Most of the Acer Aspire One's that were sold in the UK have the 3-cell Li-Ion battery pack (2200mAh), and some XP 160Gb versions come with the bigger 6-cell battery (4400mAh, longer battery life).
On auction sites etc. there are also 9-cell batteries available, but dealextreme sell these 9-cell batteries cheaper (but before ordering, check the size: they are bulky).
Blurbs from the dealextreme website:
- Type: UM08A72/UM08B74/UM08A71/UM08A73/UM08B31/UM08B32/UM08B71
- High capacity heavy duty battery pack
- 100% compatible with original batteries
- 7200mAh capacity
- 11.1V output