Voltage versus capacity

tas99

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I have a 48V 20 AH LiFePO4 battery and a chart showing nominal voltage to remaining capacity.

My question is should the voltage be measured under load and if so how much of a load.

Thanks for any help.

Tom
 
The voltage should be measured when the battery has been at rest at least 10-15 min. Your inference is spot on. The battery's voltage will be lower under load. Once your battery is low, you can risk setting off the low voltage cut off if you use too much juice. When mine gets to around 51 volts (52V batt) I can begin to trigger the low voltage cutoff on the steeper hills. My low voltage cutoff is 43V.
 
LiFePO4 packs often have a different full charge and LVC values, and they don't translate to the kind of numbers almost everyone is used to with 18650 and 21700 Li-NMC cells. This 48v 20ah LiFePO4 pack is 16S1P and its full charge is 58.4v, which makes it more like a 52v pack. You'll have to dig into the graphics on the page to see this.


A 48v 20ah Li-NMC pack would typically be something like 13S5P and have a max SOC of 54.6v. The starting and peak voltages of the cells would be different, too.

To do a chart for yourself you are going to have to know the starting and ending voltages of your cells, the cell count in the serial group and the number of groups combined together. From there you will have to do some math to figure out your individual percentage voltages.

The battery voltage charts I did actually lay out the entire process in detail. You will need to know your cell characteristics (low and peak cell charge), and the pack characteristics (S and P count) and then you can follow the method used on any of the charts on this page to make one for yourself. Click on any chart and follow the directions and examples.

 
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