Need help sizing an electric lawn mower motor/battery pack

[TylerDurden]

I like the 24V CMM1200. It is a bit tall, but the one handed cutting height-adjust is nice. (I mulch, not bag.)

http://phoenix.craigslist.org/evl/grd/2381133016.html

 

[dogman dan]

Thats' what I have. Nice now that I diched the lead. Easier to charge too, since I don't have to bring the whole mower to the plug.

 

[x88x]

Heheh, so, not exactly on the topic of lawnmowers, but in the theme of 'lipo; not just for toys', I did this today and thought someone might enjoy it.

Runs a lot stronger than on the normal NiCad pack (which was dead), but I need a more reliable connection. Those alligator clips don't stay on either connection very well...

Oh, and while I'm posting pictures, a bit more on topic, here's my mower with the LiPo pack.

 

[auraslip]

ooooh weee - enough free room in that thing for more batteries and a charger!

 

[x88x]

Well, I'm not going to be integrating a charger partly because I don't have electricity in the shed I keep the mower in, but more importantly because I'm planning on also getting a 24V weed whacker and hopefully a chainsaw and maybe some other tools, and putting together a compact pack that I can use for all of them. What I'm planning on is getting three 2S packs and putting them together with these three 6S packs (or maybe a different set of three 6S packs if I have my bike working by then) and putting together a circuit that, when I flip a switch, will dynamically reconfigure the pack from a 6S4P to a 8S3P. This just happens to be one of those sweet spots where I can actually do the switch without wasting any cells in either mode. I'd post the circuit diagram, but I can't think of an easy way to translate it to a digital version right now...and I don't feel like learning Eagle right now.

 

[x88x]

Ok, I had more caffeine so I got the energy to try my hand at figuring out Eagle (been up since 9:30 last night and been doing lots of physical labor today). I'm not sure I'm really using it right...and not being able to make or easily find exactly the part I wanted (like, say, a 6S battery symbol) is annoying...as is it not automatically putting in non-intersection notation for the leads that seem to cross in the schematic... -_^ So instead of going through the much longer and fiddlier process of manually making the notation, I just specified all the junctions. Not that way I would have liked to do it, and maybe there is a way to make it do it, idk. Anyways, here's my first rough draft of the circuit. Oh, also, I can't figure out how to modify the labels... :x

The socket at the top left is the pack +, the socket at the bottom left is the pack -, batteries 1-3 are 6S, and batteries 4-6 are 2S. Everything else should be self-explanatory.
Normally it sits at a 6S4P config but when the switch is closed it reconfigures itself into an 8S3P config. This is sort of a primitive but cheap (I priced out 30A rated components to ~$30 new or ~$20 using 25A surplus parts), durable, and simple 2-speed throttle to let me boost the speed of the blade if I hit a really thick/tall patch.

 

[dogman dan]

Looks cool. Exactly where I'm headed with tools too. I have two 24v drills, and I garantee there will be no more crappy nicad packs for em. I'm thinking either take the old nicads apart and cram in HK lipo, or convert to Makita cells from the doc. Another way would be a quick and easy to use plug on the tool, and a cord that runs down your sleeve to a fanny pack battery. That's how I do the weedwhacker now. I have two old 9v makita drills too, I bet the old battery cases on those would hold a lipo pack nice.

Looking for a 24v chainsaw for light pruning.

 

[auraslip]

ack - I broke the pull cord on my cheap hand me down leaf blower. It's so poorly made - I had to remove every screw on the damn thing to get to it - and know it won't go back together. Looks like I may be upgrading to battery powered weed eaters and blowers sooner than I thought!

Ideally I'd like a battery powered trimmer with swappable parts - a trimmer head, an edger, and a blower. BUT they don't make this. They don't even make a battery powered edger at ALL. They need too much power apparently.

This kinda got me thinking - maybe it'd be better to use standard 110v equipment and just stack enough batteries up to make that work. But golly, that is like 5x 6s packs!

 

[auraslip]

So here is an 18v leaf blower for $40 sans battery.

The question is, will 6s lipo destroy it?

