If you have the type of battery( may be that they aren't made this way any more), with the connecting straps between cells on the top of the battery, it is easy to charge the battery 3 cells at a time with a 6V charger.
   I have an electric forklift in the barn with a 36V battery and the battery has the top straps as I mentioned. I use the 12 volt charger and charge 1/3 of it at a time. By the way, the battery weighs 2200 pounds, so it takes a couple of days continuous to charge it.  I could probably get a regular forklift charger, but I don't want to spend the bucks.
    I was given a generator for my OC-3 that had come off an OC-4 and this generator has a small lever on the back of it to adjust the voltage. I have never seen or heard of such a thing. I had the generator rebuilt and the fellow who did the rebuild says it has a pretty wide range. He set it for 6V, as this is what the little crawler has.
               3 inches of rain yesterday.
                        Cal