Bad BMS. . . what to do?

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Bonwit   1 µW

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Bad BMS. . . what to do?

Post by Bonwit » Mar 18 2021 5:57pm

It appears the BMS on my Luna Wolf-Pack battery has gone south. The unit cuts out way early, and exhibits other odd behavior. Anyway, Luna's diagnosis is a bad BMS. Of course, they want to sell me a new pack, but it seems a shame to throw out an entire pack because of one board failure. My question is, is there anything else to be done? I know that these batteries are potted, and I understand the dangers of shorting out one of these things, but is it possible to get in and extract the BMS and replace it with a new one without setting the garage on fire? Has anyone done this?

Thanks!

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Re: Bad BMS. . . what to do?

Post by LewTwo » Mar 18 2021 6:46pm

You might be able to cut away some of the potting but that is pretty much a non-serviceable battery.
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Re: Bad BMS. . . what to do?

Post by amberwolf » Mar 19 2021 1:18am

Bonwit wrote:
Mar 18 2021 5:57pm
It appears the BMS on my Luna Wolf-Pack battery has gone south. The unit cuts out way early, and exhibits other odd behavior.
A bad BMS usually results in no output, or not charging, or some other pretty much non-functional behavior.

Cutting out early is typically a symptom of a cell group that has lost capacity, either from fail(ing)(ed) cells, or an interconnect problem between cells so some of them are no longer participating in current flow.

It is *possible* for a BMS with a stuck-on balancer to *cause* a cell group to have lower capacity than the rest...but you have to have access to the BMS sense wire connector to disconnect it and test some things to find out if that is the case.

With a potted pack, you would need to remove the potting at least around that connector sufficiently to measure voltages on all of the sense wires in pairs, starting from the battery negative to the most negative sense wire, then moving up one wire at a time, moving both meter leads at the same time, and writing down each voltage as you go. Any that are different indicate problematic cell groups.

If they are all identical, then the entire pack could be losing capacity, or a group could have sufficiently high internal resistance (or disconnected cells) that it causes votlage sag under load but not just sitting there.


I recommend asking Luna what specific resin or other compound was used to do the potting, so that you can find out what solvent to use to remove it, without experimenting or just hacking around in there.

Bonwit   1 µW

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Re: Bad BMS. . . what to do?

Post by Bonwit » Mar 20 2021 10:46am

Thanks for the replies. I'll check with Luna, but I suspect they aren't going to get involved with my experiment.

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Re: Bad BMS. . . what to do?

Post by goatman » Mar 23 2021 1:54pm

what cells are inside

if you can see inside the pack like in the video, id take a 1/8" drill bit and drill a hole so you land on where the blob of solder is for the bms lead, not through it, do all the bms leads and grab a dmm and check the p-group voltages and if theyre not bricked at zero then try and balance the pack groups with each other, with a 4.2v charger through your access holes, if thats 3.2v, then get them all at 3.2v and plug in the charger. if it works, silicone the holes up

https://youtu.be/qBOgEcBU2qA?t=87

if you decide to try just dont drill into the battery like this guy

https://youtu.be/pRaGIHFLeUg?t=1

or this guy

https://youtu.be/M_VtmTXhrNU?t=1

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