ContextFrom time to time with a few friends we play with these RC cars but the problem is always the same : the transmitters require 4 AA-sized batteries. Since we have 4 cars, it means we need to find 16 charged AA batteries before going outside. It often takes much more time to find and test them as the time required to charge the cars themselves! This made me wonder if it would be possible to replace them with a set of USB-rechargeable lithium batteries that are more easily available and easier/faster to charge.
Hardware inspectionThe transmitters look like this (outside and battery holder) :
After removing the 7 screws holding the two sides together, the internal board is accessible. Following the wires with a multi-meter shows that the jack plug is connected to the battery through a diode, and that the battery directly connects to this small LDO voltage regulator through the power switch :
The marking "CA33G" indicates an AIC1734 LDO voltage regulator providing 3.3V from up to 12V in. It is said to feature only a 250mV drop at 300mA. I tried with an adjustable power supply and found that the board consumes only 61mA and that the regulator provides 3.3V starting with 3.44V in (144mV drop only). Better, this voltage drop is maintained for lower input voltages and the board seems to work fine down to 2.5V!
Thus I don't need these 4.8-6V input. Using a single lithium battery from 3.5 to 4.2V will be far more than enough. No need for any DC-DC converter either. And given the low power draw, a small one is usable so that I don't have to modify the battery holder.
SolutionI decided to go with some salvaged cellphone batteries. The one photographed here is a BL-5B with a capacity of 890mAh. It includes discharge protection. I simply had to glue a small dirt cheap USB lithium battery charging board to it (you can have 3 of them shipped for less than $1). Then just solder it to the battery connection pads with hanging wires slightly longer than the battery itself. I measured the self-discharge caused by the charging board, it's around one micro-amp so it will need 100 years to discharge the battery. This saves me from having to install switches or connectors.
It is necessary to put some solder tin on the pads before soldering the wires in order to reduce the total heating time and prevent the plastic around from melting. But that's really all. Oh, and of course, it works :-)
I closed everything, installed the battery inside a small plastic bubble protection bag to save it from moving inside, and I can now quickly charge it simply by opening the battery holder and connecting a micro-USB connector to the battery.