Need Help with Battery Capacity Tester. Am I doing something wrong?

Thread Starter

bigjoncoop

Joined Feb 1, 2019
83
I recently bought two battery capacity testers in-order to test a box of 50 used Molicel IRC18650H cells. I purchased them from Alarm/Battery_Hookup. They are one of the biggest retailers of used batteries in the U.S.A. and have great reviews. The batteries were claimed to be all between 1400mah - 1600mah. Which I believed to be true.

The testers I am using, use a resistive load to determine battery Ah. They also fluctuate the amp draw as the battery voltage decreases. I have one 5W 7.5ohm resistor connected to each tester which applies a load of no more then .5 amps.

I am fully charging each 18650 to 4.2v and I set the tester cut off voltage to 3.0v under load

I have tested 10 cells so far and the tester is showing that all of the cells have been between 0.850Ah - 0.930Ah (850mah - 930mah) on a .5a load

But when the test is over the cells are immediately bouncing back up to 3.6v - 3.7v - Is this causing the low mah readings? I know that under load the voltage is lower then resting voltage. but is 3.7 to high? thats there nominal voltage... Should I set the cut-off voltage lower then 3.0v?

I kind of want to believe the sellers claim that all 50 cells are about 1500mah. The seller is extremely highly rated. Im thinking that my testing method may not be right. but I have also used my B6 RC charger to do a discharge test on a few of the cells and it basically has the same results.

Ive also tried using a 1amp load and the capacity reading is only around .620mah

Im using about half of these cells to make a 4s5p or maybe 4s6p battery pack to power a max of 10a load. So i dont really need the actual mah ratings in-order to group the cells as close together as I can but would still like to know there mah capacity...

I have the feeling that i need to set the cut-off voltage lower so the cells dont immediately bounce back to 3.7v when load is disconnected. But I know that 3.0v is the lowest voltage for lithium ion. ????

Here are the testers I am using:
https://www.amazon.com/HiLetgo-Lead-acid-Capacity-Discharge-Analyzer/dp/B0716ZL1SM/ref=sr_1_fkmrnull_1?keywords=HiLetgo+18650+Li-ion+Lithium+Lead-acid+Battery+Capacity+Meter+Discharge+Tester+Analyzer+1.5V-12V+Battery+Capacity+Meter+Discharge+Tester+for+18650+Battery+Li-ion+Lithium+Lead-acid+Battery&qid=1550563561&s=hi&sr=1-1-fkmrnull



P.S. If I have a 4s5p Battery pack and a 10a load on it, what is the amp draw per cell?

*** I have messaged the seller, and asked for there testing parameters. And my concerns. Waiting on a reply ***
 

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Picbuster

Joined Dec 2, 2013
996
Any battery has a serial resistor.
By putting a load to the battery a current will flow and create a voltage over the internal resistance.
Result voltage on the battery = voltage battery element - voltage over resistance.

Keep in mind that;
If the resistance of the load >> internal resistance you will measure the voltage on the battery element.
if however the load resistance = internal resistance you will measure 50% of the battery element.

The internal resistance of the battery is a directly related to the status of (dis)charge.( chemical process)


Picbuster
 

AlbertHall

Joined Jun 4, 2014
9,628
The measured capacity of a battery depends on the discharge rate. At high discharge rates the measured capacity will be lower than at more moderate discharge rates. Your 1500mAh battery discharging at 0.5A might be expected to take 3 hours to finish. Commonly battery capacity is specified at the 10 hour rate, that is for your batteries a discharge current of 150mA. You do need to check with the manufacturer/retailer what their specification is based on.
 

oz93666

Joined Sep 7, 2010
737
I have the same tester ... I test cells with an initial discharge current of 2 A ... so 0.5A discharge is very conservative and should result in a high reading for capacity ...

Yes , discharge down to 2.5 V , it can do no harm , it's what I do.

There is slight variation in testers but nothing can account for your low readings , I think the seller is exaggerating capacity....

"If I have a 4s5p Battery pack and a 10a load on it...." 4s will give a voltage of around 14 V .... the A is shared by 5 cells (5p) so each cell outputs 2A .. doable for all 18650s . If this is the planned usage for these cells you should measure capacity at this (2A) current to know what discharge time you can expect in use.

This forum deals with recycling 18650s and will answer all your questions .. https://secondlifestorage.com
 
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