Replace 3.7V flat tablet battery with a 18650 cell

Question

I am building a device using cheap Chinese tablet. One of the problems I have is weak built-in battery. I'd like to replace it with a few 18650 cells.

As far as I know, 18650 cell can be charged up 4V but tablet's source voltage from the built-in battery should not exceed 3.7V +/- 5%.

So, my questions are following: 1) is it true that built-in battery's voltage should not exceed 3.7V +/- 5%? 1.1) if yes, then what are my best options to always have stable 3.7V from 18650 cells? 2) will my tablet's built-in charger manage to charge 3-4 18650 cells connected in parallel?

Thx for your help in advance!

Answer 1

Just make sure 18650 battery cells mechanically fit, properly elecrically connected to tablet or protection circuit from original battery (if present) and recommended to have end discharging voltage of at least 2,7V ( must be written in datasheet ). Tablet will charge the batteries to voltage it can accept anyway and upgrading charger to more powerfull one is not neccesary, it will only use more time to charge a bigger battery.

Answer 2

Where did you read the battery specification about 3.7V +-5%?

Usually, All these batteries are same with 4.2 Vmax.

You will need more powerfull charging circuit to handle higher current of 4 batteries, and balancer board also.

Maybe you will have to deal with protection and identification circuit, if the original battery has more than two wires (NTC, version).

Edit:

No need balancer for parallel connection.