Paralel charging two Lithium Polymer batteries on a MP3 player Colorfly C4

Question

Is it a good idea to replace a built in lithium polymer battery rated: 3.7 V, 2000 mah with TWO x brand new 3.7 V, 1800 mah (much thinner stacking them together) connected in PARALLEL and place it in my mp3 player model (Colorfly C4)? As it turnes out the built in battery is of very poor quality hence the question.

1. Is this perfectly safe to parallel charge two exactly same brand new lithium poly batteries, or will there be a danger of overheating/fire?
2. Will the capacity truly then be 3600 mah then, meaning longer playbacks as well as longer charging times?
3. What things should I take further into account (aside volume/packaging/fitting etc)

Answer 1

2 1.8Ahr cells in parallel is the same as 1 3.6Ahr. Its called a 1s2p cell.

Sometime LiIon chargers stop charging after a fixed time. Something like the capacitry / charge rate * 1.5. So you might not end up with a fully charged pack after a single recharge.

Using a single cell LiIon batteries in parallel (with the same specifications and age) is generally acceptable.

When they are initially connected though, you need to make sure they're nearly at the same voltage. When connected the current flow will be I = (V1 - V2)/(ESR1 + ESR2). So long as I is less than the peak discharge and charge current you're fine.

If the batteries have the same specs, there is nothing more to worry about.

If the batteries are different (but operate over the same voltage range) then one just needs to make sure that during charging the current sharing doesn't violate the charge rate of 1 cell.

Even have a multicell stack, putting them in parallel is fine. But here it is a bit more tricky as the stack needs to be connected at the cell level, and the parallel capacity of each cell in the stack needs to be nearly identical.