I would ask what sort of PM does the original poster have in mind.
I think there is no question that regular and PROPER maintenance of a motive power battery is the one thing that goes wrong in most USA based battery powered forklift fleets, and the one thing that if done correctly can save the most money (as oppossed to doing it wrong or not at all).
Further, there is no question in -my- mind (after about 35 years of working primarily on electric forklifts) that improper battery maintenance is the root cause of solid state devices (motor speed controller components) going bad/failure.
However, if the goal is to just insure what should be [as per the battery manufacturer's design expectations] daily routine maintenance, having a battery company come in and water your batteries once a month is not going to "cut the mustard" so to speak, but will be much better than never bothering to check. There is far more to correctly servicing batteries than just adding water once a day/week/month.
I would also strongly suggest that you also have the truck PMed as per the manufacturer's recommendations, and to check to see just how much "overlap" the 2 PMs actually have, as a lot of the PMs done to the truck will only do a quick 'look-over' of the battery (usually due to time/cost restraints of the pricing of the forklift's PM)
I used to be aware of a major grocery chain that did not assign a forklift to an operator, but they assigned a battery to an operator, and it's daily maintenance was that one operator's responsibility. when that operator went on vacation, that battery got an "annual inspection, load test and acid (specicific gravity) adjustment. The operators drove different forklifts every day, but the same battery for years. An average battery life was 5 to 7 years for this company, and most of their competition was getting about 3 or 4 years [if lucky] on a battery.