FEIN battery shows "dead" with Fein charger after cell replacement - Page 1
Forum section: electronics / beginners

author: passengert78
author_rank: Newbie
timestamp: May 14, 2024, 04:03:13 pm
content: Hello!I have a FEIN ABS14 cordless drill, with two batteries. I've used it quite a lot, and decided to change the cells in one of the batteries. I replaced the original cells (they were LGDAHD21865) with 2500 mAh Samsung R25 cells. I've charged them beforehand with a Li-ION battery charger to full capacity. After assembling the battery, when I pushed the charge indicator button, it showed that it's fully charged. "Well done", I thought. I used the drill for a couple of times, and decided to try to charge the battery with the original FEIN charger (type ALG50). However, when I put it on the charger, the red "battery damaged" indicator lights up instantly. And when I push the charge indicator on the battery while it's on the charger, it shows the red led (fully depleted). When I take it off it shows that there's still some charge in it (two green lights or so).I disassebled the battery, and checked voltages everywhere it was possible: at the battery connection points, at the leads, at the BMS connector input, and all voltages are fine. I even checked the PTC thermistor, which is also fine (shows a small resistance of around 6 ohms). The charger also seems to be fine, because it charges the other battery I have flawlessly. I guess there has to be some kind of problem with the BMS of the battery, but I can't figure it out what. Is it possible that the BMS somehow "remembers" the former cells' voltage, or keeps track of something that's mismatched with the new cells, or what? Can the BMS be reset somehow to match the new cells? Like pushing the indicator button for a specific time at a specific voltage level or whatever?I've done this several times before (replaced cells in batteries), with many different brands like Hilti, Dewalt, Bosch, Ryobi, etc. But I've never met anything like this that a charger is so "picky" on the battery's cells.Does anyone know a solution to this?Thanks in advance

author: hugo_g
author_rank: Newbie
timestamp: April 27, 2025, 01:19:47 pm
content: I've been in the same situation with 18vI've suspected that the BMS has some kind of register that once it drops below a certain voltage it knows it's damaged.I never went down the path of trying to rebuild a Fein battery pack, and have thousands invested in their tools.Fein have now moved to the Bosch battery system, so that leaves me needing new tools or finding an adaptor that works. I've found adaptors that work with some tools but not others. I suspect it's the resistor value they are using in the adaptor.The tool is pretty dumb, 18v + & - and a simple resistor between - and T (NTC). Based on testing my newest battery packs , 90-110ohms was the value I was getting. So I flipped the resistor on the battery adaptor and all tools works. I'm now I'm using Bosch batteries and the Bosch charger.

author: benj38
author_rank: Frequent Contributor
timestamp: April 28, 2025, 05:59:13 pm
content: Quote from: passengert78 on May 14, 2024, 04:03:13 pmWhile I do not know about Fein battery packs, some manufacturers (Dyson comes immediately to mind) build the BMS board in such a way that once it decides that any of the cells is bad (e.g., due to over discharge), the BMS locks up and is effectively dead with no known way to bring it back to life.One possible solution I have employed (e.g., for Dyson battery packs) is to buy a compatible Chinese BMS, or a compatible cheap full battery pack (which would invariably contain inferior cells and/or would highly inflate the specs of these cells) to salvage the BMS board from.Thankfully, I have never seen a Chinese BMS board that has this lock mechanism.

author: amyk
author_rank: Super Contributor
timestamp: April 28, 2025, 07:39:34 pm
content: Post some pictures of the BMS and/or the IDs of the chips on it.As others have mentioned there is likely an undervoltage lockout that gets triggered. This is common knowledge in the laptop battery recelling business, and one way they get around it is to change the cells "live".