Easiest battery lawn mower to convert to LiFePO?

[Samson]

MADE IN CHINAAA hehe

I guess these brushed motors are good since theres no need for a controller... If only there was some kind of circuitry that would limit the current spike on start up... how bout a to stage switch on start up, where the first stage has a big resistor in line, allowing the motor to get spinning, and then you switch it to the second position "on" which is go! (no resistor)

This unit is assembled in the USA according to the labels, no doubt many of the parts are from Asia.
Probably no real need to limit start up current as long as you get the motor spinning before you attack the grass. The battery internal resistance, brush resistance and wiring resistance as well as the 40 amp fuse and circuit breaker adds a few tens of milliohms to limit the start current some.

 

[wb9k]

I have just acquired a Troy bilt model 188, 19" electric cordless mower. It was sold cheap as the 2-3 year old 9 AmpHr Lead acid batteries were done. What I like about it is it uses four 12V batteries in series to give 48 volts same as the pack(s) I use for my Ebike. I have an A123 pack with the 20 AmpHr cells that should work well with it. The motor label says 48 V at 15 Amps. It has a circuit breaker but no controller as far as I can tell.

Let us know how it goes. I bet it'll scream!

dh

 

[wb9k]

Well, I did a brief test of the 8S8P 26650 setup this morning and it fared better than I expected. The additional series element seems to have effectively cancelled out the effects of the additional impedance in the smaller, more aged stack. This was with the 26650's at ~40% SOC. Basically, I saw no discernable difference in performance between these repurposed automotive cells in 8S8P and a brand new module of 20Ah pouches in 7S3P. Capacity for the 26650 pack when brand new is 18.2 Ah. I balanced and ran a capacity test (pulling 30A continuous down to 16V pack voltage--not a standard capacity test; more of a "worst case use cycle" test.) SOH across the pack is pretty even and capacity is right around 12 Ah. A standard 1C test would have yielded a higher number.

I was encouraged enough to think an 8S4P pack might give decent performance, but probably not enough run time to cut a yard quite my size. I'll try it some time, but first I want to use this new pack to cut the whole yard. It'll be ready by the weekend.

dh

 

[dogman dan]

Re brushed motors and voltage spikes.

You could easily put a controller on one of these mowers. But it would have cost em an extra 20 bucks. Buying retail, you'd still need one able to do 40 amps or so, then just rig it to run wot.

I just never bothered to, since once I switched to lipo, the batteries can take it.

The big spikes aren't starting the mower. The blade is very light, and spins up very nicely. Not like getting a lard ass on an ebike moving. But when you hit the tall thick grass, the mower can bog down and stall the motor. Then, with no controller, the amp spike is only limited by how fast you can yank it back.

Mine can stall pretty easy at 6s lipo, but on 7s it really does take a lot to make it spike. Just run the B&D on 28v vs 24v top of charge, and it will do fine. Like with hubmotors, I bet the amp spikes are less on 36 or 48v, than on 24v. I keep looking for that nearly free 36v mower.

 

[wb9k]

Well, I tried a couple of smaller battery packs out yesterday. I used an 8S4P pack to start with, and it cut about 80% of my yard before the lowest cell hit ~2.5V. At that point, the mower was noticeably slowing down. These were used cells, with about 2/3 of their original capacity, about 6 Ah for the pack. I dropped my 8S8P pack in to finish and will probably be sticking with that form factor if I build any more packs for this mower. That will be plenty to cut the whole yard, even tall and a little wet. The 6Ah pack is VERY light, and would be great for somebody with a 1/4 acre or so.

The only gas-operated piece of home equipment I have left is a generator. Good riddance!

dh

 

[etriker]

For sure !

Electric lawn mowers are nice and quiet and so easy to start !

I have an 8s 6p for mine and I run out of power before they do.

 

[wb9k]

Were I using brand new cells, the 4P would have been adequate in grass that's not too tall or wet. Not sure if it would have cut the whole yard if it were really tall. 6P would probably be enough for me on most days, even with used cells.

dh

 

[Gab]

how about this mower as it already comes with a lithium battery of descent size ?


http://www.amazon.com/GreenWorks-25242-Amp-Hour-Lithium-Ion-Cordless/dp/B0074EBU9U

 

[dogman dan]

The mower should be ok, but the battery appears to be a 4 ah size. So you might need a couple spares to cut a big lawn. I'm assuming that it uses the same battery as the greenworks system uses in the other tools.

