Originally Posted by
rapattack
..we are thinking that i could put the new batteries on the trailor
That'll work, but isn't really appropriate. You can lose as much as a couple of volts in the wiring. Not ideal when you only have 36 V to start with.
Originally Posted by
rapattack
.... he used a car battery charger to test them.
That's not really appropriate either.
First, almost all car chargers are 12 V, so you'll have to break the batteries apart electrically to use a car charger.
Secondly, a car charger can push a much higher load current than what your comparably small batteries are meant to deal with.
It's marginally acceptable if the batteries are (almost) fully charged, but a definite no-no on an almost empty battery. Either way, I'd recommend against it - unless you're able to monitor the charge current.
If the 36V charger that you have isn't up to the job, get another one meant for the task.