So, I understand that a 3Cell LiPo battery can output 12.6V when fully charged and that the cells must remain balanced to guard against Fire/Explosions/and general bad stuff happening.
However, each cell can only store a maximum of 4.6V. These Cells are then connected in Series to produce the 12.6V.
Is it possible to Charge a 12.6V LiPo from a 5V (or 4.6V) supply using a circuit that resembles something like the one below:
I understand that the ground plane on the charging circuit would have to isolated from the ground plane of the battery.
The MicroController would just cycle through the 3 cells charging one of them at a time. Some feedback loop to 3 Analogue inputs on the same microcontroller could monitor the current voltage of each cell and ensure that more time in the cycle is provided to the cells that are lagging behind until they are again balanced.
Would this work?
Any thoughts and suggestions would be MUCH appreciated. If any circuits like these exist already it would also be super helpful if you could point me at them.
Best Answer
No that doesn't work. If you want to charge the top cell switching Q1 on (without current limit!), your Q2 (also no current limit) will pull Q1's base below the cell's negative voltage.
This is your circuit for one cell. The signal coming from the microcontroller switches on Q2, but also is pulled down to 0.7 V. So the base of Q1 is also at 0.7 V, while it needs at least 5.1 V (0.2 V saturation voltage of Q2 + 4.2 V battery + 0.7 V base-emitter junction).
A solution could be to have a current output boost converter. This can generate a higher voltage from a lower input voltage. Constant current boost converters are often used as LED drivers.
Replace the LEDs with your battery, and set the charge current by choosing R1 = 95 mV/I.