[moved] Safe USB battery extender

[thezombiekat]

Safe USB battery extender

I am building a custom case for my ASUS tablet PC. I would like to include a supplemental battery so it will last a full day of use.

The tablet charges from USB but came with a wall charger (that takes a USB cable) its output is 5V 2A.

I have found many methods on the net for making this on the net all are very simple, almost identical and most carry a warning that you cant just leave the unit charging untended because the batteries could overcharge, over heat and leak.

Commercial units don’t have this problem so I assume they have a circuit built in that prevents overcharging.

Is it possible for me to build that circuit into my tablet case, where would I find instructions.

Thankyou

The Zombie Kat

 

[Audioguru]

You must use a safe charger circuit to charge your supplemental battery. The battery will probably be a single "3.7V" Lithium cell that charges to 4.20V maximum. The charger circuit should detect the charging current dropping much less than when it began charging then it turns off the charging.
But how will it connect to the tablet that expects 5.0V to power the charger circuit it has inside? Will you remove the dead original battery and replace it with your charged supplemental battery?

Maybe you want to make a "power bank" that has a battery, a charger circuit and a voltage stepup circuit so that its output is 5V to feed the charger circuit inside the tablet?