"Robert Green"

Thanks to all who contributed to the thread. Here's how it wound up. B&D
had virtually no parts for the mower or schematics for it so I decided fix
it myself or scrap it. Nothing to lose. The grass was getting so high I
went back to my old lawn guy because he did a much better job then my
neighbor's kid, whose heart just wasn't into it.

I decided to localize the buzzing sound with my stethoscope. I hooked up
the original B&D battery that, according to the charge light AND a voltmeter
was fully charged, and rigged a bypass for the deadman switch (a trashbag
twist tie defeats it nicely!). When I turned it on, searching for the part
that was buzzing, it turned out conclusively to be the relay.

During the test, however, the whining noise from the relay dropping in
pitch. I took that to mean the battery voltage was dropping. The voltmeter
kept dipping downward when I connected it to the battery terminals. Not by
much, but definitely a downward trend.

I took out my test headlight from my '71 LTD that had a burned out low-beam
filament and hooked the hi-beam filament up to the mower battery. It glowed
brightly for a moment, and then quickly dimmed down to a dull red glow. I
did the same test with the supposedly good wheelchair battery which had
recently come out of a working chair. It, too had tested above 13VDC when
fully charged with a standalone SLA charger. And it too glowed for a second
and faded to red.

The B&F original battery was a 24AH 12VDC unit. The wheelchair battery I
tried as a test had 34AH 12VDC. But neither appeared able to supply the
current needed to spin the blade. So I whipped out a battery from my spare
wheelchair that's 55AH and was working well the day before and, well, you
know the rest. Mower hummed away like it was new.

What lead me to not suspect the battery was that it worked fine for the last
two outings, both within two weeks and then it failed catastrophically in
heavy, wet grass. Just stopped. The unit would squeak and the rotor would
spin a degree or two every few starts, but otherwise dead. From what we
could see of the controller board, there's a voltage regulator with a hefty
heat sink. I believe that's why the unit just stopped cold. I wasn't
seeing a gradual slowdown of the motor as you might expect with a
deteriorating battery. The overload, the dead drop off, the moisture and a
good voltage reading all pointed in the wrong direction, at least for me.
Kudos to those of you who stuck with the simplest explanation: "It's the
damn battery!" (-"

I've put the mower back in storage. No sense in buying a spare until I'll
be doing the yard work again since they do die over time, used or not. This
was a good lesson in not over-thinking the problem. Now my friend that I
give gas to for hooking up a hard drive's Molex power connector the wrong
way and frying its circuit board can rib me about insisting this was
something other than a simple, garden variety dead battery.

Sheesh.

--
Bobby G.

"I have yet to see any problem, however complicated, which, when looked at
in the right way did not become still more complicated." Poul Anderson