After Christmas, my sons got remore control car. During these 2 weeks, I have to replace several time the Alkaline batteries, then I put NiMh battery but always need under charge and in the car.
Last issue, when we played outside, I never carry the NiMH charger (cannot work on power bank) and I do not have enough NiMH for the 2 cars +some charging.
As these cars are with 5 cells (7.5v), it is close to the 2 lithium battery in series. For the size, I found 2 old prismatic batteries 1000mAh which can fit inside the battery location.
Now, for the charging, 2 pack in series is not trivial. The best is a balance charger (which I have) but too bukly.
Beside that, I often use Max1555 (but 300mA max on Dc input) or LT4054 (up to 800mA), all in TSOP23. Now, these IC can charge only one cell, not 2 in series.
So, on my first mockup (pictures), I add a switch which make the connexion in serial for the car usage and in single cell charge (one IC by battery) in the other position.
I add a small connector, solder the other connector in the car + and - spring and run...
What is nice is that with 2 like that, I can have a power bank with me to recharge one pack when he is using the second one.
I'm making a PCB with the 2 options of IC but I prefer the LT4054 with 500mA adjustment (2k resistance) which can fulfill most of 1A USB charger. The main difference with the mockup is the switch. I replace it with a 5V relay connected to the USB plus. As soon as the plug is connected, the relay change the configuration of the battery and goes in the charging configuration.
I could use only one IC and put the 2 batteries in //. But these pack are old and I'm not sure about their caracteristic now. Moreover, it is more clean to charge them separatly. During the usage, even if the charging current is same, I can see that the charging time is always different between both.
Each IC has an LED to display the charging status (Red ON is charging).
I'm thinking to add an adjustable regulator for another car which is in 4.8V (too high for one battery and too low for 2.
Enjoy the Christmas gift of your kids...