Making a Bantam BC6-DC balancer adapter

TL;DR?  It's built like this. But read on for details.

I use a Bantam e-Station BC6-DC to charge various battery packs, including the low self-discharge NiMH pack I use with my FT-897d when operating portable, the internal LiPo batteries I used in the same radio earlier, my bicycle headlight battery, as well as the occasional other battery pack.

When charging individual cells, everything is simple. But when charging multi-cell lithium packs, it's good practice to balance the cells at least now and then. Otherwise a single cell that self-discharges a bit faster than the others will lower the overall voltage of the pack, and the charger will not cut off until the other cells are already in an overcharged state. Or if a single cell begins to degrade and loses some capacity, it will reach full charge before all the other cells, and will become overcharged, likely degrading it even more. Many battery packs contain a balancer circuit (and other protection) built in, but e.g. RC hobby LiPo packs most often don't. Instead, they have a balancer connector that allows compatible chargers to do the balancing.

The BC-6DC has a balancer connector, but to actually connect a battery pack's balancer connector to it, requires a small adapter board, which is not included with the charger for some weird reason (or have I just lost mine, I don't know). But the adapter is easy to make. The connections are adequately explained on page 10 of the manual, with examples shown of a 6-cell pack and a 3-cell pack. It is also possible to connect two 3-cell packs or up to three 2-cell packs in series, and charge them at the same time by using the balancer function, as explained on page 21 of the manual. This requires a special "Multi-adaptor" board (which is definitely not included with the charger), whose connections are not explicitly explained, but which are quite obvious.

The connectors are "JST XH" connectors. For the 2–6 cell balancer board you need male connectors (the ones with pins, which solder onto a PCB) with 3–7 pins, one of each. If you want the "Multi-adaptor" functionality, you will need two of the 4-pin connectors and three of the 3-pin connectors. In addition, you need an 8-pin female connector (the one with holes, which you crimp onto wires, the one that will plug into the BC6-DC).

Since I do not have a JST XH crimping tool (and using the wrong tool is a pain and produces totally sub-par results), I wanted the 8-pin female connector preassembled with wires. I could only find 5-pin females preassembled, so I bought a couple of those, carefully dismantled the contacts from their housings, and refitted them in an 8-pin female housing.

I built the thing on a piece of Veroboard measuring 10 strips × 35 holes. The picture should be clear enough as to how everything is connected (click for a bigger picture). Just note that the picture is drawn from the component side of the board, the copper strips are on the underside!

The female connector has two "ridges" on one side (so it cannot be reversed when plugging it in). These ridges are upwards when the board is component-side up. This is also the orientation in which it plugs into the BC6-DC. The male connectors are all oriented so that the slotted sides (where the corresponding ridges fit) are towards the female connector end. Note that the two black wires leading to the female connector are also connected together! (Perhaps this is how the BC6-DC detects the presence of the balancer board?)

Here's the completed adapter board. You would do well to protect the underside against short circuits, but precisely how you want to do that, I leave up to your own discretion.

Note that there is no magical color coding to the wires. The black ones are the negative end of the battery pack, but after that I simply used what wires I had available. I had five different colors to work with, as I had disassembled two preassembled 5-pin wires to get the materials.

To use the adapter, just follow the manual: plug in the battery pack's balancer connector to the board, plug in the board into the BC6-DC, connect the main charging leads, and select the "BALANCE" function and correct cell type and parameters. Only connect a single pack at a time, and for 2 or 3-cell packs, use the connector nearest the black wires' edge of the board, unless you're using the "Multi-adaptor" functionality. If you are using that, only connect identical battery packs which are approximately in the same state of charge. You'll also need suitable wires to connect those packs in series, but I leave that one up to you. I don't think I'll ever need that functionality, but I built it into my adapter board just in case.

Do read the manual fully, be aware of the hazards of lithium batteries, always verify you've selected the correct battery chemistry and charge current, and never charge any batteries unattended. And don't blame me if you do something stupid or if something else goes wrong! You have been warned.

Antti J. Niskanen <uuki@iki.fi>