If removing the load and re-applying it does not bring it back to life, but attaching a charger does, then it's probably shutdown due to under voltage rather than over-current.
Typical protection circuits will re-enable from under voltage only when you apply reverse current (charge).
Assuming you have actually charged the battery packs, then most likely one of the cells in it has gone dead.
If you do want to test it under load, then use an incandescent light bulb (12v car brake/indicator one, say 5 or 10w) will be the easiest way.
But if it's potted, there's not much you can do about it either way.