Woha, some pointers.

Do you know if they're literally sealed for life, if I need to fill them up with water
For the robots we need Sealed Lead Acid batteries, but not with liquid filling. Gelcells (were the acid is an very thick ooze) or AGM Cells (glassmatt fiber as retainer for the acid) are mandatory, since the start of Robot Wars.



7.8
The following battery types can be used without any specific precautions although care must be taken when
any battery particularly during charging:
 Pb (Sealed Lead Acid, SLA), non-spillable gel type. (e.g. Yuasa, Hawker)

Imagine a liquid acid battery springing a leak during a fight. I don't want to clean up that mess.

Weight of SLA batteries. A 7 to 9 ah SLA battery (PBQ was our favorite brand, cheap and dependable) weights around 2.8 kg.
Hawkers come in so many flavers it's difficult to give a weight, but the cheaper genesis (rebranded Yuasha) sets are about the same. The High End pure lead technologies are a tad lighter for watts stored. But the pricetag gets high fast.

Weight of a heavyweight
2.1 Weight classification
Heavyweight: 55kgs to 100kgs [220lbs]