The Traxxas TRX-4M comes with a proprietary connector for its battery that plugs directly into the receiver/ESC and the supplied USB charger. This means that you cannot simply use another charger that you might already have. While the USB charger is certainly usable (much more so than the 12B charger Traxxas provides with other models), the main disadvantage is that you cannot charge/discharge a battery to storage voltage.

It is however relatively simple to make a suitable cable, as the three wires are simply the three wires of a 2S balance plug – i.e. plus, minus, and the center wire that is used for balancing. There are two ways to go about making a new cable:

1. If you want to be able to use other batteries as well, you can snip off the connector of the supplied battery, solder on a connector of your choice, and solder on a connector and balance plug onto the battery. This cable then remains with the car.
2. If you’re fine with using Traxxas batteries, but want to be able to charge them on another charger, you can snip off the connector from the charger and add a charging connector and balance plug on the other side. That’s what I did.

I bought the cheapest XT60 charging cable that I could find (XT60 is what my charger uses), which came with a Deans connector that I snipped off. I also bought a 2S balance extension that I snipped in half. You also need wire strippers, heatshrink tube, and of course a soldering iron.

Here’s the wiring scheme in case it’s not clear:

The plus and minus of the Traxxas connector (use a multimeter to figure out which is which when in doubt) are soldered to both the charging connector and the outer leads of the balance connector. The center connector is soldered only to the center of the JST connector.

Here’s the end result, which works perfectly: