Backup battery selection...

Hi Folks,

I'm running a 23.5ah 52V EM3ev battery on my Big Dummy to drive a BBS-HD set at 30 amps. Despite the battery's large capacity I ran out of juice the other night and had to pedal a very(!) heavy, loaded bike home unaided for the last 6km - too far for an old bloke! So I am now considering a smaller frame-mount 48v backup battery - maybe 11 or 13ah.

What's the best way to select a backup battery? Have others addressed this question at all?

Thus far I've thought of 3 alternatives (and their potential problems):
- get a 2-way on-off-on selector switch like the ones used in boat ignition systems (may not be able to handle 30 amps!)
- just unplug from one battery and plug into the other (loose cables wandering about)
- construct a 'plug box' to plug one or the other battery into (may not be able to get XT-90 panel connectors, still have loose cables).

Ideas and experiences would be very much appreciated.

Sam.

If you're going to add a battery, get one the same chemistry and number of series cells, regardless of capacity, and directly parallel it with the other one.

That way the load is distributed between them, so neither one has to handle as much, and both will last longer and be less stressed than if used separately.

Just make a Y-cable using whatever type of connectors you are already using on the first one, so that you can just plug it in when you will be riding where you need it, if you don't want to carry its weight all the time. Also allows you to plug it in by itself without the Y cable or other pack if you ever have to for any reason. (like troulbeshooting).



If you run them individually, then the smaller one will take more of a beating. If it's half the size of the other one, it has to supply twice the current for it's size, and so it's twice as hard on it; it'll sag in voltage more and it'll perform less well than the other.

If you don't want those problems you'd have to get one that has the ability to handle twice the current of the other one, and that will probably cost more.

(it might not be necessary, if the pack you get can handle the current without that issue, but some of the cheap packs cells can barely handle 1C. :lol