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MetroMPG 06-27-11 02:01 PM

Reviving a free, dead 12V cordless Black & Decker mower
4 Attachment(s)
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 02:17 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 02:24 PM

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

MetroMPG 06-27-11 02:27 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 02:33 PM

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


MetroMPG 06-27-11 02:52 PM

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


Ryland 06-27-11 11:10 PM

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 06-27-11 11:58 PM

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 10:30 AM

1 Attachment(s)

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 06-28-11 11:36 PM

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.

Travis 06-29-11 05:04 AM

After doing the same thing, only then did I realize how much money I saved at a local battery shop that was clearing out floor space. I got 4 40AH 'matched' batteries for $1/AH. They became the backbone of my power pack, and I ended up with almost double the storage for 1/3 less cost than the single 90AH battery recommended for my panel.


Originally Posted by Ryland (Post 14311)
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.

MetroMPG 06-29-11 10:07 AM

I need a lawn-mowing room-mate!!

I talked to my alarm company technician friend last night and it sounds like I can have as many used 12v 7Ah batteries as I can take, probably gratis. He says they replace many each week. Some are actually worn out, but others are still good (e.g. replaced when one battery in a parallel string setup croaks before the others, but they replace the whole string).

I'm meeting him at their shop this afternoon, where he says they have a milk crate full.

Then I'll just need to set up a testing rig to figure out which ones still have life in them & measure their capacity.

Daox 06-29-11 11:01 AM

Woo, good news.

Testing shoudl be pretty simple, charge em up, put them on an inverter and time how long it takes to discharge them at ~1C. A 75W bulb would do nicely.

MetroMPG 06-29-11 11:20 AM

That's the plan!

12v battery > DC-AC inverter > Kill-a-watt meter (to record time & watts used until inverter disconnected) > 110v load

= good battery condition data

Weed Dog 06-29-11 03:49 PM

Cast-off Electric Mower
Awesome...I hope to acquire an electric mower by similar means, and replacement batteries using tips from the other posters. Then I take one more step away from the prevailing petro-culture...:cool:

MetroMPG 06-29-11 04:01 PM

Update: batteries won't be free! Sadly, my guy found out that someone already comes and buys the used batteries from his company on a regular basis. Pays 200 bucks for about 3 milk crates full (about $2 per 7 Ah 12v brick, by my estimate).

I'd happily pay that amount, but I'm not sure if my guy has the authority to sell them to me.

Still investigating!

Travis 06-30-11 04:41 PM

and that guy probably takes them to the local swapmeet and resells for more

MetroMPG 06-30-11 05:07 PM

Probably something like that. There aren't any swap meets around here that I know of, but I wouldn't be surprised if it's someone from out of town making the rounds of the alarm companies.

Anyway, I got one freebie used 12v/7Ah from my friend. Haven't tested it yet. One isn't really useful to me at all.

I asked him to ask his boss if he would sell me some (I'd take a dozen for sure) at the same price he gets from the other guy.

MetroMPG 07-01-11 01:31 PM

an embarrassment of lawnmowers
Mower #2 ....

OK, maybe I need to start another thread...

This morning, there was another electric mower at the curb - literally at the neighbouring house to the one that chucked out the battery mower last week.

This one's a small "corded" 110v AC machine, 7A rating.

The handle is pretty bent, and has been repaired (not very well) a few times. I knocked on the door to make sure they were actually throwing it out:


"Does the motor work?"

"Yup -that handle is no good. Squashed it with the car."

So I rolled it over to my place, plugged it in and cut the grass with it :). The handle definitely needs some patching/welding - it's work fatigued in several spots (not too far from breaking) and has been lightly patched up with tin & screws.

And 7A might be small enough that the inverter at the shack will be able to fire it up so I can cut the grass there with the solar power setup.

Gives me something to use while I'm sorting out the battery situation.

Travis 07-01-11 02:09 PM

At 7 Amps, you'd need 1000W inverter minimum to supply the constant current, and since that supply would be continous you're going to need heavy cabling to handle it. Drawing that constant current from a smaller battery may exceed its chemistry.

MetroMPG 07-01-11 02:20 PM

It's a 1500w inverter, with 2/0 cables to four 225Ah golf cart batteries (that are half worn out, at least). So the system should be OK for light mowing.

But the constant current isn't the problem, it seems to be the starting current.

I already tried a 12A rated mower and 9A rated mower and the inverter wouldn't start either of them. Maybe the staring draw of this 7A one is low enough to work.

Travis 07-01-11 02:23 PM

Is your 1500W inverter one of those 'soft start' types?

