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Connecting different-sized lithium batteries in parallel.

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xani:
Hi,

I have a bunch of batteries (from recycling used laptop batteries) that are fine (tested for excessive self discharge and such), same type but of varying capacity (say ~20-30% difference). I want to connect around 6 of them.

Due to nature of use (it's a backup power supply for rPI) I do not really care about max current or getting full capacity out of every cell.

My gut tells me they will just charge/discharge proportionally to their capactity and as long as I match voltages to avoid battery-to-battery flow during connecting it should be fine.

Am I missing something ?

Wimberleytech:

--- Quote from: xani on November 05, 2019, 04:33:06 pm ---Hi,

I have a bunch of batteries (from recycling used laptop batteries) that are fine (tested for excessive self discharge and such), same type but of varying capacity (say ~20-30% difference). I want to connect around 6 of them.

Due to nature of use (it's a backup power supply for rPI) I do not really care about max current or getting full capacity out of every cell.

My gut tells me they will just charge/discharge proportionally to their capactity and as long as I match voltages to avoid battery-to-battery flow during connecting it should be fine.

Am I missing something ?

--- End quote ---

I would not put them in parallel unless they were mounted in a fireproof box!

xani:

--- Quote from: Wimberleytech on November 05, 2019, 09:03:56 pm ---
--- Quote from: xani on November 05, 2019, 04:33:06 pm ---Hi,

I have a bunch of batteries (from recycling used laptop batteries) that are fine (tested for excessive self discharge and such), same type but of varying capacity (say ~20-30% difference). I want to connect around 6 of them.

Due to nature of use (it's a backup power supply for rPI) I do not really care about max current or getting full capacity out of every cell.

My gut tells me they will just charge/discharge proportionally to their capactity and as long as I match voltages to avoid battery-to-battery flow during connecting it should be fine.

Am I missing something ?

--- End quote ---

I would not put them in parallel unless they were mounted in a fireproof box!

--- End quote ---

And why exactly? They were in parallel to begin with, just in pairs of 2

mariush:
Can you sort them out by capacity and then use 3 of the lower capacity in parallel, and then in series with two of the higher capacity in parallel ?
Use a switching regulator to step-down to 5v.

xani:

--- Quote from: mariush on November 05, 2019, 10:47:26 pm ---Can you sort them out by capacity and then use 3 of the lower capacity in parallel, and then in series with two of the higher capacity in parallel ?
Use a switching regulator to step-down to 5v.

--- End quote ---

Can't, the device I'm using it with (UPS Pico, a raspberry Pi ups board) is the one doing the charge/discharge and it supports only single cell work.

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