You forget about other factors. capacity/volume/weight ratio's and price.

• Since it's a robot, discharge is controlled and not usually influenced by the user.
• NiMh for a given capacity is cheaper
• Heigher weight is actually an advantage for this type of product.
• Less electronics for protection and possible liability issues for the manufacturer or distributor.

All these or a combination of several of these probably tip the scale.

You forget about other factors. capacity/volume/weight ratio's and price.

• Since it's a robot, discharge is controlled and not usually influenced by the user.
• NiMh for a given capacity is cheaper
• Higher weight is actually an advantage for this type of product.
• Less electronics for protection and possible liability issues for the manufacturer or distributor.

All these or a combination of several of these probably tip the scale.