The mower could be run on a variety of batteries, from lead-acid to lithium polymer (LiPo), but lithium-ion (Li-ion) batteries could be the sweet spot. Li-ion batteries are lighter and can store more energy than lead-acid and are cheeper and safer than Li-Po batteries.
You could buy ready made battery packs on the Internet, but it is also possible to make your own battery packs (if unsure, don't do it!). Either way the battery pack should be able to deliver at least 4 ampere continually and 6 ampere temporally to support the power hungry motors in the mower. It is also important that the battery pack is equipped with a protective circuit for overcharge, over-discharge and short circuit!
Read up on Li-ion functionality.
If you take on the quest to build your own battery pack, then these battery cells could be suitable Samsung INR18650-15M and Samsung INR18650-30Q.
For optimal use the battery packs should be fitted with a BMS-card to protect the cells and help balancing cells during charge-phase.
Suitable BMS-cards:
- "cf-4s30a-a", more information about the charger here (in Russia).
- yh2204a
- Smart BMS