The 3 cells are now assembled into a pack, which will speed up testing compared to doing each individually.
6 cycles have been performed on the pack so far, and the performance has been consistently around 19Ah. On cycle 3 the pack was accidentally over discharged, causing a temporary reduction in capacity that appears to be recovering over several cycles.
Cycle Capacity Notes
3 19.20Ah 1
1. Accidentally overdischarged to <1Vpc on cell 1. Ah value includes discharge only to 2.5Vpc
The cell internal resistance averages 2.5mOhm with some spread over time and cell that’s measured. The load was varied from 20A to 36A and voltage deltas used to calculate the internal resistance. X axis on the graph below is Ah discharged from full charge.
Raw data: A123 Battery Data
I’m designing a battery for our go-kart team ArcWolf Racing. Basic specs are around 88V and 30-40Ah, and one of the prime candidates are the 3.2V 20Ah A123 pouch cells, configured at 28S2P for 89.6V 40Ah. Local sources are about $70 per cell which is very high compared to the ~$20-30 per cell available on Alibaba or AliExpress.
After looking at reviews, the seller ShenZen FHT Co. Ltd. has the best feedback, so we ordered 3 cells for testing at $30 each. They arrived within 2 weeks and were packed well (box in a box in a box!).
The test setup consists of a modified Delta-Q charger acting as a power supply, a shunt, one of my BMS prototypes as a data logger for voltage and current, and a “MacGyver” resistor consisting of bailing wire in a bucket of water.
Figure 1 – Test setup
Figure 2 – “MacGyver” resistor with resistance controlled by moving green alligator clip tap
Cell 1 was charged to full capacity at 1C (20A) to 3.6V, putting 13.5Ah into the cell from its shipping state of charge. It was then discharged using the resistor down to 2.5V, resulting in 19Ah capacity.
Figure 3 – Cycle 1 on Cell 1, starting from shipping SOC. Total discharge 19Ah. Raw data: Cell 1 cycle 1
A quick test of the internal resistance was performed, which is probably highly inaccurate. The voltage drop from rest was measured when a 25A load was connected, resulting in a 50mV drop, putting the internal resistance at about 2mOhm.
Further testing to follow with more cycles on more cells. Looking promising initially though!