Custom Li-Ion battery pack. voltage and charging

[fastline]

I am working on a custom light source that will need to operate at standard 12V automotive voltages so it can be charged from a car. The original power source was lead acid was is way to heavy to be practical so I need something much lighter.

I am currently looking at a pack that has 20x 18650 bats which have an individual cell voltage of 3.7V and surface voltage of 4.2V. in series/parallel, I should be able to make an 11.1V/20AH bat. surface voltage would then be 12.6V which I 'think' is about right for charge voltage.

However, I realize charging Li-Ion presents some challenges in terms of voltage, current limiting, etc. I am not sure how to make this all work right. I want to make sure the charge current is at least what the device uses so it can run directly off car power if needed. That is approx 7A. That current seems reasonable enough to charge the pack as well so possible 7-10A charge current?

I am most concerned with direct charging with no current limiting. IIRC, Li-Ion can pull big current when drained and both causes problems with chargers and is not good on the batteries. I am not sure if a simple series resistor can work here or if I need to make a circuit for this.

 

[ronv]

They are tricky to charge especially with several in series/parallel. Look at something like this. Might be a bargain.

**broken link removed**

 

[alec_t]

I realize charging Li-Ion presents some challenges

You're not wrong! If you don't follow correct charging procedure you are likely to have a FIRE

 

[dr pepper]

Dont know much about em but some 18650's have a built in chip that manages charge and discharge, might be worth a google.