Bafang Battery Charge Port: Internal Wiring

Short-circuiting the Bafang battery’s charge port may have done anything from completely destroying the battery management circuit to just welding a brass nugget onto the port’s center pin. The main output to the bike motor remained functional, so my friend used it on rides over the next few days to reduce the charge level.

Meanwhile, I peeked inside the undamaged battery on Mary’s bike:

Bafang battery interior - overview
Bafang battery interior – overview

The battery pack is neatly shrink-wrapped and firmly glued into the plastic shell, with the battery management PCB on the other side of the battery. Some gentle prying suggests it will be difficult to disengage the adhesive, so getting the pack out will likely require cutting the blue wrap, extricating the cells as an unbound set, then cutting the blue wrap to release the wires.

A closer look at the nose of the battery:

Bafang battery interior - front
Bafang battery interior – front

The large red wire entering on the left comes from the motor connector, loops around the nose of the battery, and probably connects to the battery’s most positive terminal or, perhaps, to the corresponding BMS terminal.

The medium black wire from the side contact of the coaxial jack (atop the pair of red wires) burrows under the battery and likely connects to the most negative battery terminal. This is the charger plug’s outer terminal.

The small red wire from the center contact of the coaxial jack (between the medium black and red wires) goes to the charge indicator PCB in the nose of the battery. This is basically a push-to-test voltmeter with four LEDs indicating the charge state from about 40 V through 54 V. The small black wire from that PCB burrows under the battery on its way to the BMS.

The medium red wire from the center contact goes to the BMS.

There is no way to determine how much damage the short might have done, although the silicone-insulated wires should have survived momentary heating, unlike cheap PVC insulation that slags down at the slightest provocation.

Removing and replacing the coaxial jack requires Cutting Three Wires then rejoining them, a process fraught with peril. You must already have a profound respect for high voltages, high currents, and high power wiring; this is no place for on-the-job learning and definitely not where you can move fast and break things.

With this in mind, the only hope is to remove the nugget and see if the battery charges properly.

The trick will be to do this without any possibility of shorting a metallic tool between the center pin and the side contact.

7 thoughts on “Bafang Battery Charge Port: Internal Wiring

  1. I’m thinking, something like a ceramic pipe that would fit around the inner pin of the female plug. This could be glued onto a metal pin and become an abrasive non-conductive Dremel compatible plug-restoring-tool.
    Quick search confirms that these pipes exist. Ebay and Alibaba sells them as heat resistant alumina ceramic tubes, Etsy sells them as beads that improve your water quality and help your dog against tick bites if worn as a necklace. No word however on compatibility with your copper anti earth radiation armband.
    On second thought, you might already have a suitable ceramic tube hidden in an old laser printer fuser. (That probably never got tick bites!)

    1. They’d surely aid my digestion, too! [grin]

      Amazon, of all places, lists little ones with 4 mm OD and 2 mm bores in lots of 30 and 50, which is likely Close Enough for the purpose. They might be too smooth to abrade the brass, although wrecking one end to get a few sharp edges should be straightforward with enough practice.

      If the shell drills I made don’t work out, a ceramic “saw” is definitely next up.

      Thanks for the suggestion!

      1. Cut the wire that goes to the outside contact of the charger jack. Remove the bead from the center pin, then splice the wire you cut.

        1. That’s basically what I’d do inside a battery on my bench. The gotcha: getting it on my bench!

          Can’t wait until telepresence robots become cheap, readily available, and able to perform fine manipulations, so I can tinker with things at a (sometimes safe) distance. [grin]

  2. “You must already have a profound respect for high voltages, high currents, and high power wiring”

    I used to work as a handyman at Sears in AZ. The manager was an *** who required me to work on electrical circuits in the store, live, because he didn’t want the lights out. Commercial buildings often use higher voltages than 120V. I would be moving 270V fluorescent light fixtures around on the top of a 13 ft. ladder with the circuits live. Realizing that a mistake would probably kill me via a fall but only after stopping my heart, I had a very healthy respect for the danger in what I was doing and developed a set of skills to protect myself. These became valuable later as a homeowner not wanting to de-energize a circuit to replace a wall switch or whatever.

    1. Eeeeeeeek!

      Watching the power company guys do hot-line work is always impressive: they deploy insulating sheaths and work carefully, but it’s basically just another day at the office.

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