I hooked up the EVD5 BMS to a set of 5 Thunder Sky LFP40AHA cells and did a first charge on them. These cells have been sitting for a little more than a year now, with no charging or other maintenance. I learnt several things.
- Making up wiring looms is extremely tedious, I really envy BMS designs where a board sits on each cell. Bob designed this BMS and he has this problem solved with his A123 packs -- he has a circuit board down the side of the pack connecting the junctions between cell layers to the BMS on top. Sadly prismatic cells don't lend themselves to this approach. My loom has three wires, one for voltage sensing, one for current shunting and one for the temperature sensor (the temperature sensor is only hooked up on one cell). I'm not yet sure if a Kelvin connection is required.
- The quality of your connections matters a lot. While I've got a separate shunting wire, these wires connect on top of the strap taking the full charging current. If the cells charge at 3.450V, and you start shunting at 3.550V, and you're charging at 15A you only need 7mΩ in the strap to cell joint (actually there are 3 joints in the charge current path, so they have to be even better) to make the difference between normal charging and the start of shunting. I could see this by tightening the cell bolts and watching the shunt lights go out.
- The noise problems I reported while charging in the car are still present with only 5 cells. This might be exacerbated by the big ball of wires between the BMS and the cells -- each one acts as an antenna.
- With 5 cells and a constant voltage charger, no intervention was necessary to turn down the charging current, at least during the charge presented below -- it looks like cell 0 and cell 3 might get out of hand soon. I don't yet have the BMS monitor in control of the charger, this will be the next improvement.
- I need to measure the charging current -- when I was charging in the car this was done with my EVision, out of the car with only 5 cells the EVision won't work (it needs 50 or 75V or something to work)
- There is a zero error in the shunt current measurement, this could be the firmware (it recall it doesn't turn down the current shunt properly), but there is a bug in the prototype hardware which certainly isn't helping.
Edward lent me an isolation transformer to make the PFC-30 charger safe. Without the transformer, the battery is connected to the mains and you'll get a shock if you touch it.