Idiot-proofing a 24V lithium pack.

[x88x]

Ok, so a little background. My dad found a couple refurbished Homelite cordless lawnmowers (a UT13222 and a UT13126) this weekend (at a Habitat for Humanity ReStore if anyone's familiar with those) for $50 each, so since he's in the market for a new lawnmower and I want to get an electric one...and since they normally retail for $300-350...I convinced him to pick them up.

The one catch is that neither has a charger. I know, not a big problem. For now, I told him to just open up the battery case, disconnect the batteries (24V20Ah SLA, I'm assuming two 12V 20Ah in series), and use his car battery charger on each separately. Even picking up an official Homelite charger for these would only bring them to ~$100 each, which is still a bargain.

Still though, that SLA is only going to last so long under lawnmower abuse. I'm going to be converting mine to use a 6S LiPo pack (the mowers are 24V) in a removable unit that I can repurpose to run other equipment (that I don't have yet..but I'm going to be getting or converting an electric weedwhacker at least) and just charge it with my iCharger, but for his I want something a bit more idiot-proof (not that he's an idiot, but you know what I mean).

Now, I'll have to do some testing to confirm, but I'm guessing that a 10Ah lithium pack would do just fine for his yard. They have a ~0.75acre wooded lot, but there's not really much grass on it. I'd say maybe ~0.25acre of thin grass, max. So, I'm thinking for his pack, probably 7S1P 10Ah (or 14Ah if 10 looks like it won't be enough) HiPower LiFePO4, for the long cycle life and peace of mind, and since he won't be using the pack anywhere else, the extra bulk doesn't really matter. It should still fit just fine in the old SLA battery pack enclosure. I think he's going to be picking up an official charger for his, so I'm thinking that will probably work fine to bulk-charge the LiFePO4 pack (might need a little tweeking, idk, we'll see once I can examine one instead of just reading online descriptions), but for keeping it balanced, I don't want him to have to worry about having to check anything, so I want to put in a simple BMS.

So now we come to the real question. For a zero-maintenance pack a 2-hour drive away from me if it ever breaks, what would you all recommend for a BMS? For context, I'm perfectly comfortable building my own electronics.

And before someone mentions it, yes, I am aware that Ping makes a 24V pack in 10Ah and 15Ah sizes, but since the HiPower cells come out to ~$100 less for each size (before shipping for both), I'd rather go with those and build the pack myself unless the circuitry to make it bulletproof is prohibitively expensive.

 

[auraslip]

This is very relevant to my interests!


Based on reviews of this machine running stock with a run time of hour I can guess it'd drawing around 12a. How? Because a 20ah SLA will last about an hour at 12a.
So a 10ah lifepo4 pack will cut for an hour of use. Barely. And remember what we always say here? If you can't afford twice the capcity you think you'll need, then at least get 1.5 the capacity you think you'll need! So 15ah lifepo4 sounds good, and at 12a the 1~2 C hipower stuff should work fine.

However, reviews of this mower have said it's a little bit lack luster in the power department. So instead of 7s lifepo4 (22v - under stock) perhaps 8s? I'd be tempted to go higher, but then your drawing lots more current. Less run time, and the possibility of burning out the motor (and then upgrading it ?)

As for the official charger. $100. Yikes. Tell him to just get one on ebay for $20! Same damn thing. Charging to a lower voltage is perfectly fine(good for life cycles too). However, the cells won't balance unless you modify the bms appropriately. This won't be an issue if your only running it every other week. You'll just lose some capacity as the lowest cell shuts off first. You can balance them at Christmas or something. Something like this might work. You might want to test it on you're own rig to be sure though!

What might work for you and I though, is a cell-log with the alarm port tied to a relay to switch the pack on and off for HVC/LVC. Since you only need one cell log, it wouldn't need any fancy stuff. Just a relay, and you could bulk charge for most of the summer and balance when you need too. Super simple. I've been trying to do something similar here. Add a watt-meter and you got a LA - lawn analyst

I've toyed with this idea many times as a way to get my business completely "green." Quieter too. I like the idea of a modular battery. But for weed whackers and blowers it'd almost have to be in a back pack. I need to find away to keep the back pack from getting me hot and sweaty as hell.

One last point - the mower weighs 100lbs. 30lbs which is batteries. A 24v15ah ping weighs only 8lbs. Lipo would be even less. Pretty good upgrade I'd say

 

[auraslip]

http://www.arttec.net/Solar_Mower/
http://www.builditsolar.com/Projects/Vehicles/LeeMower.htm
http://visforvoltage.org/blog/jdh2550_1/2076

The problem with all these is they use a 1.5hp motor. My toro recycler has a 6.5hp motor, and it occasionally bogs down in the thick. I know motors are rate differently with gas and electric. The 1 to 3 rule. Meaning you'd need a 2.3hp electric motor to match the 6.5hp gas motor. Hrm...

