Question Laptop battery-cell replacement problem ?


Mar 29, 2006
I'm trying to rebuild the battery of an old Dell C400 (from 2002) laptop.
It has 6 prismatic 3.7v cells, adding up to ca. 12v when fully charged.
I removed the old cells and I measured 9.4v, without the BMS. Yet the battery was completely dead, it wouldn't charge at all, Theres'a button at the back of the battery pack, which, when pressed, lights up 5 LEDs that show you how much charge there is. The old one had zero LEDs lit.
I put the new cells in (using the same type as the original ones), and when I press the button now some lights are blinking, but it doesn't show a number of LEDs in sequence, which I would have expected, as the cells were all at 4.02v when I put them in, so fully charged.

When I put the pack in the laptop, it provides no charge to the laptop. When plugging in the charger cable, the charging light on the laptop comes on as green first, but then after 15 seconds or so goes to a blinking amber light, presumably indicating an error.

It is possible, of course, that the BMS is not working right, but does anyone have any other ideas why it's not working and what I could try to get it to work? Would it be possible to swipe a BMS from a different battery and connect the cells to it? The battery connector looks like a similar or even identical one could be found.


A lot of packs have tamper proof connections that once tripped brick the charging and power delivery capabilities. This a safety feature more than tamper protection.

Never made the attempt myself, not really worth it to me, but you might want to do some research on that specific BMS chip. There may be a way to reset it, or you will need to replace it.