What is going on with this cell?


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madsci1016

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Dec 13, 2019
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Ok. And I have seen that most of these probably do have high internal resistance too, just empirically though as seen in my OP. I took another one that was sitting full for a month just now and set it at a 2C discharge and it immediately drops to 3.0V. The good ones handle it well and stay at 3.2V.

Thanks everyone, I think I'm ready to approach BH about getting these replaced.

BTW, i know you can't tell who you are talking to through a forum. I'm actually a Electrical Engineer who specializes in embedded systems designs for robotics. I'm just cutting my teeth on battery chemistry physics on the side for fun and too learn. I'm only book smart, trying to get street smart on them. I'm actually disappointed in all the BMS options on the market and probably about to design my own.
 

Redpacket

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Feb 28, 2018
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madsci1016 said:
I'm only book smart, trying to get street smart on them. I'm actually disappointed in all the BMS options on the market and probably about to design my own.

We all get "street smart(er)" by learning & building :)
re the BMS, take it you've seen the others being developed out there? eg Stuart Pitaway & this thread:
https://secondlifestorage.com/showthread.php?tid=8540
Be great to see something come together.
Cell monitor comms via a common isolated bus & "off board resistors" ie not surface mount seem key features I've felt missing!
 

daromer

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off board resistors is made by diy bms as an example and several others. With that said its actually cheaper with surface mount components nowadays :)
 

Redpacket

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daromer said:
off board resistors is made by diy bms as an example and several others. With that said its actually cheaper with surface mount components nowadays :)

Yeah surface mount are cheaper but I'm worried about the thermal stress on the joints & other parts of having the main load resistor on the board itself.
All kinds of factors in it I'm sure:
- surface mounted powerdissipation is limited
- surface mount better with EV market (better withvibration, smaller)
- cheaper to make
- monitoring temp harder on external resistor
and more I haven't thought of too.
 

daromer

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Power disipation is the only thing but to be honest thats not an issue really. You have built in tempereture sensor and if properly done the board is stressreliefed. You can also glue on heatsink if you really need to.

But.. When you need that amount of balancing you have other issues like i tend to say :)
 

madsci1016

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Dec 13, 2019
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26
So i reached out to battery hookup and asked for replacements for the cells that show self discharge, and they are sending me replacements. SO moral of the story is battery hookup is awesome.
 
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