Dewalt Mower & 10ah 20V Max pack shenanigans

I had a strange incident a little while go I figured I’d share. When the storm Elliot came through in December last year temperatures were expected to drop well below freezing. I normally have some of my power tool batts in my outdoor workshop and shed but the expected temperatures were unusually low so I figured it was prudent to bring them all inside before the big bad freeze hit. I rounded them up, took them inside, and put them all in a queue for charging…but strangely, neither of my two very recently purchased 10ah packs, which had been sitting in my DCMWSP244 mower, would take a charge. They both appeared completely dead on my 8ah fast charger, none of the various other batteries I had kept in identical weather conditions had that problem, and those batteries ranged from 2ah to 9ah Flexvolts, some old and some new.

I recalled reading that another Toolguyd user had mentioned a similar issue with a 10ah pack, and that it could be resolved by charging on one of the more basic model chargers, so I tried a couple different ones. One of the two packs would charge on a basic cheapo charger. The other would not. I left it in a warm place to see if that might help. It did not. I then figured I might be able to manually charge the battery, albeit without the benefit of doing any balancing, using a power supply. This is not ideal of course, but thought I could do it in a careful controlled manner. I have a precision DC power supply with a constant-current function on my test bench; I figured I could use that to charge at a slow controlled rate so I wired it up to the battery, set it to 500mAh and turned it on. My cheapo charger is 2 amps, so I figured 1/4 of that would be quite safe even without the balancing. At first it was only taking about 9V, over the course of about 15 minutes this slowly crept up to about 14V. Once I saw that I tried putting it back on the 8amp fast charger, and then it took a charge no problem.

Since then I’ve made a point to use those batteries a couple of times to see how they performed and I’ve had zero issues with them. I’ve never had this sort of issue happen before with any other Dewalt batteries or tools either. So, I suspect that the mower–DCMWSP244–is the culprit, and it constantly draws down its batteries if you leave them plugged in.

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Thank you for the detailed post information. Well, I don’t have these batteries I think it’s great that you didn’t just abandon them in a landfill and work to try to solve the problem with some ingenuity. Kudos to you.

So I had something happen with a 5ah that I got free in a promotion a while ago. Dewalt battery.

so it seems there is a lock out in the battery circuit of the newer batteries (ie since like 15 or such) where a battery will decide not to take a charge. for some reason and I read alot to do with temp. So perhaps it’s over temp or undertemp protection.

point is - the way you reset the battery is to hook up the main positive and the main negative to another charged dewalt battery and the minor jolt is a signal to the battery to reset the circuits and test again.

Or something like that - it’s apparently what they do at the service center when you have a battery that won’t take charge.

Key thing - I did this and it worked for mine. wired up and for a short moment - then plugged in and it took it. what you did with your weaker charger and the low current would have done similar I think which is why that worked out.

there are a few youtube and other posts on something like this. Apparently Milwaukee and Ridgid batteries do a similar thing.

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I don’t know the exact logic behind it, but I suspect this lies in the charger. My theory is that if the charger doesn’t detect at least X volts then it won’t turn on. The chargers do monitor temperature as well, but if you try and charge a pack that is too cold or too hot the symptoms are different: the lights on the charger will turn on, and they will blink a code to indicate a “hot/cold delay”. That explains why one of my batteries would charge on one charger and not the other: the two chargers have different thresholds for that minimum voltage. That also jives with what the previous Toolguyd user had written (and I’m sorry I don’t recall the name).

I think the “reset” procedure that you mention is really working by charging the battery a little bit, raising its voltage over the threshold for the charger to think it’s still good. That’s basically the same thing that I did with my power supply, just with a lot less control over the amount of juice flowing into the discharged battery. I don’t believe there’s a lockout in the battery, I think the chargers are built to refuse to charge a battery whose voltage has dropped below some internally set value.

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that is another dewalt issue I have 3 chargers of the various generations but none of them new I think my newest is the 112 model. Anyway they do act a little different too.

I also find - with one I have to really seat the battery home for it to charge right - but not so in the others. Hell the oldest one the battery just hangs there. I keep all of mine mounted vertically - actually on a shelf box I made for my cordless drills and drivers and etc.

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