It might beat buying one with lead, then throwing out brand new lead.

 

[dogman dan]

Well, maybe 4 ah is big enough after all. I just mowed my lawn with 3.9 ah. It was very tall and thick, since I was being sick again last week. Almost two weeks between the cuts. I ran 7s lipo, with a starting voltage of 29 volts. 4 ah of 40v would of course be more watt hours.

Here is a pic of the lawn, about 3/4 of it in the picture.

And a pic of the mower itself. Older model with non removable battery. I just chopped a hole, and put a box in there for the battery. Originally made the right size for two NiCad packs. This is year 4 for the mower, and year 3 for the lipo I run it on.

Thick lawn today, so some interesting numbers from the wattmeter.

3.9 ah used to mow the smallish, but overgrown lawn.

26.7v ending voltage

20.7v minimum voltage. That's some healthy sag. The battery is old as hell though.

134 amps maximum. At one point I did put a full stall on the mower in the thick grass, almost bringing the blade to full stop. But it pulled 80 amps plenty along the way as I was mowing along.

2774 watts peak. Again, that moment of full stall on the blade rpm. 1200w -1400w was being drawn continuous.

101 watt hours used.

Boy, those spikes, easy to see how no controller on this B&D mower makes it just a tad harsh on the sla's they come with.

29v was much better in the thick grass than 25v has been in the past. No motor overheats, even with those stalls. It ran at 1400w with no signs of problems. I should have stopped running 6s and gone to 7s RC lipo years ago.

 

[RedLine19K]

interesting thread. lifepo4 has the reputation for durability and surviving abuse but lipo seems to hold up well.

I'm wondering, instead of heavy metal blade, why isn't string used? It would basically prevent stall spikes, and with the torque of electric, recovery would be instant. Spinning a 3lb metal blade back up after a bog takes a lot more energy and string basically self feeds through heavy cutting without losing rpms. There are ICE heavy-duty string trimmers already, so the hardware exists but they seem intended for weed cutting and have minimum height issues - not intended for lawns. Maybe the hardware could be adapted. One concern is how it would clear clippings, and another is how much cooling the metal blade/moving air. Line replacement is always a hassle, but there are solutions
http://www.alexbrushcutterheads.com/turbo3.html
http://www.amazon.com/Cyclone-155-Inch-Commercial-Fluorescent-CY155T18-24-24/dp/B001NMT2JY/ref=pd_sim_lg_19

Has anyone tried string? if it works on gas, it would actually be better on electric, at higher rpms

 

[wb9k]

I don't think you would have mulching, bagging, or side ejection capability with string. Would likely leave lots of long clippings on the yard and have a very messy appearance. It would work, but most people would probably not use it.

dh

 

[dogman dan]

Yep. For what it's worth, my battery mower has the thinnest lightest rotary mower blade I ever saw in my life. (which incudes about 20 years of professional mowing) It couldn't be a much lighter blade. I just sharpened mine too, so that wasn't the problem. It might be that it stalls easy because it's light. There sure is no flywheel effect in that tiny motor.

Over tall common Bermuda grass is some thick stuff. Somewhat like mowing a lawn made of baling wire. I have the mower set to mulch, so it does stall easier than bagging would be. Lotta hay spinning around in there today. But the stalling problem is much much less at 29v. So for sure, if using RC lipo, 7s is the magic cell count for the 24v B&D's.

 

[Samson]

The 48 Volt Troy mower I acquired recently has 4 x 12V 9 AmpHr lead Acid sealed batteries. My intent was to replace the Lead acid with the spare A123 pack for my bike but I thought I would test the Lead Acid first. A discharge test at 10 Amp load showed Battery #1 to be weak, Battery #2 to heat up to 45 deg C near the red post and be down on volts some. #3 and #4 had about 2\3 new capacity. The other Batteries stayed a bit above ambient temperature during the discharge test.
I did a balance charge for several days, 15V at 10-20 mA on Batterys 1 and 2.

