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Change batteries from serial to parallel configurations with switches? Answered

I have a 5s4p battery pack.  The cells are 3.7 nominal voltage lithium ion cells.  In a serial configuration it is 18 volts nominal.  If I wire it in parallel it is in a 1s20p configuration and 3.7 volts nominal.  I had this awesome idea to use switches to change the configuration from 5s4p to 1s20p.  I want to do this because then I could charge whole pack with a 3.7 volt charger.  Better yet, I would not a balance charger. Once I got to trying to design the switch configuration I was amazed at how complex it would have to be.  The only way I can think of at the moment is to use 4 single throw single pull switches to isolate the cells and then use 5 double pole single throw switches to connect all the pos and negs together.  

Does anyone out there know how to make a circuit which could work with with one circuit board and one switch?  It could switch between 18 volts serial to 3.7 volt parallel?

Thanks

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frollard (author)2012-01-15

You always want to use a balanced charger with lithiums in parallel. Slight manufacturing defects mean they won't charge equally and the overcharged cells will be damaged, likewise the overdrained cells will be damaged also.

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Noblenutria (author)frollard2012-01-16

Don't you mean you always want to use a balance charger with lithiums in serial?

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frollard (author)Noblenutria2012-01-16

serial in particular, it's true that its difficult to balance a parallel orientation ;) another reason parallel is bad unless you can balance each series strand separately.

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Noblenutria (author)frollard2012-01-19

I thought batteries could never get out of balance while in a parallel configuration. If you connected a charged battery and a discharged battery together then the charged one would drain and the discharged one would charge until they met somewhere in the middle. All batteries connected in parallel have no choice but to be at the same voltage as all the others.

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