MetroMPG 07-01-11 02:34 PM

Not sure.

I do know it's not a "pure" (sine) wave type (so that makes it a square wave style?). There is a warning in the manual about it not playing nicely with certain motor types.

All I can say is so far it's been OK with these hand tools: a skillsaw; 2 different drills (hand & bench); electric chainsaw; angle grinder; dremel; shop vac.

EDIT: when I tried the other two mowers, it triggered the inverter's overcurrent warning & auto shut-off. One of them almost started up (spun, but didn't reach cruising speed before the breaker tripped). So that's why I assumed it was a starting current issue.

MetroMPG 07-02-11 08:39 PM

3 Attachment(s)
Mower fleet update!

1) Mower #2 pics (the "before" ones):

It's also a Black and Decker (this one made in China), model LM115, 7A rating.

After working on the handle today, I think the problem was someone backed into it with a car, and bent the crap out of it. Looks like they made several attempts to straighten it, and that's where at least some of the metal fatigue came from.

Yes, it's actually duct taped. And, yes, those are hose clamps holding the handle on. There was also some tin tying the two sides together.

Metal failure at a weak spot.

2) Welded - this AM, I patched up the handle with the little welder and metal from the junk bike pile (head tubes). Just need to bolt it back together now, and touch up with a bit of paint.

3) Inverter test - plugged this one in (briefly) on the solar inverter at the shack and... success! It starts and runs! If it doesn't piss rain tomorrow AM as forecast, I'll try it out.

4) More batts for mower #1? I may have another source for used gel cell batteries: my brother knows someone who works for a company that sells/services those emergency lights (for power failures, eg. in apartment stair wells). He'll ask the guy what they do with the ones they swap out.

MetroMPG 07-04-11 10:12 AM

Mower #2 success!

I mowed the lawn with no problems using the inverter. It was even a somewhat heavy (dewy) cut.

Glad this mower came available while I'm waiting to solve the battery situation for Mower #1.

When I was finished cutting, my shack next-door neighbour asked, "how'd you power that mower?" I pointed at the sky and he laughed.

Daox 07-05-11 08:17 AM

Haha, very clever. Glad it worked out for ya. Whats the plan once you get the other one running?

MetroMPG 07-05-11 09:26 AM

One mower for the shack, the other for the mansion!

Daox 07-05-11 09:59 AM

But, you already have another plug in mower, right? Or is that one yours?

MetroMPG 07-05-11 10:16 AM

I do, at the mansion. But it's got a 12A motor (power hog :D ), and it's pretty heavy for an electric. So it'll go into the backup fleet.

MetroMPG 07-13-11 01:07 PM

So, one of my battery sources says he's coming through for me with 5 batteries in the near future: three are 7 Ah bricks, two are 12 Ah. All are supposedly in decent shape.

I'll test them for capacity when I get 'em.


I've mowed the shack grass twice now with Mower #2 -- the small 7 amp, 110v one using the solar setup. Works fine.

MetroMPG 07-13-11 04:23 PM

1 Attachment(s)
Batteries! (Used batteries.)

The 2 on the left (12 volt, 12 Amp hour) read 13.1v each. That's a good sign.

The 3 on the right (12v, 7.2 Ah) are a dog's breakfast: one dead @ 0.0 v, one at 13.1 and the other at 12.6.

Let the capacity testing begin!

If the pair of 12 Ah ones are close in capacity, I think that's what I'm going to start using in the mower. They'd make 24 Ah total (rated) capacity vs. 28 Ah from the OEM battery.

MetroMPG 07-13-11 08:37 PM

Capacity testing, part 1...

So far it looks like all the smaller 7.2 Ah batteries are probably duds.

I tested the "best" one first (the one with the starting/resting voltage of 13.1), using a 40 watt, 120 volt incandescant light on an inverter. It pulls 3.7A from the battery at the start (when batt voltage is high), but it goes as high as 4.15A as voltage falls close to 11.0. The inverter shuts off automatically when the battery gets below 11.0v. So let's call it a 4 amp load.

Note that different inverters vary on their shut off points. And some capacity tests go as low as 10.5 volts to evaluate a 12v battery - eg. How to Measure Capacity – Battery University

The "best" 7.2 Ah batt could only deliver 4 amps for 7 minutes before the inverter automatically shut off when the battery slid down to 11.0 volts.


The 12 Amp hour batteries look more promising. I've been pulling 4 amps from one of the pair for 45 minutes now, and voltage is at 12.06 and falling reasonably slowly.

