So a friend of mine brought over a drone he ordered off Ebay. He complained that it doesn’t fly anymore. So I looked around and saw that the battery was all bloated up. Told him that I need to replace the battery, but first I’ll take a look at the charger. So I opened it up… Basically, it was the comparison of connecting the 3.7V battery to the freakin’ USB 5V! But hey, it had a pretty red LED.
Later on, after the initial shock of the amazing things you can get off the web, I learned that this is called a “linear charger”. It (kinda) works, but mostly it kills the battery. So I went on to fix it using my remainders of the BQ21040 I used in the previous project. In order to connect it I had to hack and slash a little bit:

This basically yielded the following scheme

The charge current resistor was chosen to be 3.9K Ohm as it sets a ~130mA charge current, suitable for 300-600 mAh batteries.

I definitely recommend this charger fix as it will elongate the battery life and give and keep you away from bloated batteries. Try it out!