David Scheidt wrote:

18V lithium ion batteries consist of 5 cells
in series, to make the the 18V.
Bigger batteries (the 4 and 5 AH ones,
probably; I don't know ryobi's line well)
have two sets of five cells in parallel (5s2p
is the common notation). If the design
supports it (again, I don't know ryobi), you
can draw twice as much current, which lets
you have more powerful tools.

At the back side of the 2.5 Ah battery, it says:

Ryobi

18.0V

-----
- - - [the/a symbol indicating DC]

45Wh (2.5Ah)

5ICR19/66 [???]

RB18L25(130429057) [1st - Ryobi 18V Lithium 2.5Ah,
2nd - random product/version number?)

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