Reviving dead over-drained power tool batteries
I’ve always wanted to understand electronics a bit more. Mostly so I can repair them. We’ve been using a battery powered fan because we have no power to our house (intentionally, see this post here for details if you want), and the fan recently died. So I took it apart to see if I could figure anything out. With a volt meter I could see that the power was coming in under the battery connections at 20.8 volts. Inside there was a little circuit board connect to the switch with two wires going from the battery to that thing, and then from that switch board up to the motor. I tested up at the motor and there was nothing going from the switch to the motor.
The internet was just about useless for help. I couldn’t find a replacement switch + circuit board even though I had the part number. All the online sources were not giving me much to go on to repair it myself. And used broken fans to try and salvage parts were almost the same price as a new functioning one. It’s pretty lame living in a throw away culture.
So I ended up just cutting that switch out and hard wiring from battery to fan motor. I felt wires as it was running and nothing felt hot so it didn’t seem like any major problem. The biggest issue I have with it is that it is now on full blast when the battery is plugged in, where we like running it at about 30% speed for quietness and longer battery life. But it’s been so warm we wanted to run it and have been.
Just a warning to everyone out there who may try something like this. It turns out that the battery does not have a low voltage cutoff built into it. I ran the fan last night and it ran until it completely killed the battery. The battery was so low that it wasn’t recognized by the charger and wouldn’t charge. I found out that you can jump two batteries together by putting a wire from positive to positive and negative to negative and revive the dead battery to the point of being able to charge it again.
Here are a couple videos on the idea:
If you do this be careful. I noticed that the wires I used got warm. It’s better to copy the method used on video two. Touch the wires on and off to pulse some power to it rather than just hook it up and let it sit. It was hard to get wires in there, so I had to use too small of wires for the job. Doing this is risky and if you just left it there the wires could start on fire. So I only hooked it up for about 5 seconds and could feel it getting a little warm. I think it’s also smart to copy the method in video one where he’s using the same kind of battery to jump the dead one if you can. When I pushed the button on the battery to see how much power was left, it still did not register – the battery was still pretty low. But when I plugged it into the charger it worked! So now it’s out there charging.
If I use the fan anymore I’ll be monitoring the battery level more regularly. If you run batteries to very low voltage (beyond what it is built to do) you can do permanent damage. I may have shortened the lifespan by doing all this. However it’s an older battery and overall I’ve learned a lot through this.
If you think your battery is dead because it was over-drained for whatever reason you may be able to revive it without any specialized “smart charger.” One reason would be if you left a very low battery uncharged somewhere for too long and then came back to it thinking it was completely dead. This trick might give it a second life.
Hope this helps, have a great one!