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kennybobby · 03-08-2014, 07:21 AM

It looks like subpack #3 is bad by your data--all the rest settled down to 8.1-8.2 except that one dropped to 7.5, it's not holding.

i have done cell capacity checks on some batteries, the largest pack being 44 cells of lifepo4 with 100Ahr capacity.

For example i recorded voltage, current, temperature and time while discharging at 30 amps until the voltage reached 2.5V, then recorded same while charging at 25 amps until the voltage reached 3.6. After resting the voltage settled to 3.33-3.34, which is considered the 100% state-of-charge voltage for this chemistry. The cell capacity is calculated by the current multiplied by the time, e.g. 30Ax3.4hrs=102Ahr.

You need to get the specific upper and lower limits for your cell chemistry, the cell capacity rating, the charging procedure, etc to make sure you stay within the box and don't damage them.

There is not a real resistor in the battery that you measure, but a so-called internal 'resistance' can be calculated as the difference between the unloaded and loaded voltage divided by the load current. For example if a 12 V battery drops to 10V with a 100A load, then Ri is 2/100 = 0.02 Ohms. Some people swear by it, but i haven't found it as useful as actual cell capacity, especially when trying to determine expected range.

03-08-2014, 07:21 AM	#3 (permalink)
kennybobby Southern Squidbillie Join Date: Aug 2012 Location: Heart of Dixie Posts: 97 Thanks: 50 Thanked 26 Times in 22 Posts	what are your units of charge value? It looks like subpack #3 is bad by your data--all the rest settled down to 8.1-8.2 except that one dropped to 7.5, it's not holding. i have done cell capacity checks on some batteries, the largest pack being 44 cells of lifepo4 with 100Ahr capacity. For example i recorded voltage, current, temperature and time while discharging at 30 amps until the voltage reached 2.5V, then recorded same while charging at 25 amps until the voltage reached 3.6. After resting the voltage settled to 3.33-3.34, which is considered the 100% state-of-charge voltage for this chemistry. The cell capacity is calculated by the current multiplied by the time, e.g. 30Ax3.4hrs=102Ahr. You need to get the specific upper and lower limits for your cell chemistry, the cell capacity rating, the charging procedure, etc to make sure you stay within the box and don't damage them. There is not a real resistor in the battery that you measure, but a so-called internal 'resistance' can be calculated as the difference between the unloaded and loaded voltage divided by the load current. For example if a 12 V battery drops to 10V with a 100A load, then Ri is 2/100 = 0.02 Ohms. Some people swear by it, but i haven't found it as useful as actual cell capacity, especially when trying to determine expected range.