EDIT: pulled 4 amps for 53 minutes before the inverter shut things down @ 11.0v. Obviously voltage falls much faster at the end of the discharge - but you probably already knew that!


It'll be interesting to see how the mower treats the batteries: what amperage the motor pulls under normal conditions, and at what voltage it shuts things down to protect the batteries (assuming it has an automatic shut off).

MetroMPG 07-13-11 09:47 PM

Capacity testing, part 2...

Results of the other 12 Ah rated battery:

- pulled 4 amps for 57 minutes before the inverter shut things down @ 11.0v.

Since the first 12 Ah battery went for 53 minutes, that means I've got a reasonably well-matched pair and can use them together in parallel without one running out of juice significantly faster than the other.

I'm not sure what the mower motor pulls, but I have a feeling these two are going to be able to do the job.

Next step: recharge & install the batts in the mower, then do a test-mowing!

(That'll be next week, because I mowed with the 12A plug-in beast today. And the grass isn't growing terribly fast these days.)

Daox 07-13-11 10:46 PM

Nice work! Did you top off all the batteries before discharging them?

MetroMPG 07-14-11 07:12 AM

Nope! The three batteries I tried had resting voltages of 13.1 when I got them, so I assumed they were fully charged and just stuck them on the inverter.

(They came out of a system this week that was keeping them charged up.)

I realize a better test would be to partially discharge, then fully charge the batteries and let them rest for a day before doing any testing.

I was impatient. :P

MetroMPG 07-14-11 01:57 PM

Capacity testing, part 3...

Did another one of the 7.2 Ah rated batteries, and this one's good!

- pulled 4 amps for 45 minutes (!) before the inverter shut things down @ 10.9 v (why not a 11.0v cut-off like the others, I'm not sure - maybe a function of how "stiff" the battery was - voltage dropped much more slowly & steadily than any of the other 3).

Compare to 4 amps for 7 minutes, which was what I got from the other 7.2 Ah batt. (The one I thought was probably best, based on its higher 13.1 resting voltage when I got them.)

So, as a proportion of rated capacity, this one's the healthiest of the four batteries tested so far.

Unfortunately, one good 7.2 Ah battery does me no good. Need 3 or 4 of these to put in the mower. (Or 3 to put in the electric car... or...)

Piwoslaw 07-14-11 02:24 PM


Originally Posted by MetroMPG (Post 14575)
Unfortunately, one good 7.2 Ah battery does me no good. Need 3 or 4 of these to put in the mower. (Or 3 to put in the electric car... or...)

If all of these batteries are 12V then why not add this one the two 12Ah batts in parallel? Then you'd have 12+12+7.2=31.2Ah, which is more than the OEM 28Ah.

And the discharge times aren't that different. How long would you be mowing with this mower, anyway? 30-40 minutes?

MetroMPG 07-15-11 05:14 PM

Not exactly sure what my mowing time would be (minus dancing with the extension cord, that is) - but definitely no more than 30 min.

"the discharge times aren't that different."

True, but we also don't yet know the current draw of the mower's motor. The capacity test was with a 4A draw. That's a big wild card.

Also, if we do exceed the run time of the battery with the lower capacity, the mower will probably murder it before it does an auto shut-off.

I've murdered a few batteries in the forkenswift this way. The healthy batteries will continue to work after the weak one has given up the ghost, so it masks the damage you're doing -- unless you actively monitor the voltage of the weakest batt(s).

Anyway, we'll have our answer next week!

MetroMPG 07-19-11 01:30 PM

Battery murderer!

So, a few mysteries solved:

  • First, the motor pulls about 9-10 amps once up to speed - that's just spinning the blade (not cutting grass).
  • Second, and most suprising, the mower doesn't seem to have a low voltage cut-off!

I hooked up one of the 12 volt, 7.2 Ah batteries, and when I started the mower, it initially dragged it down to 4.8 volts, pulling ~20 amps!

Of course, as the blade speeds up over the course of 2-3 seconds and RPM stabilizes, current drops, and battery voltage rises. But the voltage only recovered to 9.5 volts @ ~10 amps, which is way too low to safely use a small lead acid battery (risk of cell reversal = battery murder).

Of course, with a larger pack powering the motor, voltage sag will be much less.

I'm trying to track down an owners manual. Maybe it'll shed some light on the subject of how owners were supposed to know when to shut it down for the health of the battery!

MetroMPG 07-19-11 01:33 PM

Oh, and some good news: the two 12 Ah batteries will fit under the hood of the mower with some slight modifications (need to trim back some plastic bits in the housing).

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