Tried to cut the lawn but the circuit breaker kept popping though the load seemed light, I confirmed the Amps were low with an inline ammeter so replaced the 20 Amp circuit breaker with a 20 Amp fuse which solved the problem. I have ordered a new circuit breaker.

I had also checked the LED battery charge indicator prior to see how it was scaled.

While cutting long wet grass the voltage indicated about 42 Volts after 3 minutes (due mainly to Battery #2s problem) but I was able to cut about 3000 sq feet before running below 40 volts. I then checked the motor temperature with an IR thermometer and got only about 32 deg C on the case and 64 deg C at the commutator, quit cool!
Three batteries read 11.79 volts at no load with #2 at 10.4.

The life of the VRLA (valve-regulated lead–acid battery) battery is not acceptable, the charge time at 1-2 Amp (max rating) is to long and the battery weight makes the mower heavier than a gas mower!

Locally here in Canada they seem to be quite expensive.

I think with a full (no sag) 48 Volts from the 20 AmpHr A123 Lithium pack, the mower will cut moderately high dry grass fine and have a range of about half an acre.

I also found as Dogman mentioned the blade is much lighter than on a gas mower and uses a crude plastic slip clutch that is very delicate. The motor vibrated terribly at first until I straightened the clutch tabs which probably became bent when the blade hit something solid. This caused the blade to be offset about 1\4 inch, hence the vibration. It is a good idea to check the blade run-out once in awhile.

All in all I think this design may be adequate for a small level law but the same size gas mower is about 1\3 the price locally. I doubt they will sell many, especially with the poor battery life.

 

[dogman dan]

Yeah, for shit lead, the cost is way too high. WAY WAY too high for the cost of the motor in my mower. It's not a big powerful motor that cost them much.

You have to find some kind of a deal on these things, buy them used, or like I did, get the floor model from last years display at half price. When I bought mine, It had no power supply to run the on board charger, and was missing the on off safety key. They wanted $300 for it, vs $350 full price.

I told the guy there was one guy in a 200 mile radius of the store with the skills to make that thing work without spending hundreds on replacing the overpriced replacement parts. Explained that the lead batteries had a scrap value of 4 bucks apiece at most. I offered them $200, and in retrospect, should have paid less.

But paid too much or not, it was unused when I got mine, so it's lasted fine and looks like it will last me another 4 years easy. The lead sucked for me, too heavy to push, too slow to charge, and I had to charge it in 100f temperatures, or bring the mower into the living room. 2009 models had the removable battery box, but mine was built in. Once I went to lipo on the thing, it's been a great mower, so light and easy to push. That's what I have really enjoyed, the light plastic body, combined with less than 5 pounds of battery. I've pushed snapper mowers professionally, and those are like pushing shopping carts full of bricks. Much better to push a mower that weighs nearly nothing.

 

[wb9k]

All in all I think this design may be adequate for a small level law but the same size gas mower is about 1\3 the price locally. I doubt they will sell many, especially with the poor battery life.

Well now, that sounds familiar...just change "mower" to "EV"...

I think the biggest thing holding these things back in terms of price is economies of scale. I can't imagine it costs more to make those little brushed motors and a LA battery than it does to make a 3.5 HP Briggs and Stratton ICE. While the mowers aren't cheap, a lot of gas mowers are priced similarly, though you can get a very basic gas mower for under $150. This really is almost an identical conversation to whether EV's can be viable for most people. The answer is yes, but not without some real growing pains to get there. This won't happen at the flip of a switch.

dh

 

[Samson]

I replaced the bad #2 Lead Acid Battery with a battery made from four used Headway cells. It allowed me to finish the tall wet lawn before the Lead Acid Batteries died. I will have to charge the Batteries separately, a pain until I replace the Batteries with lithium. This mower is heavy, it has a steel deck and elaborate wheel height adjust mechanism. I read somewhere it is over 100Lbs vs 80 for a gas mower. The Lead acid Batteries contribute 25 Lbs. My lawn has steep hills I may try to reduce the weight further from the 15 Lbs the A123 pack would give me. Might be time to consider an 18650 pack.

 

[dogman dan]

Lipo it. 14s 5 ah of lipo is only about 4.5 pounds.

