Recycled power tool/laptop packs charge settings for mppt 75/15

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shlfn1 asked Jan 15, '19

Recycled power tool/laptop packs charge settings for mppt 75/15

Hi, I have a 7S 16P battery pack made of reclaimed 18650 Li ion cells with an ebike style 20 amp bms pcb.

Cells tested from ~1300 mah to ~1800mah with nothing over 100 milli ohms internal resistance used.

Should I be sticking with the LiFePo4 settings as a preset or can you suggest some more apt parameters.

The system is running my camp/beer fridge (waeco 95L) and also a dc power tool battery charger (Milwaukee) seeing a max load of about 5 amps when both are operational.

Looking for battery longevity so would be happy to discharge to 3.1 or 3.2 V per cell and maybe charge to 4.0 or 4.1V per cell.

Hope this is enough info and thanks for any help in advance, I understand there are a lot of people doing this now and it may benefit others to have this info as well. Regards N.

li-ion

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Answer 1 · 2019-01-15T11:12:58Z

wkirby answered · Jan 15, 2019 at 11:12 AM

7S works quite well.
LiFePO4 settings are not ideal, you should use a custom profile and sure, 3.2V to 4.1V per cells seems a sensible range to use.
Is your BMS wired to stop the MPPT in a cell overvoltage / emergency event?

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Answer 2 · 2019-01-16T09:59:33Z

shlfn1 answered · Jan 16, 2019 at 09:59 AM

These are the details from the eBay listing, I know eBay right but was enough to get me up and running, happy to look at any bms pcbs anyone would care to recommend, I know batrium is out there and have that in mind for a larger project but this one suited the pack configuration as well, thanks for the help, will keep checking back.One more question, because charging lithium is CC/CV is it necessary to float after the bulk stage, thinking 1/2 an hour float after reaching 4.1V per cell maybe enough?

screenshot-2019-01-16-20-45-19.png (329.1 KiB)

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boekel ♦ commented · Jan 16, 2019 at 10:25 AM

This BMS will work somewhat, but a few issues will arise:

-bms only balances at fixed point (4,18v), when charging to 4,1v per cell this means you'll get quite an imbalance in the pack.

-cuttoff voltage 4,3v can be too high for some chemistries, these cells will wear out faster, making them reach 4,3v even faster.

Please keep the battery at a place where it cannot do damage if it catches fire.

If you want this to work well for a longer time (years), you'll have to use a bms that balances to the average voltage in the pack (or to the lowest cell). And that communicates to the users / chargers in the system to stop / start

I knowh for such a small pack this does not make much sense financially.

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Answer 3 · 2019-01-16T10:24:57Z

thanar answered · Jan 16, 2019 at 10:24 AM

You should use 28.7V as bulk/float voltages on the battery settings and configure load output with a user def. algorithm so as it cuts off at 22.4V. That way, your cells will stay between 3.2V and 4.1V. Problem is that your BMS will never balance the cells, unless one cell group goes off balance quite a lot, in which case 80mA balancing current might not be enough. That's why you better get a customizable ("smart") BMS, where you can set the balancing to start much lower, i.e. at 4.0V.

And here's the question I have on another thread on the forums: if a cell group goes off balance while charging and the BMS disconnects the battery, will this damage the charger?

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