On 9/16/2013 2:52 AM, John Grabowski wrote:

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.

More details.
Assuming the batteries are in series...
If you turn on the switch and BOTH batteries measure negative,
you've discovered something new.
If only one goes negative, that's the result of the other battery.

Bottom line is that you need new batteries. They're sulphated
and they ain't comin' back.

Problem with most garden tools powered by lead-acid batteries
is that they sit in the garage over the winter without charge
and sulpahate themselves to death.