Category: Recumbent Bicycling

My friends in Raleigh sent a small box with the various tools I made, along with the scorched Bafang battery charge port. As it turned out, none of the tools were useful and the real fix required opening the battery cover enough to remove and replace the charge connector.

A view looking straight into the connector, with the side contact on the top of the image:

Gutting the connector shows why my homebrew shell drill wasn’t going to work:

There’s not much left of the central pin: the nugget hanging on its side is much larger than I expected. Most of the pin melted into that nugget, with a bonus droplet on the near side.

The rectangular chunk (upper right) is the switch terminal, with the tab from the side contact (on the right) welded to it.

Fortunately, none of the mayhem (including a few small sparks during the connector replacement) damaged the battery management circuitry or triggered a shutdown, so the reset tool wasn’t needed.

It’ll make a great 3D printing show-n-tell exhibit, in the unlikely event I ever do an in-person talk …

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:

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:

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.

The Bafang mid-drive e-bike kits I installed on Mary’s Tour Easy recumbent and a friend’s Terry Symmetry used the “Ultra-Slim Shark” lithium battery, a rectangular lump with a tapered snout:

The battery has a key lock on its left side:

The lock might deter casual thievery, but really prevents the battery from bouncing out of its mounting plate while riding.

The right side has a charge port closed with a rubber plug:

The cover protects a coaxial jack with a 5.5 mm OD and a 2.1 mm center pin:

My friend in Raleigh generally removes the battery before hoisting the bike into the back of her car to haul it to a friend’s house for their companionable rides: not lifting an additional seven pounds is a Good Idea™.

A momentary distraction in the middle of that process caused her to insert the brass key into the charging port, rather than the lock. The key put a very short circuit between the coaxial jack’s side contact and the center pin, melting the key tip and welding a brass nugget onto the side of the pin:

The charger plug normally sits almost flush to the port’s surface:

The nugget keeps the plug out the damaged port, preventing the plug from making electrical contact:

She owned the problem and immediately bought another battery, which tells you the value she places on riding her e-bike.

Verily it is written: let someone who is without whoopsie cast the first shade.

Any takers? Yeah, the way I see it, someone who says they’ve never done anything quite like that is either not doing anything or not telling the complete truth. For sure, I’ve done plenty of inadvertent damage!

Here’s the problem:

• The damaged battery is the better part of 600 miles away from my shop
• Civilians cannot ship 560 W·hr lithium batteries through any parcel delivery service
• Civilians cannot fly or take the train with such a battery, either
• Driving 1200 miles twice is out of the question for either of us

How would you proceed?

More to come …

For reference:

• USPS – Hazardous Material Info – section 349.222
• UPS – Lithium Battery Pack & Ship – PDF page 9
• FedEx – How to Ship Batteries – PDF page 3

Basically, it is possible to ship lithium batteries up to 100 W·h.