Laptop battery power supply for extended life

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Hello,

I want to extend my laptop's battery life without tinkering with the battery itself. I read that a battery power supply could be plugged into the power input on a laptop to increase battery life.

My question is: Is it possible to create a lithium-ion battery power supply that charges batteries when they need to be charged, but automatically stops charging the batteries when a certain voltage is reached? Is there a PCB for it?
 
If you are charging plain batteries, you can use a comparator with hysteresis to determine when to charge the battery. This, however, I believe will not work with a laptop because...

laptops have their own battery chargers built in that regulate the 19V typical to a constant current (until a voltage threshold is reached) that charges the batteries. Once the batteries are charged, then it only supplies power to the computer, with possibly a trickle charger to the li-ion pack .

If you can get into the computer battery terminals while it is installed, you might be able to do what your thinking, but it's a bit risky with an expensive laptop.

The other alternative is to hook up the external charging battery to a commercial power inverter ($30 typical) that your laptop wall supply is plugged in to.
 
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