Lately we’ve been working on the performance of our battery packs. Though a large component of this performance comes from the cells themselves, the bus wire material and thickness, quality of welds and solders, number of bus bars and resistance of the BMS all contribute to the overall peak discharge performance a battery can supply.
In nearly all cases, though, the way this energy is lost (the sign of decreased performance) is heat. An “ideal” battery pack would not get warm to the touch under any discharge conditions, and the voltage would stay “flat” through the entire discharge and only drop right as the pack has run out of charge.
Such a pack doesn’t exist, and so we can only improve our existing designs to “move the bar” in the direction of better performance. Lipo battery packs are pretty good in terms of not producing heat, and LiFePO4 packs have very flat discharge curves, but Lipos lose voltage as they discharge and LiFePO4 only packs about half the capacity of Lipo.
This is why we choose to use LiMn cells – while they have a voltage curve like Lipos, it’s much more manageable and they have very low internal resistance so less energy is wasted as heat during heavy discharge.
Of course, the best way to evaluate performance is with testing. 🙂
Above, the pack was discharged continuously at a 3C rate, meaning the pack when from fully charged to dead in 1/3 of an hour, or 20 minutes. The pack hit about 50C, which is indicative of a heavy but manageable discharge rate.
Cells individually can be heavily discharged when they have the ability to dissipate their heat in all directions, but put a bunch of cells in a pack, where the heat can’t be dissipated in all directions (because for some of these cells, there’s other hot cells on all sides except top and bottom) and you have to derate your battery’s discharge rate. In this case, our new high performance cells are actually rated at 9C continuous discharge (a full discharge in 7 minutes) but due to this heat dissipation issue, we have to derate our cells to 3C. 9C is still very much a possibility for these cells to handle, but only on a time frame that doesn’t result in a lot of heat getting built up in the pack.
If my design tests well, I should see the components of my pack maintaining about the same temperature as the cells themselves – meaning the resistance of my bus bars, wiring and BMS don’t produce as much heat as the main “action” of the cells, which is the chemical reactions going on inside those cells.
So far, so good! Under thermal, the cells are indeed the main contributors of heat. The BMS has transistors that get hot too, but in this image my 45 amp discharge actually pushing the BMS a little past its rated limit (of 40 amps), so I expected some heat at the BMS.