These are all referred to as 2S batteries, but they are clearly not all the same.
S is for series P is for parallel X is the number of cells
you will also see hobby batteries have a C rating, this is the discharge rate
hobbyist batteries typically use 18650 Li-ion cells or equivalent LiPo cells with a nominal voltage of 3.7
| 1S | 3.7 |
|---|---|
| 2S | 7.4 |
| 3S | 11.1 |
higher energy density
smooth constant voltage
~ 3.1-4.2 volts
Many shapes and sizes. Can be very thin
slower self discharge
safer
good all round
broader usable voltage
~ 2.5-4.2 volts
longer lifespan ~ 400 cycles
cheaper
Cheapest
very heavy
very long lifespan ~ 1000 cycles
very safe