BM2 Cells at Rest
At the end of the cold-temperature BM2 battery testing mentioned in the previous blog post, the BM2 was allowed to rest. This simply means that the overall power system was not drawing any power from the BM2, and the BM2 was fully charged to its terminal voltage (16.2V in this particular test). The resulting cell voltages illustrate the efficacy of the cell balancing algorithm -- there's a maximum spread across the four cells of 5-10mV.
The BM2 intelligent battery module includes an active battery balancer that automatically seeks to equalize the voltages of all two (2S4P) or four (4S2P) cell groups in the BM2's total of eight 18650-size Lithium-Ion cells. The balancer is active when the battery is charging and when the charge current has dropped below a prescribed limit.
Most Li-Ion batteries cannot be charged at temperatures below 0ºC, and the BM2's LG MJ1 cells are no exception. The BM2 cell heaters for this cell chemistry automatically enables the heater at 5ºC as the temperature drops and disables it once the temperature rises to 7ºC.
As part of low-temperature testing being performed at Pumpkin, a neat telemetry capture came up this evening. The thermal chamber is set to -5ºC, the BM2's heater was in the default AUTO mode, and the charging current into the BM2 was limited to 1A to simulate a wimpy charger. The cell heaters consume around 8W; battery energy is 100Wh.
As the heaters turn on and off, the current flowing into the cells decreases and increases, respectively (because charge + heater current is limited to 1A). Each time the charge current increases, the cell voltage increase with it, but individually. The telemetry capture shows the four individual cell voltages as the BM2 approaches its final pack voltage. You can see the active cell balancing in cell 2 (the purple trace): it started out around 20mV higher than cell 4 (the yellow trace), but within 45 minutes of all of the cells charging, these two cell voltages have converged. All four cell voltages will converge further as the BM2 continues operating like this. The cell balancing is slow but effective; we typically see mismatches of under 5mV across four cells, when the balancing algorithm is allowed to run its course.
The cell balancing, cell heaters and real-time telemetry (and nearly 100 other data points) are available to all BM2 users as standard features of each BM2.