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Charging Li-Ion batteries

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Nigel Goodwin

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I thought members might find this useful?, I'm currently working on controlling charging (and discharging) of a Li-Ion battery pack with a PIC, for a project I'm creating at work. As part of the testing I sent the voltage and current readings out the serial port every five seconds, and then graphed it using Excel.

The battery pack is four 18650's in series, and are actually cells I reclaimed from old battery packs (as we've been waiting for 18650's to arrive, and ended up getting the supplier to resend the order!) - but they seem pretty decent. So for 4 cells I was charging from 16.8V and current limited at 520mA (0.2C), with the cut-off point at 130mA (0.05C). The actual 'charger' was a bench power supply, set to 16.8V and 520mA, with the PIC disconnecting it when fully charged via a relay - a second relay then connects it to the actual 'load' for the project, the start of which is the RHS of the graph - but the load is pulsed ON and OFF, and this seems to prevent the current sensor (a MAX471) being read correctly, it reads zero all the time?. However, the scope on the output of the MAX471 shows a nice 1.2A 50mS pulse.

Incidentally, the load is switched OFF once the battery drops down to 12V, but it's a bit tricky to know how well the battery is doing without the current monitoring working, but it does seem to run for far longer than I expected.

Li-Ion_Charge.png
 

rjenkinsgb

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Most Helpful Member
Just be careful with cell balance, when using cells in series.
If the voltage on one cell gets too high because others are still only part charged, and the total pack voltage is low, you can end up with a burst cell or fire - that's the common problem with cheap gadgets.

You must have a balance charge system that monitors and limits the voltage on each cell fro safe operation.

There are places on ebay selling balance boards if you don't want to bother building your own, but they tend to have quite low current ratings.

eg.
https://www.ebay.co.uk/itm/1PC-4-2V...ull-Charge-Battery-Balance-Board/263158542570
 
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