EV "Buddy Batteries" - Series and Parallel

by Mike
(Bay City, TX USA)

I have a 94 Metro I am planning to convert to EV. I planning to get a 72V kit for the conversion. Can I use 6 12V batteries in this sequence?

...plus additional 2 or 3 12v batteries connected in parallel for additional amps,not volts.

I live is south Texas,no hills,just flat land. (That's why I'm thinking about the 72v kit instead of the 96V.) I plan to drive the car about 25miles per day at city speeds (35 mph).

Hi, Mike -
Good questions!

  • 72v should be fine for what you want to accomplish. The EV Photo Album is chock full of electric Geo Metros in voltages from 36v to 156, so depending on the amp-hour (AH) capacity of the batteries you choose, you should have no problem meeting your modest range and speed goals with a 72v system.

  • 12v batteries would not be my first choice with a DC system. I'd be leaning toward 6v golf cart batteries (unless you're worried about weight) for greater range and cycle life.

  • About the "buddy battery" proposal? What you're thinking of doing is not compatible with the laws of physics. (Yeah, THOSE again; )

    Mike Willmon, president of NEDRA put it succinctly in a recent discussion at the EVDL:

    "If you are going to parallel any of them, you have to parallel all of them."

    If you've got 6 batteries in series for 72 volts and you want to increase your range (AH), then you've got to have an equal number in parallel.

    Why, I hear you asking?

    Because of the nature of electricity. The effect of adding 2 or 3 parallel "buddies" to the string will increase the AH of the ones they're attached to, and it will be just like having 2 or 3 batteries of 200 AH in a string of 100 AH batteries; it will over-discharge the 100AH batteries and kill them off quickly.

Good idea though, keep them coming!


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