 

[wineboyrider]

I have this cyclone motor laying around and was thinking at 36v it would make a great lawn mower motor????
8)

 

[swbluto]

This kinda got me thinking - maybe it'd be better to use standard 110v equipment and just stack enough batteries up to make that work. But golly, that is like 5x 6s packs!

Yeah, I was thinking kind of the same thing. The battery powered equipment is just plain weak compared to the corded models (The corded models start to rival gas power) and upgrading battery-powered lawn-mowers would seem ... difficult while also retaining longevity (I think it'd be hard to install a regular hobby-city outrunner into a lawn mower for a non-machinist like myself and upgrading the voltage on brushed motors just shortens the motor's lifespan.), so I was thinking about hooking the batteries upto an inverter to run the AC motor. Btw, I think you'd need to use an AC signal to run the motor because it is assumably an AC motor which requires an AC signal.

And, I was sizing up inverters, but I found that the only "appropriate ones" weighed like 8 pounds and cost ~$200. Of course, by appropriate, I'm probably just looking at the ridiculously oversized ones as I'm not exactly sure how to do the AC calculations and I'm staying on the "safe side".

For the calculations, I'm using my lawn-mower's specs as it's the worst out of all the electric power equipment. It claims to be 12-amps and I'm assuming that's the peak? Or maybe that's just the motor's continuous rating...

Anyways, using 12 amps and 110 VAC, the VA is 12*110 = 1320. Plugging that number into the calculator at http://www.generatorguide.net/watt-acpower.html and using .8 for the PFC (worst case value resulting in the highest wattage for a "motor driven application" which typically ranges from .6-.8), I get 1056 watts. I'm guessing that this $100 "1500W continuous" inverter at http://www.amazon.com/Cobra-CPI-1575-Power-Inverter/dp/B00126IDDC/ref=pd_cp_e_1 would probably work? Anybody have any insight?


EDIT: I stand corrected (I asked this same question an electronics forum and they corrected me). It's true that induction motors won't work on DC, but these tools use the cheapest motors available which are the series wound or permanent magnet with a bridge rectifier, which both types run on DC. So, yay, no need for an inverter! (I just tried out my corded lawn mower using a power supply and it spun.)

 

[auraslip]

I called worx because I saw this blower made for 18v lithium for only $42.

The worx tech/parts guy said he regularly runs his 18v tools with 24v packs. I'm not sure if the battery packs are compatible, or if he was just frocking with me.

I know it's not a full strength blower like the old gas one I had, but all I really need to do is shoot some grass clippings into the street for few minutes. At 24v it should have a bit more power too.

Anyways. Fire away credit card.

 

[dogman dan]

Just like we overvolt our hubbies some with no problem, I'm confident that you can safely overvolt your corless saws, drills, blowers, etc. I'd say for a blower, which won't experience spikes cutting something hard, 10s might be just the thing.

I found overvolting the mower too much made the rocks, pine cones, whatever fly out of the blade shroud awfully fast, so I'm sticking to 24v on the mower unless the grass is just crazy thick.

Just running my string trimmer on a rock solid 24v caused some problems with it. It has automatic string advance, and it blew it's string real fast with no voltage sag. So I think I'll go with 5s for that tool.

Looks like cutting my entire yard is taking just about 4 ah of 24v. Interesting that the same mow could not be done with one charge up of the original 18ah sla batteries last summer. And now the mower is about 30 pounds lighter.

 

[auraslip]

hehehehe

this electric lawn tool stuff is pretty cool. I'm amazed by how poor the battery tech is that they use. do they not realize their is better stuff out there? or do they not care?

24v string trimmers are pretty cheap, so I probably won't build my own this year....

 

[x88x]

this electric lawn tool stuff is pretty cool. I'm amazed by how poor the battery tech is that they use. do they not realize their is better stuff out there? or do they not care?

I'm gonna say they just don't care. SLA makes for the simplest, cheapest charging system, the cheapest cost/Wh for the manufacturer, and it keeps customers coming back and buying replacement batteries. Plus, every company I've seen that makes electric yard tools also makes ICE yard tools, so pushing the range envelope probably isn't as important to them. By my best estimates (made a bit more difficult since I can't charge my batteries past ~50% atm), it takes about 50Ah at 6S to do my yard. So, nobody currently makes a mower with that capacity, but tbh I don't really care...I don't really want to spend probably ~4 hours straight pushing a mower around my yard anyways.

