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How do you tap into a series battery pack to have the full series voltage AND half pack voltage from 2 parallel taps? Answered


I have a 320V lithium battery pack (100 x 3.2V in series) that I need to tap 160V from to run a couple of heating elements with. The main 320V is needed to run a motor. I was hoping to have the 160V taps in parallel (50 x 3.2V in series x 2 in parallel) to keep the pack at a uniform discharge level while still being able to run the motor at full voltage and at the same time.

Is this even possible?  Is there an inexpensive way to do it?

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Jack A LopezBest Answer (author)2010-11-18

Can you put the two heating elements in series to make one heating element that runs at twice the volage? i.e. make one 320V heating element. I'll draw a diagram of this if you want me to. BTW, what the heck is this thing? Is it a battery for an electric car?

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orksecurity (author)2010-11-19

Doesn't apply to this case, but a general safety advisory seems needed here:

Note that you generally do NOT want to tap from the middle of a battery stack. That means one set of cells are being drained faster than the others, which means that they will go dead first and then be subjected to reverse voltage from the others. This is a Bad Thing and can damage the batteries. And Lithium batteries can object rather violently to being abused; exploding casings and metal fires are nothing to trifle with.

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Nashty (author)orksecurity2010-11-30

This is why I am even asking this question. I do not want ANY unequal charges in the pack. Even though I am using a chemistry (LiFeMnPO4) that does not have terrible thermal runaway, these are expensive cells that I do not want to have to replace any time soon.

This said, do you have an answer for me? I would like to use full pack voltage if possible, or some other way to connect it up so the pack keeps a uniform charge/discharge.

Want to see the thermal runaway of these cells?
Go to: http://media2.evtv.me/archives.html
And watch the September 10th, 2010 Episode...it is about 105 minutes long...at about 95 minutes in Jack shows the GBS battery after it had runaway.

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steveastrouk (author)orksecurity2010-11-19

I think I'd clarify that to say you must NOT have UNEQUAL loading of the tapped battery, or running with one end as the reference, and not the centre

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lemonie (author)2010-11-20


Can you not re-jig the heater to run @ 320V?

L

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steveastrouk (author)2010-11-19

I agree with Jack, put both in series across the whole stack.

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