On-orbit BM2 Battery Telemetry

It's always interesting to see battery behavior on a cell-by-cell basis through telemetry from an on-orbit system. In one of the SUPERNOVA buses currently in a LEO orbit, we see the eight individual cell temperatures of one of its two 4S2P (100Wh) Pumpkin BM2 batteries as the BM2's integrated heater cycles automatically over its user-programmable +8C setpoint and +2C hysteresis:

We see all eight of the BM2's 18650 Li-Ion cells heating and cooling as the integrated BM2 heaters on each cell heat up, and then cool down during automatic heater operation. The small reported temperature differences between the cells is due in part because of the overall thermal gradient across the satellite's internal layout, and the distances from one cell to another within the core of each BM2.

It's important to keep these 3500mAh Li-Ion cells above 0C to ensure that the system can charge them at all times. One could perhaps fly with a lower heater temperature setpoint, but this setting ensures that if the satellite gets colder (due, e.g., to shutting off some active components elsewhere in the system), the BM2 heaters will always be fast enough to keep each and every cell above 0C with more aggressive, passive satellite cooling.

Here we see the individual cell voltages of this 4S2P BM2 battery over the same time period, during an eclipse of the satellite. The battery voltage is trending downwards (note the scale — grid marks are in 10mV increments) and there's a noticeable cell voltage dip when the heaters are on, followed by a noticeable rise in cell voltages when the heaters are off. That's because during eclipse, the BM2 has to take (internal) power from the cells themselves to run its heaters; when charging, the heater power comes (automatically) from the (external) battery charger (here, the Pumpkin EPSM1).

Also of note is the cell voltage match to within 11mV at the time of interest. When discharging as above, the BM2 is not in a state of rest, and so there is no active balancing in progress. The cells tend to diverge in voltage as a discharge progresses. Once the SUPERNOVA bus gets back into the Sun, the solar array power will go mostly towards running the bus, and the batteries will recharge automatically. Once the battery charging current drops below a defined threshold, the automatic cell balancing will begin, and will typically balance the cells to within 5mV of each other.

Cell balancing is extremely important in advanced battery management systems (BMSes) like that of the the BM2, because most of the BMS safeties for Li-Ion batteries are on a per-cell basis, not on a pack basis. For example, if one cell diverges in voltage considerably from the others, then the overall pack is likely to encounter a cell undervoltage (CUV) or a cell overvoltage (COV) fault prematurely as the battery approaches its discharge or charge voltage limits, respectively; this limits access to the full theoretical SoC of the battery. By balancing the cells, the battery has the best chance of delivering its full SoC while on orbit.