In order to make the robot as hassle free as possible, i want it to be charged without removing the battery.
I used pololu zumo robots for teaching computer science, and those have 4 NIMH batteries. Not only the single lipo battery i used provides roughly the same energy for half the mass and a a lower price, but LiPo is simple to charge, hence the vast amounts of cheap LiPo charging boards. I selected not the cheapest, but still a cheap board, at less than 1.5$ a piece, with over discharge protection (and of course over charge protection!).
I bought a pack of 10 for 13$ with free shipping, from Aliexpress seller Shenzhen factory. This board is based on the very common TP4056 chip. You can see a high quality and detailed review of a more basic version of this board following this link.
Compared to the one tested above, mine adds tabs for load, thus you can have the battery and the load plugged at the same time. The charge maximum output current is given at 1.2A, and it is supposed to support simultaneous charge and discharge.
From the same seller, a single board is less than 2.5$, with free shipping.
The circuit changes in order to add this board are simple : battery wires goes on the charger, and the out + and - goes where the battery wires previously went.
So no modification of the circuit board (that was the plan). This circuit board goes under the electronics board, on the base of the chassis. Room was planed for it, and fixation holes are available. However, i'll have to design a small plate with claws to hold the charger, and with the corresponding screw holes to secure it to the chassis.
If one wants multiple batteries in a version of this robot, it is possible to do so without increasing charge time by adding multiple chargers and diodes on the output pins. More on that on R.Hasika project, which has 4 lipo batteries in parallel.
Anyway, it also has input pads, so it is easy to add a barrel jack, and why not a 5v regulator so that the robot can be charged with various power bricks with a wider voltage range. This is not something i'll include in the default robot (to keep costs down), but certainly an option or a mod that can be done.
Here is the look of the underside of the board :
Another use of the input pads could be to have some kind of pads on the robot in order to enable it to go to a charging station on it's own. I'll design such a station and work on this later on.