I know that 2 cells in parallel doubles the mah. My question is when a battery is advertised as 5000mah, is it really a 2500 or can I expect 10,000mah?

You can expect 5000 mah from a battery that's advertised as 5000mah.

A 2s2p battery has 4 individual cells in it. the 4 cells are then broken down to 2x2 in parallel for 2 cells at 5000 mah. Those 2 5000 mah cells are then wired in series to give you 7.4 volts.

A 2s2p pack is made up of 4 x 2500 mah cells

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