Can I use a LiPo battery while charging it?

The simple reason is this: a battery being charged is subject to a voltage higher than that of the battery’s voltage. That’s basically why the current flows from charging circuit into the battery, rather than vice-versa.

If you put a load on the battery while under charge, two things will happen. First of all, the battery likely won’t supply the current, the charging circuit will be powering your load, when it can. Secondly, the battery charging circuit will at times stop charging to monitor the state of the battery. When that happens, the battery will want to supply the load, if permitted. Unfortunately, the battery under load will report a different voltage than the battery not under load, and that’ll screw up the charging process.

It’s technically possible to design a charging circuit that allows both your charger and the battery to power the load. In my experience, most smartphones do this, but with Lithium cells, it’s more complex than simply just dumping power into the system. There’s a specific cycle of charging one must observe to ensure the Lithium cell is charging for maximum lifespan, and of course, not overcharged. There are various ways to do this, but bottom line, if the battery is supply any power to the system, it’s not charging.

There things are never a practical problem in today’s devices, because because devices that do charge their own batteries, and do so completely under software control. So the phone could be charging when you’re on a power dongle with enough power, then cut off if the phone load started to peak. And in fact, it’s even possible to change the charging behavior entirely. Samsung just released a new update for the ill-fated Galaxy Note 7 that completely disables the charging circuit. So once your updated Note 7 runs down, it’s bricked.

It’s good to know that the “charger” you put on your phone is not actually a charger at all, it’s a power supply. It will power your phone during the charging process, if there’s enough current to both power the phone and charge the battery. Otherwise, it’ll just power you phone.

In fact, some cheaper laptops come with power bricks that don’t provide as much current as their batteries. When you have these running from the power brick, they may actually perform worse than if the power brick is disconnected.