Power tool battery (li-ion) specs/wiring/etc

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Joined Oct 10, 2014
I have one of the newer dewalt 20v Max which uses a 5S li-ion pack the smallest being no parallel and 2 models up, with 2P and 3P (1.5, 3 & 4.5 Ah). The 20v part is basically the voltage of the battery fresh of the charger but after a minute or 2 of use it is at 18.5.

The thing is this is the same voltage as the 18v drills and I'm trying to see if I can do some conversions from NiCd to Li-ion as they save a lot of weight.

I found a "dead" pack at a recycler and I opened it up and saw a broken tab which was the beginning of the series of batteries (so it was the negative terminal -> wire -> tab connected to battery --- the tab has broken in half causing a break in the circuit. The only means of fixing it was to solder a new wire to the batter tab (part that hadn't broken off) and then splice the wire with the existing wire (replacing on the terminal wasn't possible as too much plastic to close - it would have melted). Everything went fine and I put it on the drill and it works great.

When I plug it into the charger the red charge light comes on for ~1/4 a second then nothing. The charger has 5 connections labeled:
C1 C2
C3 C4

B+ & B- are where I get voltage but I don't know what the others are, I'm thinking temp (TH ID) and maybe Circuit 1 & 2, Circuit 3 & 4? This battery has 10 cells so it is 5S2P setup and my smaller battery of 5 cells has the same labels. I'm sure the charger handles these batteries as it should work with all 20v max batt's. Does anyone have experience with these batteries or chargers that could maybe shed some light on this? Note - I checked each cell and all were 3.895 +/-.01 of each other, so I don't think there are any bad cells & the temp on the battery was room temp when put on charger (I think the bat drew too much current and made the metal tab break).

I'm not sure if there is a way I can get this to charge and if not would charging it without the special charger be acceptable (I am aware of the needs of li-ion in not over-charging - I'll charge slow and monitor voltage). I'm wondering if I can convert my old 18v NiCd charger to charge this pack. I know I got readings of like 36-38v when no pack was on it but I think that would be way to high for li-ion and a more sensible 4.2v/cell would be appropriate.

Can anyone shed some light on these issues?


Joined Jan 1, 2017
I know this post is a year old but by any chacne, did you find any info on the th/I'd pins?

I have fixed and/or rebuilt a few of these batterys. I have ran across 3 batteries that I cannot seem to figure out. I've spent hours searching the Internet for info on the circuitry in the board of the battery's. I even called dewalt. (Complete waste of my time)

Here's what I know.
B+/b- = 20v
C1-c4 measure independent cells. (For balancing while charging I'm sure)


Joined Jan 1, 2017
I'm not sure how I doubled that post up and didn't finish it. First post here, I'm still figuring it out.

Anyway. That being said, on the bad packs I've come across.

I get Full voltage (whatever I have between B+ and B-) across B- and TH

The problem lies at the ID pin. I wind up with anywhere from 10.5 to 12.5 volts across B+ and ID.

I am by no means very fluent in lithium charging circuitry to begin with, but I have tried to pull the putty off of the board to get an idea at least of what components are inline with the ID pin. Every time Ive wound up ripping the components off of the board.

Also, I have swapped the pcb out and it fixed the problem. So i know I am at least on the right track. Although all that means is now I have a different pack that doesn't work.

I have tried (pure despiration) hooking 20v across the ID and B+ and letting it sit. It barely draws any current and the highest I've gotten it is 12.8 v.

If anyone has any more info/suggestions it would be greatly appreciated. It kills me to have perfectly good battery packs go to waste over such a simple fix.
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