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MetroMPG 06-27-11 03:01 PM

Reviving a free, dead 12V cordless Black & Decker mower
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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! :D

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 :D ...

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!

MetroMPG 06-27-11 03:17 PM

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Oh, forgot to say:

One reason I'm particularly happy to have found this 12v rechargeable mower is because I can use it at my workshop/shack, which only has solar power (panels on the lower right side of the roof in this pic):

I've already tried, and the 1500w-rated inverter I have isn't powerful enough to start up a small 110v / 900 watt AC electric corded mower.

And I don't want to use gas, if I can avoid it. (Noisier, stinkier, less fun to tinker with.)

Daox 06-27-11 03:24 PM

Nice find Darin. Hook up two of those 6V - 220Ah Forkenswift batteries to it for SUPER long run times. :)

MetroMPG 06-27-11 03:27 PM

Ha! You know I already thought about it... nothing like 130 lbs of batteries on a 45 lb lawnmower! :D

MetroMPG 06-27-11 03:33 PM

Looks like the OEM battery is $85 USD (Digikey, plus shipping)


MetroMPG 06-27-11 03:52 PM

WOO - yikes! OEM replacement battery: $133 CAD! (plus taxes & shipping)


Ryland 06-28-11 12:10 AM

how are you searching for new batteries?

I'm a big fan of used, free and cheap electric mowers because they almost always have a simple fix, like new batteries, the last one I fixed needed a new fuse on the charger along with a repair on the relay that worked with the safety cutout, I keep thinking that I'm going to have to bypass the printed circuitry on one, but so far it's all been pretty solid and I have yet to see a motor that has failed.

Piwoslaw 06-28-11 12:58 AM

Great find, Darin! I hope it serves you well.


Originally Posted by MetroMPG (Post 14273)

It always amazes me how much free space there is inside electric appliances, compared to their over-complicated, gas-powered counterparts. Why would anyone prefer those heavy stinkers:confused:

MetroMPG 06-28-11 11:30 AM

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Originally Posted by Ryland (Post 14286)
how are you searching for new batteries?

I was just sticking that BB Battery model number into Google. Digikey was one of the first results. Then I searched on Black & Decker's site by the part number.

There's no dedicated battery place in town, unfortunately.

But I sent an e-mail to one of my two alarm company contacts, and I'll be seeing the other guy this evening at the sailing races. So hopefully I'll have some cast-off 7 Amp-hour bricks soon.

Confirmed (well, Google Sketchup-ed) that I can fit 3 of them in the space of the OEM 28 Ah battery.

There might even be extra room under the hood to stick a 4th one (or a 5th).

Ryland 06-29-11 12:36 AM

How I found lower priced batteries for my mowers was to do a search for the amp hour of the battery that I was looking for, then look for one that was the same size, ended up finding some that cost almost half as much, they are about a year old and seem to be holding up fine, my room mate mowed the lawn last night and made some comment about how much she loves our electric mower.

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