The heavy deck will last forever and then some. But bummer if it's really heavy. I used to loath those snappers I pushed around for years. They were twice as heavy as homeowner grade mowers.

My plastic deck will not last forever, but for now it's super light. When it dies, if the motor lives I'll just look to mount the motor into an old gas mower deck.

 

[67spyder]

Has anyone used a BMS in converting one of these?

I have a yardworks (cheap crap) that I bought for $25 with dead SLA batteries. I happened to have a pair of the exact batteries laying around from rebuilding UPS's so I put them in but that was 4 years ago and I expect these will be toast soon.

I have an almost unlimited supply of Sony 18650's from makita drill packs and was thinking of a 7s15p pack but it has to be plug and play as I don't mow the lawn. If you make sure the lawnmower works perfectly it is easier to get someone else to mow the lawn :lol:

 

[Samson]

Lipo it. 14s 5 ah of lipo is only about 4.5 pounds.

The heavy deck will last forever and then some. But bummer if it's really heavy. I used to loath those snappers I pushed around for years. They were twice as heavy as homeowner grade mowers.

My plastic deck will not last forever, but for now it's super light. When it dies, if the motor lives I'll just look to mount the motor into an old gas mower deck.

I ordered some Lipo yesterday. Building a 18650 pack is time consuming for only a small weight benefit over the Lipo. I usually mow the lawn in three stages a day apart, so I will try to get by with 5 AmpHr 12S. If I need a few more volts I will use 1 or 2 Headways in series. The Lipo will reduce the weight by 21.5 Lbs and charge in 20 minutes or so using the ebike charger at 12-15 Amps compared to 6-14 hours now . My only concern is the need to make sure the Lipo does not burn the garage down. Perhaps a metal battery tray with fiberglass lining will replace the original plastic tray.

I have bulk charged 3 times with the Headways in series with the Lead Acid. The individual Battery voltages are all correct. The 1 amp charger for the mower backs off to a float charge and seems to work fine so far.

Now if I can only avoid hitting the street water shut off with the mower blade!

 

[999zip999]

How many sla's and how many headways to what voltage. Buck charging to what voltage ?

 

[Samson]

How many sla's and how many headways to what voltage. Buck charging to what voltage ?

3 SLAs (12V.5 nominal) and 4 Headways (13.3 nominal). Charges to about 14.5V on the SLAs and 3.33V on the Headways 56 V total before going to float charge of 13.5V on the SLAs each. This is not a permanent solution, it will keep me going until the LiPo arrive.

 

[dogman dan]

I think you will love the lipo once it comes. Every pound you take off that mower will be priceless. A smaller pack can fit in the charging bags they sell.

RE the bms question,,,

In my case, with the brushed motor and no controller on the 24v black and decker, it would take one hell of a bms to take amps spikes that big. But if you were careful and not waiting too long to mow, you could mow without popping a 60 amps bms. Plug in the battery and mow is easy enough for whoever you hire. All you have to do is charge the battery for the guy doing the mowing. I don't see why it wouldn't work. Just get a high rate bms and he won't likely pop it. Just make sure he understands that if the blade does really slow down, he has to back up a bit.

 

[67spyder]

so far I have always used BMS from Paul at emissions-free.com, would drawing more than the rated current damage the BMS or would it just limit the current to 60A or whatever it is rated at? OR is this a question for Paul?

 

[dogman dan]

Typically a bms will trip and shut off if amps exceeds a certain set limit for long enough.

I was seeing about 1,400w continuous when I did that mow the other day, when I waited too long to do it. Thin grass uses more like 1000w. At 29v, that's 48 amps in thick stuff. So it should be quite possible to mow normally at less than 60 amps. Those higher amp spikes are very short unless you keep jamming the mower in. So mow careful and a 60 amps bms should be fine. But not with 10 ah of 2c cells. That would only be good for 20 amps.

When I was using two NiCad packs parallel, I was using 30 amps fuses in the discharge wires, and never blew one. But if I tried to use just one pack it blew the fuse instantly. So 60 amps at 24v is enough.

If you have a higher voltage, you get the required 1000-1400w at much less amps of course. Why they chose 24v for the early mowers beats me. 48v would have worked better at half the amps for 1000w or so.