How do I build a simple charger for 3 or 4 LI-Ion cells ?

[enocknitti]

I have a number of 18650 Li-Ion cells I want to use in some home-made stuff.
What I want to build is a charger for the cells. Input is 12-18 V. Preferably something that can be built-in.
I need overcharging protection, deep discharge protection I can do without..

Any suggestions ?

 

[Harald Kapp]

Do you want to use the batteries single or in combination (parallel or serial connection)?
For single use, you can buy inexpensive charger modules (example) that operate from 5 V, so you'll need an additional step-down module (also not expensive. Example) to create the 5 V from 12 V ... 18 V. These charger module are used to build a 5 V power bank from the Li-Ion battery.
For a parallel connection of the batteries the same setup can be used. But paralleling Li-Ion batteries bears a certain risk, read e.g. here.
For a series connection you will need a charger with charge balancing to ensure long life of the battery pack.

 

[enocknitti]

I need series connection.

 

[Bluejets]

How many in series..??

Here is one for 3S, couple of dollars just about antwhere online.

 

[Harald Kapp]

Note the connections at B1 and B2? These are for charge balancing.

 

[enocknitti]

Thanks for your info. With the help of your image and goggle image search I was able to find a seller in EU.