infinkc you may have answered your own question about what is happening.
you say it drops from 24v to 23v when you press the lift button?
24v system right?
what battery pack does it have in it? 2 12v batteries? 4 6v batteries?
in either case the complete battery pack should read a lot higher than 24v IF the batteries are charged up with a fresh charge. A fully charged 24v battery pack should be reading 26v-27.5v or there abouts, the voltage drop should not exceed 1.5v drop during operation.
Yes i know the BDI is reading full but it can do that just from seeing a higher voltage right after the charge, if you plug it in right away the surface voltage can reset the BDI. Even if there are bad batteries the BDI may not react quickly and might show a higher voltage than what's really there at first, it may take time for it to level out to what the state of charge actually is.
Also if you have a bad battery cell in one of the batteries it may not pick up on that until you put a load on it.
What i would do in this case is isolate each battery and do a load test on each one to see if you have one that is bad.
From your description when you hit the lift button tells me that you have a bad battery and when you hit the lift button the problem is showing up because of low voltage, it does not have enough voltage to engergize the lift system but may have enough to drive the unit. (i'd be willing to bet the drive performance is not very good for very long). The drive uses less load because it is variable.

In short, there is something wrong with your batteries or the charger, both need to be checked more thoroughly.