although lawnmower batteries obviously would not work well in this application they did prove the truck is operating as it should once they were connected. The BDI reset and everything went back to normal. You can now rule out any problem with the lift itself.
Now back to the charger and batteries....

To check to see if the charger is going to start charging correctly you can take some jumper cables and connect the charger to those lawnmower batteries in a 24v configuration and turn the charger on
(ONLY BRIEFLY), just long enough to see if the charger is going to ramp up and try to charge them.

If you see the voltage jump up to what you would expect to see on the normal batteries (26v or higher) then you can verify the charger is working.
Do not leave the charger on those smaller batteries very long, just long enough to see what the charger is going to do. You could damage those smaller batteries if you leave the charger on them too long.

The reason i say to try this trick is that i have seen batteries that were so damaged they would not let the charger work correctly and the appearance of what the charger is doing in charge mode is a deceiving symptom the batteries are causing making you think the charger is also bad due to the lack of output voltage caused by the damaged batteries prohibiting the chargers output because of their internal damage most likely an open cell or something. One tell tell sign is to look at the battery(s) themselves, if the sides of the battery(s) are bulging any like they are swelled up that would be a strong indicator.

So... if you get good output from the charger using those smaller batteries then its just the batteries in the lift battery pack being bad.
If the charger does not charge correctly during that test then it also has a problem and needs to be repaired.

If you wind up replacing the batteries keep this in mind, replace them with the same type battery that is in the lift. DO NOT cheap out and try to buy a cheaper knockoff of whats in there. They have to be a specific amp rate and type to run correctly and to interface with the charger correctly. If they are lead acid, replace them with lead acid type, if they are AGM type then replace them with AGM type. If they are lead acid type do not try and go with AGM type because the charger might not be capable of charging AGM batteries and visa versa... an AGM charger may not charge a lead acid battery correctly either. It's just easier to stay with what you have. :o)

good luck