor for 24 hours you will find V threshold where your cell like to be in real world charging max cell voltage with absorption.
Unknown are instruments used for voltage measurement. How voltage is set before Charging and how accurate is his power supply that reads Voltage. Cheap voltmeter are not something you wanna use when assembling expensive 48v battery pack. There many Chinese power supplies that don't like voltage adjustment under load even they are capable because they don't show true V values. I cannot stress enough how important is to become familiar with tools you are using. Even simple voltmeter you should read manufacturer manual to know how accurate it is and how you should operate or clear to zero before taking measurements.
 
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