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Questions about battery capacity tests and test equipment

ericfx1984

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Oct 10, 2021
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In this video Will uses a neat little piece of tech... I can't find it for sale anywhere


I have something similar, but it has a built in load... 200w... that will take a while on a 304ah, 4p4s battery...

Couldn't i just attach a 200 amp smart BMS and an inverter with a heavy and consistent load? Wouldn't the Smart bms app be able to tell me if the capacity is correct/high/low?
 
In this video Will uses a neat little piece of tech... I can't find it for sale anywhere


I have something similar, but it has a built in load... 200w... that will take a while on a 304ah, 4p4s battery...

Couldn't i just attach a 200 amp smart BMS and an inverter with a heavy and consistent load? Wouldn't the Smart bms app be able to tell me if the capacity is correct/high/low?
You could use a BMS to get a general idea of the capacity. When the amps going into the BMS sum up to be the capacity that you have specified (in amp hours), all amp hours after that will not be seen. Likewise, on discharge, when the BMS reading is to zero amp hours, the remaining discharge would not be recorded. This is the way the BMS keeps in sync, but resetting at zero and at full after X flow has been measured. If you could set the capacity large enough and trick it into being at 50% (or some middle range number) you could make it work.

These little meters work but are not bi-directional. I use one for charge and one for discharge, both connected (parallel to each other).
 
I just got done playing around with trying to get an idea of the capacity of my 304ah 12 volt battery.

I have one of the cheap constant-current/constant-voltage power supplies that can provide about 10 amps. The battery bms is a 200A JDB smart bms. I also have this hall effect DROK charge/discharge meter, which is bi-directional. I also have this Kaweets DC clamp meter.

I wanted to get an idea of how the meters that i have compare, considering they are all on the bargain-bin side of test equipment. I decided to compare them while charging because the power supply is less likely to have a lot of high frequency noise or pulsation as compared to an inverter. So I connected everything and started charging with the power supply display saying that it was delivering 10.3 amps. At the same time, the JBD BMS reported a charge current of 10.2 amps, which is acceptably close. However, the DROK was showing 10.6 amps of current, and this was backed up by the Kaweets clamp meter was showing 10.55 amps. So I have four meters, and still a split decision on what to believe.

One of the uses for this battery is for power backup for my deep freeze. So I put it on the inverter as a load. I thought I would see higher current draw, but it turns out to be pretty efficient, and only pulled about 10 amps from the battery also. However, this did let me confirm that the same reading differential exists between the JBD bms, the DROK meter, and the Kaweets meter while discharge as while charging.

So considering it is a nominal 304ah battery, this is a 12ah difference in results on a full discharge. Not the end of the world, but it could be the difference between thinking you got what you paid for or not. Mine came out between 310ah and 322ah at very low C rate. I need to conduct another test where I keep the C rate closer to 0.2.

Also, @DThames has a good point about setting the BMS nominal capacity higher than the actual capacity for testing. The JBD will only read from 0 to the nominal capacity I set, so unless I set it higher, I can't read the full potential capacity of the battery.
 
I finally got around to figuring out how to use an iCharger X8 to cycle a battery cell to compare with the capacity that I got when testing the 12 volt battery made from Docan 304ah nominal cells.

When it has a power source that it can discharge back to, such as a battery, the iCharger x8 that I have can charge and discharge at 30 amps maximum, so with 304 ah cells, this is about a 0.1C rate. I ran the test by charging the cell up to 3.6volts until the charge current dropped to 3 amps. Then I had it discharge to 2.5volts until the discharge current dropped to 15 amps. I did not pause between the charge and discharge cycle.

Then end result was a capacity of 317ah, based on the metering in the iCharger X8. This is about midway between the 310 to 322ah my previous test gave. I tested a cell with a production date code from October 28, 2021. All of the testing was done in a room that is about 75°F or 24°C.
 
In this video Will uses a neat little piece of tech... I can't find it for sale anywhere


I have something similar, but it has a built in load... 200w... that will take a while on a 304ah, 4p4s battery...

Couldn't i just attach a 200 amp smart BMS and an inverter with a heavy and consistent load? Wouldn't the Smart bms app be able to tell me if the capacity is correct/high/low?
It's a no-name Chinese power meter. Will has a link to ebay on his site: https://www.mobile-solarpower.com/battery-capacity-testers.html
 
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