How to charge a lipo battery inline with load

Question

How do you charge a lipo battery while it's still connected to it's load (e.g. like all modern mobile consumer electronics) using an off the shelf lipo charger?

I have an Accucell S60 battery charger and a 2S 4000mAh 25C Lipo battery. Both are designed for high-current RC cars where you typically remove the battery when you're done, but for my project, I want to embed and wire the battery so it doesn't need to be removed, nor the load completely turned off. How can I do this with the minimal extra wiring?

The battery has the standard HXT4mm discharge plug and JST-XH balance plug.

When charging, my application's load will be minimal and will only require about 500mA of current, so it won't be drastically stressing the battery. If possible, I do not want to have to introduce a separate power circuit to disconnect the load and power it separately.

Options I've considered are:

1. Charge solely through the balance plug, keeping the discharge plug connected to the load. Comments here suggest that this charger doesn't directly support that, but comments here suggest I'd need a parallel balance extension cable to connect the mains charge lead of the charger to the first and last pins of the balance plug.
2. Create a special adapter to wire the charger in parallel with the load via the discharge plug. I think this is essentially what option #1 does.

Is this feasible? Are there other options I'm not considering?

Answer 1

Yes, it is feasible. You just need to expose the balance plug and main positive and negative terminals (using a socket that cannot be accidentally shorted out or have something inappropriate plugged into it).

One possible problem is that the charger detects a full battery by monitoring the charge current and shutting off when it reaches 1/10th of the programmed current. At the standard charge rate of 4A the charger expects to shut off at ~400mA. However if your load is drawing 500mA then the charger may not properly detect the end of charge. While this won't damage the battery, the charger could run for longer than it needs to. Luckily your charger has a safety timer that can be set to 1-120 minutes (90 minutes should be plenty long enough).

You can charge through the balance connector by paralleling the positive and negative balance wires (red and black) with the main charger outputs. However the balance plug and wiring is only good for low current (<2A) so the charging time will be longer.

Note that during charging the battery voltage will reach (or even slightly exceed) 8.4V for quite a long period of time. Make sure that your load can handle this.