Since the lithium has a higher voltage than the lead when fully charged .... the lead battery will be the "backup battery".
That means when fully charged the lithium would be supplying pretty much all the current to crank the main motor and everything else til the lithium is going dead ... and the BMS currently capability would have to be large enough to handle cranking the main motor.
It also seems to me that system would have to be on charge all the time when not being used or the lead battery will be a parasitic draw on the lithium.
Doesn't seem to me like a good idea without a device that that would regulate things in a smart fashion .... but I could be wrong.
I haven't seen that thread on BBC, does anyone on there have this operational?
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Looking for the one that makes it all worthwhile.