I create a arduino robot car and I used the power bank for the supply of arduino. And I used L293D Motor shield arduino and the supply I used is the 4pcs of 18650 battery, I series the 2 pairs of the battery and parallel the pair. In calculation, 3.7V x 2 = 7.4V and the current increase. I want to have a charger of my 7.4V battery and how to charge it?
An easy and safe way is to put a standard female RC power connector (like an XT60) on the battery pack, put the male connector on the robot car, then use a commercial RC battery charger to charge the pack. Putting a balance connector on the battery would also be a good idea and help prolong the life of the battery pack.