Hardware hacks‎ > ‎

The easiest DIY Lithium Polymer battery charger

Those pages show how to build yourself a cheap LIPO charger.
It is basic, safe and will probably cost you less than 3€ for the most elaborate version.

For a long time I searched for some DIY lipo chargers schematics or ICs.
Strangely most things you can find is either using complex schematics, specific charger ICs or microcontroller based chargers.

Most dedicated non switching ICs can handle 1 cell, some rare ones can do 2 cells, no more.

Switching chargers are great and efficient, but are complex and require more exotic parts like inductors and Schottky diodes.
Considering the precision required for the charging voltage it can quickly become a challenge, and the risk of destroying a battery or a complete house is not negligable.

Basically, a lithium polymer charger is a constant current / constant voltage charger.
But it is not both at the same time : 
-Current is initially limited up to the point where the top voltage is reached.
-Then it regulates output to a constant voltage, providing the "topping" charge to finish the job.

The constant current phase is only there to limit the stress on the battery.
Usually it is set to 1C which is 1 time the capacity : a 800mAh battery can be charger with a current up to 800mA.
Some people prefer using 0.5C to reduce the stress, whille some special batteries can support up to 10C charge.

If you search for constant current / constant voltage schematics, you can find many references to a dual LM317 setup.
One LM317 is setup to provide constant current and the second in serie is for constant voltage.
This just works and is very robust but it has a major drawback : the votlage drop.

Each LM317 can have a voltage drop of up to 3V plus the 1.2V on the current sense resistor means up to 7.2V can be dropped by the setup.
So to charge to 12.6V you need around 20V and with 1A charge current, more than 7W will be dissipated as heat.
In practice it is only usable with low voltage batteries and low charge current or you will need serious cooling to keep the setup at a working temperature.

Heat is the actual limit when using voltage regulators as high current chargers.