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?


*Sounds as though the batteries are failing. To confirm, you can take them
to a place that services cars and have them test the batteries.