 

[swbluto]

this electric lawn tool stuff is pretty cool. I'm amazed by how poor the battery tech is that they use. do they not realize their is better stuff out there? or do they not care?

I'm gonna say they just don't care. SLA makes for the simplest, cheapest charging system, the cheapest cost/Wh for the manufacturer, and it keeps customers coming back and buying replacement batteries. Plus, every company I've seen that makes electric yard tools also makes ICE yard tools, so pushing the range envelope probably isn't as important to them. By my best estimates (made a bit more difficult since I can't charge my batteries past ~50% atm), it takes about 50Ah at 6S to do my yard. So, nobody currently makes a mower with that capacity, but tbh I don't really care...I don't really want to spend probably ~4 hours straight pushing a mower around my yard anyways.

Yep. They really care about the lowest manufacturing cost to keep the price to the consumer low so that they won't immediately run to their competitors due to high prices. To get an equivalent battery system in a lawn mower requires 300-400 wh, which gets pricey with other chemistries (An extra $10 is 'pricy' to companies because of the quantity of lost sales due to higher prices.).

 

[x88x]

Not entirely on-topic, but since I mentioned it earlier.. I have to say, I'm not exactly impressed with EP Buddy's customer service. On the one hand, yes, they responded very quickly when I emailed them, but when I sent them the unit...which got there on May 21st...I heard nothing from them until I sent them another email a couple days ago asking (in more delicate wording) wtf was going on. I got a response nice and quick, with a shipping number for a replacement that shipped out yesterday, but I shouldn't have had to contact them again in the first place. I could be wrong (and from their reputation, I hope I am), but it seemed to me almost like they would have been perfectly happy ignoring my case and hoping I forgot about it. -_^ 13 business days it sits there, and as far as I can tell nothing happened until I contacted them. It's especially annoying since in my initial contact they said they would ship out a replacement as soon as I shipped the faulty unit...I notified them when I did that (on May 18th), including a tracking number, and obviously that didn't happen. I'm sorry, but this is not acceptable behavior for a business, and after this I can't say that I would feel comfortable giving them my business in the future. I have RMA'd a lot of equipment over the years to many different companies, large and small, and I have never seen this complete lack of voluntary communication....neither I have never seen an RMA take this long.

 

[veloman]

Thanks for the field report 8)

This one is surprisingly simple. And light!

I was staring at my 20" Scott's reel mower tonight, trying to think of a good way to add e-power to it. That damn thing is so efficient, it plows through thick 8" tall Bermuda grass with what I'd estimate about 400watts of human power (me pushing it). A rotary would likely draw over 2000watts at that speed. I do have to cross the grass 3-4 times to get a reasonably clean cut though.

This pics really helps, I may try that.

 

[x88x]

Have you sharpened the blades on your rotary recently, veloman? That does make me wonder though...there are a bunch of rotary push mowers currently for free around my area on Craigslist...might pick one up and see about converting it. I do want to make an autonomous mower eventually; it would be neat to make it with one (or maybe two) of those....and a sweeper.

 

[dogman dan]

I have the 24v B&D battery mower. It's not all that powerfull compared to a very similar mower in the same brand, that plugs in. With the original SLA's it did not perform for very long either. I now run the battery mower on lipo batteries, and it has enough power to mow my lawn good enogh. I do have to cut twice if it's very tall though. One pass set tall, then another at the lowest setting.

Though definitely not as powerfull as a 110v corded mower, or a 3.5 hp gas, it is good enough once you ditch the crappy sla batteries. Using 6s lipo, with three bricks of it paralell connected, I have 15 ah of solid battery that doesn't sag when I hit the thick stuff. Occasionally the mower may shut off when mowing hard, when a thermal sensor trips in the motor. So if that happens it's time to raise the mower a notch, and mow taller.

I suspect that the 36v models common now may perform better. They may be less likely to slow down so much in the tall grass.

