I have a cordless lawn mower with 2 12V sealed lead acid (SLA) batteries. The batteries are not charging properly, but what I am trying to understand is why when I test the batteries voltage without any load they read a positive voltage, but when I pull the switch (which would turn the motor on if there was enough charge) the voltage goes negative.

Does this suggest any additional problems with the mower or is that something normal?

Thanks,
Jeff