Val of EMotorWerks.com (aka valerun) has an interesting project going on over on the RAV4-EV forum
. I don't think he'd mind if I linked a couple posts.
Basic theory is that he has an auxillary pack of 109 LiFePO cells (38 kWh worth) wired in parallel with the main pack.
The aux pack is top balanced with BMS and is connected to/from the main pack with a set of high current contactors.
Because the LiFePO pack has a slightly narrower voltage range than the stock pack, when discharging, the aux pack is connected up using the contactors as long as the main pack voltage and aux pack voltages are in the same range.
So the aux pack disconnects after main pack is almost full when charging (or BMS tells it to) and when discharging, it will connect the aux pack after main pack discharges enough, then disconnect once the main pack gets down around 50% SOC.
The beauty (IMO) of this approach IMO is it's simplicity as now you don't have any expensive/complex DC-DC converter to worry about - this was always a weak point in the Enginer add-on solution.
Seems like a very similar approach could be taken with the LEAF, though with a smaller pack/cells.http://www.myrav4ev.com/forum/viewtopic.php?p=12189#p12189http://www.myrav4ev.com/forum/viewtopic.php?p=12461#p12461
Kohler Controller wrote:Val, Do you normally charge each pack separately, or did you just do it this time to capture individual pack recharge capacities?
only this time - for that very reason, yes
We have just built a power distribution unit to manage battery connections - see photo at https://docs.google.com/a/emotorwerks.c ... EhKSG1DQ0k
It will also handle our special battery arrangement for mobile CHAdeMO charger we will be doing a public demo on in ~2 weeks.
Kohler Controller wrote:I like that the RAV4EV used the AUX pack more than the main pack during its bulk discharge, thus always leaving the main pack for the bottom end where you need more accurate range estimation. I'm wondering when you bump up the AUX pack to 111 cells, if the packs will start to share more evenly, thus losing this benefit?
this will always happen due to particular shapes of discharge curves for the involved chemistries. AUX pack (LFP) has a much flatter curve relative to the Tesla pack. So almost no matter how many cells you have, you will always fully discharge the AUX before you fully discharge the Vehicle pack. With 108 cells, we have AUX discharging when 50% of the main pack is left. Moving to 111 cells would cause this to happen sooner - say, when 70% of the main pack is left.
PS. We have decided not to go to 111 cells after all. Doing so would result in difficulty to top-balance the AUX pack as no cell would reach the balancing voltage of 3.5V (CV of LFP cells we are using is 3.6V - 111 cells will have combined CV of almost 400V which is too high - we need to match native pack's CV of 382-386V). We will deal with equalizing currents through a PDU (power distribution unit) that I have shown in my previous post - it will connect the AUX pack when main pack voltage falls to AUX level and will disconnect when AUX gets discharged (based on cell-level BMS signal).