 

[Pure]

I have a 24 volt homelite electric mower. It's powerful enough to mow your yard without any trouble as long as you don't go too long between mows. The SLa has enough capacity to do the average quarter acre lot. If the grass is too high it will bog down and you have to do half passes. I agree with Dog that the new 36 volt models that are out now would probably be much better. I would still rip out the SLA and put in some lipo ASAP. SLA is just too much work with always having to top it off on the charger and not to mention how much it weighs. Pushing my 24 volt is enough of a PITA. I wouldn't want to push one of the 36 volt models.

 

[x88x]

I am interested in buying a battery powered lawn mower but I'm not sure if they are as powerful as the electric lawn mowers that you need to have plugged in.

Simple answer...they're not. As with everything, when you make the move from corded to cordless, there are compromises in power for the increased flexibility that you get. I also run my 24V Homelite on 6S 15Ah LiPo, and personally I love it. Switching from the original 18Ah SLA to LiPo gave me both a bit greater range (especially since the SLA pack was mostly dead when I got it) and a much lighter mower. Hell, the SLA pack weighs almost as much as the mower itself does. As far as range, like others have said, if you don't let the grass grow too much range is just fine. I also do two runs if the grass is really high; one at ~3" and the final at ~2", where I like to keep it normally. If I don't let the grass get too high, I can go for about an hour, about 1/4 acre, until the battery drops down to ~3.6V/cell. So, it takes me about 4-5 charges to do my whole yard (~1 acre of grass), but I don't really mind not being able to do more than ~1 hour of mowing at a shot. Plus, with the charger I have (iCharger 106B+) I can charge my pack in ~90 minutes max, so I just mow for an hour, work on other stuff while it charges, mow for an hour, and so on and so forth.

 

[robermax]

I have the 36V Black and Decker this past summer and have been happy with it. Only powered mower I ever had. I have a large yard, about 1.5 acres and there is a very steep hill. In an effort to reduce maintenance we planted a new slow growing grass seed over about 1/3 of the property last year and decided to do the mowing ourselves. We were getting it cut weekly.
The new grass wants to be 3-4 inches high. 1st cut in the spring was from about 6" and my mechanical reel mower couldn't do it. That's why I got the electric mower. It actually worked extremely well. The SLA battery lasts about an hour and it does take 5-6 charges to do the entire yard. The part with the new seed only needs mowing every 3-4 weeks so I'm not under any pressure to do it on a rigid schedule. We are converting more of the yard to the new grass so it should be easier next year.

Downsides are that the mower is heavy at about 75 lbs (part of the yard is a steep hill) and the SLA takes overnight to charge. I am looking into lifepo4 conversion so I can mow longer by switching between the 2 batteries and for the weight. I have been looking through these forums and trying to learn as much as possible to be ready for next year.

Seems the battery is 30 AH (3 x 12V 10AH cells per one poster) and the SLA runs for an hour. It doesn't have to run any longer than that (though it would be nice). One concern I have is safety since I tend to be a "set it and forget it" kind of person.
Anyway I thought I would share my experiences and thank all the helpful members of this forum. I will be studying more over the coming months.

 

[theGoPedal]

I picked up a 24V cordless mower off of Craigslist and upgraded it to Headway LiFePO4. Build thread is here:

http://www.endless-sphere.com/forums/viewtopic.php?f=1&t=31354

I increased the size of the battery from stock (15ah to 20ah) and I've been very happy with the performance so far. I have a small yard so I have plenty of range to do the whole thing on one charge. With the larger battery I haven't noticed much sag, only on the thickest spots. The Headway battery came with a BMS and charger, about as plug and play as you can get. I would definitely do it again.

 

[dogman dan]

Just to update, my mower worked well all summer. Only in the fall did I have the whole yard green though. Really dry summer, and really expensive to water a big yard. Currently the mower now runs on 20ah of 6s lipo. 10 ah is plenty, but I have a 4 brick pack for a bike that I run all paralell for the mower.

I think the 20 ah 6s lipo pack would have enough juice to do at least 4 typical houses, like 1/3 acre lots that aren't 100% lawn, or all of 1/4 acre houses.