Powell,

No diode is needed. I put the switch to prevent one chemistry from charging the other when parked. Usually the NiMH will charge the Li Ion with a few watts since the OCV of 30 NimH cells is slightly higher than 10 Li Ion through out most of its discharge cycle. Because charging is not 100% efficient I would waste ~20% of those few watts. If charging were 100% efficient it wouldn't matter as long a either chemistry doesn't exceed the charge voltage maximum of the other.

Of course, if the 2 chemistries are connected in series, then big trouble. Nothing really to do with with actual chemistries, but as you point out, unmatched internal resistances and unmatched capacities. If the capacities of series connected cells of any kind become unbalanced, weak cell may be driven negative and further damaged. Use different chemisties in series? Forget it, it will never balance.

Charging parallel connected batteries of two different chemistries is also a very bad idea. We all want to charge our batteries to the maximum possible. The full charge, NiMH procedure, would really mess up a Li Ion battery and visa versa.

But....., discharging 2 series connected batteries with 2 different chemistries, even different AH capacities, works just fine as long as both chemistries are happy with the voltage limits set by me or the electronics. I try to stay between 33 and 40 volts although I'll allow 30volts to help me climb the final hill after a long ride. Interestingly, when the parallel batteries reach LVC off, the connection between the 2 batteries is cut and the LI Ion has no load either. I continue to be amazed by the E+'s sophisticated electronics.

During discharge, the 2 chemistry batteries are self balancing. They have to be, their voltage is the same. As one chemistry depletes, it shares less of the load and the other chemistry provides more, until they both, together, reach the cut off voltage. So, the new Li Ion supplied 4.5ah at the same time the old NiMH supplied 8 to 10ah. Actually the 4.5ah Makita in parallel improves range more than just the 4.5ah it supplies. With two batteries sharing the load, their discharge rates are cut significantly which improves the output capacity (AH) of both; especially the NiMH.

In my first post I dreamed of being able to transfer the power train from the E+ to my Santana tandem and back as needed. Dumb idea, mostly because I don't want to mess with the Santana's well tuned 27 speed drive train and everything is so damn heavy. So, I got a 800W Chinese front wheel hub kit and installed it on the tandem. I powered it today with just 2 of the 18v 4.5ah Makita type batteries in series. (I plan to use 2p2s, maybe 3p3s, since there is lots of room on a tandem frame) Wife and I only rode 4/5 miles. It was fast and fun, great power. This is the wrong forum for e tandem talk. I'll post more in the right spot.