On 11/4/2010 2:40 PM, David Roden wrote:
NiMH batteries can be discharged fairly stably in parallel, but they don't play nice when charged in parallel. You need a separate charger for each string. You also need contactors to separate the strings for charging and connect them for running. Both of these add complexity and expense.
Right. I actually tried this with a surplus Prius pack; with 7ah cells I needed to have 5 strings in parallel to make the tractor move well. And charging 5 strings in parallel did not work.
I can get away with this on the BB600's because normally I only have 2-3 strings and I can overcharge and gas the cells if needed. Plus NiCD flooded can handle the heat generated by a topping off charge. However even running with 2 packs I'm thinking of isolating them with the tractor off to keep them from discharging into each other (takes about a month or two) and allow 30+a charge rates on a single string every once in awhile (they like that).
You might gain some cycle life and get longer runtime in cold weather, but at quite a substantial cost. And you lighten a tractor that you want to be heavy! I don't think it's worth it, but I suppose others might.
Much better to use the flooded NiCD. Incredible power output in the winter, they seem to last forever, and they aren't chewing apart the battery box with acid.
C