Very often I need to charge one or two series LiIon cell for my experiments.
I do have a sophisticated battery charger and analyzer but I don't want to use it every time.
I need something simple yet safe.
There fore I came to a nice shematics here.
It is a very simple schematics using just one LM317 and two NPN transistors.
I made some modifications.
The most important is R7 which is 6 Ohm for a 210 mA current.
Despite the urban myths, the Li based batteries are very safe at charging if you respect two rules:
1. No more than 4.2 V per cell
2. No more than 1.5 C (thats 1.5 x Nominal capacity in A) charging current.
The main advantage is that at the end of charging, when the battery has the desired voltage - in our case, 4.2 V/Cell, the charging current drops to zero. The bonus is that the LED lights during the charging phase.
Yes, I know, there is CAD but I like the old fashion way to make a PCB. By hand with a fine marker...
Here are some pictures....
I plan to put this with a 12V/2A SMPS into a small box.
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