Meade 18V power supply as a charger?

I have a vintage LX200, and field power is always an issue. I currently
run a 120V converter off of a 12V battery, then use the AC - 18V Meade power
supply. It's not very efficient.
I recently found six 6V sealed batteries and I thought to combine three
to give me an 18V battery, and another for reserve.
The problem is how to charge them? Anyone have any thoughts as to
whether I can use the Meade AC power supply as a charger to charge these
batteries? I obviously don't want to burn out my power supply, so I'm hoping
to hear from people who would have some knowledge as to if my idea will
work. I know there is a Meade 12V - 18V adapter, but being out of work for
some time I would like to work with materials on hand.

TIA,
Bruce