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Dell C-series battery packs - seeking for info before replacing cells.

JamieDoesStuff

JamieDoesStuff

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Aug 9, 2021
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Novi Sad, Serbia
I've got 2 Dell laptops that take these kind of batteries: a Latitude CPi-A and an Inspiron 8100. Now, I also have 3 battery packs for them: 1 holding about an hour's worth of charge (that's on the CPi, the Inspiron lasts about 20 mins shorter), a 2nd holding a few minutes and the 3rd being completely dead, blinking indicator light on the machine and all.

So, I'm looking for replacing cells in the 2 rather useless ones, but first I need to ask something before doing the deed:
Can I do this without the battery management system bricking itself when it loses power? I'm thinking that could've happened with the 3rd one, and it would certainly be a bonehead move to throw 40 Euros (80 for both of them, really) at something before doing research first.
 
Do you mean cracking open a battery pack and replace the cells inside?
I would suggest not to do it, unless you know how to correctly reset the controller thing inside the battery pack. Otherwise, the new cells may not last that long.
 
h2ospa said:
Do you mean cracking open a battery pack and replace the cells inside?
I would suggest not to do it, unless you know how to correctly reset the controller thing inside the battery pack. Otherwise, the new cells may not last that long.
Click to expand...

YMMV what may happen. Some of the earlier BMS boards would reset to factory defaults and behave like a new battery if the voltage goes low enough. Others wouldn't reset, but would keep on truckin' when the new cells are installed. There are of course the ones that have persistent flash storage and won't work even if you re-cell them, but those are hard to know unless you do re-celling regularly.

If there is some sort of EEPROM on the BMS board, I'd recommend dumping all three packs to compare them. If you examine the ROM, it may give some clues if there's a kill switch in the code. If you can't find that, you might be able to get away with flashing the ROM from the remaining good pack on the dead pack and see if it comes back to life after re-celling it, assuming just re-celling the pack doesn't work.

I've re-celled probably a dozen or so packs, and have had mixed results. More of them worked than did not, but there were still some complete failures.
 
I kinda don't want to mess with the working one, for one because it works, and two, cells aren't particularly cheap, especially not good ones. One thing I failed to mention in the post was that the third, completely dead one, was actually already cracked open, and the cells seem to have been replaced with some brown-yellowish ones. So I guess that kills the dream then. (Although I may still try it though, because stubbornness)
 
You don't need to invest in expensive batteries if you just want to test if re-celling it works.

You can use cheap 18650 cells wired in the same way the more expensive cells are and plug the battery pack in to see what happens. I keep a pile of used 18650 cells around for such purposes.
 
It's a silly question, but I've got to confirm: two 3.7V batteries wired in parallel make up for 3.7V, only the capacity is doubled, correct? So all I need to do is just order 8 of these 9800 mAh batteries. Now I'm wondering what their actual capacity is. (eh, they're dirt cheap anyways)
 
JamieDoesStuff said:
It's a silly question, but I've got to confirm: two 3.7V batteries wired in parallel make up for 3.7V, only the capacity is doubled, correct? So all I need to do is just order 8 of these 9800 mAh batteries. Now I'm wondering what their actual capacity is. (eh, they're dirt cheap anyways)
Click to expand...

Yes, parallel doubles the capacity.

By 9800 mAh, do you mean 18650 cells? Those are hilariously fake, most measure in the 300-1500 mAh range, and have a very short life. The absolute maximum capacity I've seen in real 18650 batteries is 3500 mAh.
 
I have yet to order some cells, but decided against this fake chinesium chap, as I figure even if the pack ultimately doesn't work, I'll be able to use them for something in the future. I've found a local supplier, and will be ordering 8 LG MH1 cells.
Another question: does the capacity increase with every cell added, and not with every second when added in parallel?
 
JamieDoesStuff said:
Another question: does the capacity increase with every cell added, and not with every second when added in parallel?
Click to expand...

For cells in series, the working voltage increases while the capacity remains the same. For cells in parallel, the capacity increases while the voltage remains the same. Packs use series+parallel arrangements to get both increased capacity and voltage.

You can use this battery pack calculator to figure out the specifications of your given pack.
 
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