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Old 06-27-11, 03:01 PM   #1
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Join Date: Aug 2008
Location: 1000 Islands region, Ontario, Canada
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Default Reviving a free, dead 12V cordless Black & Decker mower

This Saturday was my town's "Community Treasure Hunt" day, when residents put out large, unwanted items at the curb and half the town goes shopping! Stuff that's left over gets collected and taken to the dump this week.

I wasn't shopping, I swear!

But I had been keeping an eye on the local classified ads for a used electric mower... and I saw this one at the curb on Saturday:

It's a 12v rechargeable (cordless), by Black and Decker, model M3300. It was even made in the local Black and Decker plant (before it was shut down).

The owner of the mower had helpfully placed a sign on it which said:


Helpful? Yes! Because it kept the mower sitting at the curb, deterring other would-be owners!

But the blade turned freely (motor not seized), so I took a chance and picked it up, assuming the problem was either a case of (a) a dead battery, (b) bad wiring/switch, (c) a dead charger, or, (d) worst case: fried motor.

First thing I did was plug in the charger to check it with a multimeter, and it worked fine:

Next, I popped the hood ...

Components of interest, from bottom left to top right...

1) 12v motor, 3.25 inches diameter. Turns freely, no evidence of overheating/scorching, and the brushes look fine.

2) 12v battery

3) Small circuit board, mostly I assume it controls charging/discharging to protect the battery. There are charge indicator LED's that show through holes in the cover as well.

4) The orange thing behind the circuit board is a "key" that mechanically activates a beefy switch on the curcuit board. You also have to remove the key to plug in the charger.

The battery showed 11.0 volts when I put the multimeter on it. Aha - Primary suspect!

As you can see, it's a sealed lead acid type, with a 28 Amp-Hour capacity (at the 20 hour discharge rate). Type: BPL28-12 (BB Battery brand).

Dimensions: 6.25 x 6.75 x 5.0 in

I charged it to 12.6 (indicated), then activated the mower switch... and it dropped to 4.x volts, and the mower did not make so much as a peep, click or whirrrr. Pretty strong case that this battery is toast.

Next, I connected it with jumper cables to a car battery, and... presto! The mower started right up!

Why, oh why?

It sure makes me wonder what would cause someone to throw out a functional mower, simply because it's got a dead battery. The answer I keep coming up with is that to most people, it's not "simple". I bet there's little aftermarket support for electric mowers, especially battery-electric ones. Everyone knows someone who can tinker with a small gasoline engine (there's actually a retired guy across the street who does a brisk trade keeping people's mowers, chainsaws & snowblowers going). But finding someone to look after electric/electronic components (even simple ones)? Not as easy.

So out to the curb it goes! Lucky me.

Replacement battery?

I haven't inquired about the cost of an OEM replacement yet. Will do.

But I also know a guy who works for an alarm company, and they regularly replace their 12v SLA batteries when they're at or below 80% capacity. I'm pretty sure he can get me a bunch of smaller 7 amp-hour batteries. If I can fit 3 in parallel in the space of the original battery, I'll probably take that route.

That would give me only 75% of the original (new) Amp hour capacity, but I'm OK with that if it's free, and because I don't have a large lot to mow.

Will post further updates!

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