Capacity testing power loss?

[Maxxim]

When watching some youtube vids about lithium capacity testing, and after I made my own tests, I see that there is quite big voltage drop when compairing the tester voltage to real measured voltage(and BMS internal voltage that you can monitor with the bluetooth app).

For example on my tester I see 12.4V and 14.1A current draw and in the BMS I see 12.85V and 14.5A curren draw. The measured voltage is very close to the BMS voltage.

Wouldn't that mean that also the measured total capasity would be a little bit off? A little bit less than the real battery capacity?

 

[Supervstech]

Depends on which value is correct...
Your tester, the leads on it, the BMS, etc...

 

[Maxxim]

My multimeter measures close to BMS values. Seems that the capacity tester alligator clip wires makes the biggest loss.

 

[ArthurEld]

I have seen several claims that voltage drop doesn't effect capacity test results.
My capacity tester shows accurate capacity results.
So does my BMS and battery monitor.

That is just my opinion based on a fair amount of testing.

 

[Supervstech]

My multimeter measures close to BMS values. Seems that the capacity tester alligator clip wires makes the biggest loss.

What are you clipping the leads to? You should clip to the bussbar, or lugs, NOT the bolts themselves.

 

[Maxxim]

I had them clipped on the battery cable ring connectors. Will test with ticker wires and bolted on the cables.

 

[fafrd]

I had the clipped on the battery cable ring connectors. Will test with ticker wires and bolted on the cables.

View attachment 33766

Nice build.

Voltage measurements don’t depend on wire size or additional contact resistance from clips, etc (since a voltmeter draws no current) and capacity testers are notorious for being off on voltage measurements (probably due to internal resistance), so measure voltage with you multimeter at the battery terminals and that is probably the measurement to trust.

Current is the same all the way through, so if your multimeter and capacity don’t agree, you should calibrate your capacity tester (usually at 10A).

If you measure voltage at the battery terminals and again at the capacity tester input and they are different, that is probably due to wiring losses and you may want to increase wire size and/or decrease wire length. Capacity in Ah will be the same in any case and Wh will be off anyway due to internal resistance, so it may not be worth the trouble.

Using one of these cheapo capacity testers and a multimeter, it should be possible to get an accurate measure of Ah capacity and battery voltage, which can be translated to maximum Wh.

In terms of how many Wh can be delivered to your inverter, you need to pay attention to wiring losses. You’ll lose V=IR of voltage drop between battery terminals and inverter and you’ll lose I2R of WattHours between battery and inverter...

 

[Maxxim]

Thanks.

Measured the setup again today with the same wires. Fully carged battery after about 10min load:

Idle voltage
Battery terminals: 14.07V
Battery + BMS: 13.66V
BMS bluetooth app: 14.10V
Tester display: 13.60V
Tester terminals: 13.66V

15A load
Battery terminals: 13.09V
Battery + BMS: 13.08V
BMS bluetooth app: 13.14V
Tester display: 12.70V
Tester terminals: 12.84V

 

[fafrd]

Thanks.

Measured the setup again today with the same wires. Fully carged battery after about 10min load:

Idle voltage
Battery terminals: 14.07V
Battery + BMS: 13.66V
BMS bluetooth app: 14.10V
Tester display: 13.60V
Tester terminals: 13.66V

15A load
Battery terminals: 13.09V
Battery + BMS: 13.08V
BMS bluetooth app: 13.14V
Tester display: 12.70V
Tester terminals: 12.84V

So looks like about 16 mOhms of wiring resistance between battery terminals and tester terminals and then another ~9 mOhms of resistance within the tester itself to the point where voltage is being sensed...

You can significantly reduce the wiring resistance (I use a busbar and 3” length of 10AWG copper wire to connect my cells to the tester) but there is nothing to do about the internal resistance (which is why tester measurements of voltage and power are worst-case and should be ignored except for setting cut-off voltage).