XT30 Battery Discharger

The other day I was charging up some batteries to fly and was surprised when a ground lead snapped off of its XT300 connector when I was pulled it off of the charger. That’s bad because one it couldn’t fly it and two I’d just finished charging it up so the battery was at it’s most dangerous.

I checked the rest of the batteries and noticed that there was one other one where the ground lead wasn’t stiff at the XT30 connector like it should have been. It hadn’t come loose yet, but I figured it would soon. I left it alone at this stage though so I could fly it and run the battery down.

I joked online about resoldering the connections, but I never seriously contemplated doing that. I’ve soldered a lot of XT30 connectors but never where the wires were attached to a battery. It’s possible to do, but you have to be careful about getting the wire too hot and damaging the battery’s wiring connecting the cells together or worse damaging the actual cells and potentially causing one or more to fail. Batteries are cheap enough that I’d rather just replace one than risk causing a fire trying to repair one, especially one that I’d been flying for months and pretty much had gotten my money’s worth on anyway.

It did make me realize a bad habit I’d picked up though. I’ve gotten pretty used to using the leads to disconnect XT30’s either from the quadcopter or from charging boards. I don’t know for sure that my habit was the sole cause for those two battery leads breaking, but it’s likely that it contributed. I have four more from that set that are all fine, but I’ve still tried to be better about using the plastic housing on the XT30 and the balance plugs to disconnect rather than grabbing the wires.

Disposing of batteries requires discharging them down to zero, and that’s a bit of a trick as most battery chargers that have a discharge setting won’t go below 3.2 volts as that’s usually the absolute minimum before damage will occur to the battery. So while I could discharge both batteries down to 3.2 volts, I couldn’t get them to zero and so safe to take to recycling without making something.

There are a few different ways to completely discharge a battery that carry varying amounts of danger with them from the completely safe and very slow saltwater bath to the very dangerous and not smart puncturing the cells with a nail. I opted to a fairly safe and fast method of using light bulbs. The only danger here is the heat that the bulbs generate while discharging, but that’s easily managed by hanging it off of the edge of my desk.

One thing I didn’t want to do though was just solder a bulb to an XT30 connector and leave it at that since eventually the bulb would need to be replaced, even if it would probably take a long time for that to happen. I found a set of bulb sockets on Amazon that looked like they would work pretty well along with some Halogen bulbs.

After that it was just a matter of soldering an XT30 connector that I could plug batteries into and soldering that to one of the bulb sockets, which I decided to shorten as much as possible to limit the amount of current traveling through those thinner wires.

Once I had it soldered and heat shrink over the connections I was able to test it out on the battery where the ground lead hadn’t quite come loose yet. I tried to keep a battery tester on it but since I’d flown that battery after charging it, it was already down to about 3.5 volts and once it went below 3.2 volts there wasn’t enough juice for the tester to run.

The other battery required a little extra effort to discharge. My multimeter came with a set of cables, one red and one black, that had alligator clips on the ends, and I was able to use one of those to connect the ground lead to the XT30 connector. That seemed much safer to me than trying to hold the wire against the connector with a pair of pliers. Thankfully it didn’t take long for the halogen bulb to kill the battery, but it did produce a pretty impressive amount of heat. I could feel it just by holding my hand over the bulb, and I’m sure it was hot enough that I would have gotten a nice blister if I’d have touched it. It did cool off afterwards quite quickly though.

After killing both batteries I cut the leads and balance plugs. The trick with this is not to cut both wires at once which would short them out. It shouldn’t matter since I’d discharged both batteries down to where the bulb wouldn’t light but that didn’t necessarily mean they were at zero volts, it was just too little voltage to light the bulb. It’s just a good habit to havy anyway.

I gut the red wire short since the black ones were both snapped off at the lead. That way the wires can’t come into accidental contact like they might if their lengths were similar. Then I taped all of the wires to the battery just to make sure that nothing could come in contact with the wire ends. It’s probably overkill, especially for a relatively small 2S battery, but I’d prefer to err on the side of caution.

Discharged and taped up, I put them aside for the next time I’m near the battery recycling center for disposal.