Re-purposing some laptop cells for ebike pack

Batteries, Chargers, and Battery Management Systems.
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a8ksh4   1 µW

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Joined: Feb 13 2020 11:16pm
Location: Sac-a-tomatoes, CA

Re-purposing some laptop cells for ebike pack

Post by a8ksh4 » Feb 14 2020 12:20am

Hi all, I'm posting a little early as I haven't spent a week reading posts on this forum yet, but I'm curious if anyone has some offhand advice to help jump-start me building a pack with some batteries I have. I have enough MLP3844143-2S2P to maybe put together a 2kw pack, but I would love to be able to do it without totally disassembling every pack. They're listed on ebay, but I really can't find any info about them, so have to do some experiments. :) Ideally I'd bundle them into a few 12v packs that I can daisy chain or put in parallel depending on the project and re-use the built in protection circuits...

So the packs are 2s2p "Li-Polymer", 7.7v, 7.8Ah. I'm not sure of the chemistry yet. The one pack I took the wrapper off of has ~4.6v across the cells, or 2.3v per parallel set.

The connector looks to have three v+ and v- pins (more pins, more current). In the middle are a whilte "clk" wire, mystery yellow, black mystery "pres" wire, and mystery green wire. I'm guessing it supports a serial connection to read out voltage, temperature, cycle count, and stuff like that... ?

Anyway, some photos are attached. I'll post more info as I learn. If anyone has nougats of fun info for me, thanks!
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dogman dan   100 GW

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Re: Re-purposing some laptop cells for ebike pack

Post by dogman dan » Feb 14 2020 8:51am

Disassemble without removing the strip attached to the tabs. So you still have a strip you can solder to.

Charge em, test em. Then make parallel sections that are big enough to discharge without huge voltage sag under load.

999zip999   100 GW

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Re: Re-purposing some laptop cells for ebike pack

Post by 999zip999 » Feb 14 2020 2:45pm

Read DrAngle post's.

a8ksh4   1 µW

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Re: Re-purposing some laptop cells for ebike pack

Post by a8ksh4 » Feb 17 2020 11:53pm

I'm working through DrAngle's posts. :)

Probably will remove the cells, as dogman recommends. I have 136 cells total (2kwh), but don't know how many will be good. TBD!

Trying to be conservative since I don't know much about the chemistry of these, I could target .5C peak current, so should be good w/ a 750w or 1000w motor/controller in an ebike. I want to be able to ride back and forth through Lassen park w/ my dog in a trailer, so I'm looking for a controller/motor that supports regen since the park has a huge hill in the middle. I can pedal myself through the park, but I want the trailer to be self supporting. :P E.g. spring & variable resistor in the bike mount that keeps trailer at equilibrium w/ the bike with a little math to remove accel/decel harmonics.

I haven't figured out how to structure a pack yet. Cell test process:
* Charge cells to specific voltage (4.1v) - need to find a charger that I can use with alligator clips for li-poly cell tabs.
* Leave cells 1 week and re-check voltage to find any with shorts/internal issues.
* Remaining cells put on load for specific amount of time and separate buckets based on capacity - alligator clamp series arrangement w/ light bulb and check voltage after time.

When we assemble packs, our goal is to get same capacity into any series arrangements, right? So if I had two cells in parallel, they both need to be +-.05wh capacity? And if I have strings of 13 cells in parallel, the sum wh of each string should be about the same?

Is it best to build strings of 13s1p vs. 13s3p? 1p costs more in balancing circuits ($4ea ?) but makes it easier to test each string and identify if I have a bad cell, right?

If I put nine 13s1p strings in parallel on a 5a charger, I'm dumping 350w into the pack and get a full charge in ~7hrs, and if each string is built with tested healthy batteries and with a battery balancer on each string, I don't have to worry about under/over-voltage condition on individual cells?

Off topic - I was curious if I could use a 48v (13s) in my camper in place of the 12v deep cycle I have now. Looks like Victron makes a 100v/20a-48v solar charge controller that would work and is supposed to be programmable to work w/ lithium batteries. That's fun. 48v to 12v adapters for common camper electronics are cheap and easy.

a8ksh4   1 µW

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Joined: Feb 13 2020 11:16pm
Location: Sac-a-tomatoes, CA

Re: Re-purposing some laptop cells for ebike pack

Post by a8ksh4 » Feb 18 2020 12:10am

Well, it looks like 13s BMS boards are more like $10-$25. Hmm.

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