 

[x88x]

Based on reviews of this machine running stock with a run time of hour I can guess it'd drawing around 12a. How? Because a 20ah SLA will last about an hour at 12a. So a 10ah lifepo4 pack will cut for an hour of use. Barely. And remember what we always say here? If you can't afford twice the capcity you think you'll need, then at least get 1.5 the capacity you think you'll need! So 15ah lifepo4 sounds good, and at 12a the 1~2 C hipower stuff should work fine.

However, reviews of this mower have said it's a little bit lack luster in the power department. So instead of 7s lifepo4 (22v - under stock) perhaps 8s? I'd be tempted to go higher, but then your drawing lots more current. Less run time, and the possibility of burning out the motor (and then upgrading it ?)

Thanks, I hadn't looked into the power draw yet. That's good to know. And yeah, I've been leaning more towards the 14Ah cells...it takes the cell cost from ~$100 to ~$125, but to give it more headroom and extend the life of the batteries, I think it's worth it. Same with the voltage. I said 7s because I figured the stock SLA probably sags to less than 24V anyways, but yeah, making it more powerful for another $15-20 would definitely be worth it. I haven't actually even seen the things yet, let alone tried them, so I can't comment on their power yet. Thankfully, though, my yard is a lot bigger than theirs and I have much thicker grass (as I've been finding out the last couple days..), so if I test mine on my yard with a pack and it's powerful enough for me it'll be plenty powerful enough for them. I also couldn't find any major differences between the two models, which is good.

As for the official charger. $100. Yikes. Tell him to just get one on ebay for $20! Same damn thing. Charging to a lower voltage is perfectly fine(good for life cycles too). However, the cells won't balance unless you modify the bms appropriately. This won't be an issue if your only running it every other week. You'll just lose some capacity as the lowest cell shuts off first. You can balance them at Christmas or something. Something like this might work. You might want to test it on you're own rig to be sure though!

Actually the official charger was $50. $100 came from that plus what he paid for the mower. He ended up looking around online after I talked to him and ordering a 24V scooter charger from some random retailer for, iirc, ~$25 shipped. Thanks for the link to that BMS. That's sorta what I was thinking of, though more than I was hoping to spend. Well, whatever I end up doing, it probably won't be for a while since I don't have a job atm..so no money to spend on fun projects. Plus, if the battery it came with isn't shot I think I'll see what kind of performance he gets out of that before planning out a new pack for it.

What might work for you and I though, is a cell-log with the alarm port tied to a relay to switch the pack on and off for HVC/LVC. Since you only need one cell log, it wouldn't need any fancy stuff. Just a relay, and you could bulk charge for most of the summer and balance when you need too. Super simple. I've been trying to do something similar here. Add a watt-meter and you got a LA - lawn analyst

Heheh, yup, that was my plan for mine. Just run it off 6s LiPo with an LVC beeper, then charge it with my iCharger. Yet another one of those cases of "well, I'm perfectly happy messing with all this stuff because I know what I'm doing and how to not screw it up...but I wouldn't want to make someone else do it".

I've toyed with this idea many times as a way to get my business completely "green." Quieter too. I like the idea of a modular battery. But for weed whackers and blowers it'd almost have to be in a back pack. I need to find away to keep the back pack from getting me hot and sweaty as hell.

I'm just going to figure a way to latch them on to whatever I'm using. If you think about it, the engine in ICE weed whackers and blowers is probably about the same size and weight as a motor and 24V15Ah of LiPo, so it should fit nicely. Heck, with the weed whacker at least, depending on what all you're doing, you might be able to get away with only putting on a 10Ah or even 5Ah pack. Although, it would be pretty sweet to just have one pack that you carry with you and just plug it into whatever tool you're using.

The problem with all these is they use a 1.5hp motor. My toro recycler has a 6.5hp motor, and it occasionally bogs down in the thick. I know motors are rate differently with gas and electric. The 1 to 3 rule. Meaning you'd need a 2.3hp electric motor to match the 6.5hp gas motor. Hrm...

Yeah, and this one I'm getting, if it does only pull 12A, is only a 288W (0.386HP) motor..oh well, we'll see how it works out. Worst case I'm out $50.

 

[dogman dan]

I don't know how the homelite mowers are built. On my B&D one the sla charger is part of the mower. But it took a 24v power supply to run the charger.

So maybe all you need is a 24v power supply.

As for the motor controller, what controller? It's a brushed motor, and NO CONTROLLER. So what? why would it need one? Really, it doesn't need anything but a good on off switch to run. Ok fine, but what does this mean? It means you hit thick grass and you get a dandy voltage spike! Not real good for a low c rate lifepo4 of small size.

Last summer I did try the 36v 20 ah ping on it, and it ran great. The blade speed was dangerously fast though. Likely to fry the motor, or throw rocks right through the deck of the mower, along with chunks of the blade. I'd think a 24v 15 ah ping would run this mower ok, but you might have to reset the bms when you hit really thick stuff.

Half of last summer I ran the mower on 24v 16 ah of ebikes-ca nicads. That worked ok, but I kept popping the 25 amp fuse if I ran just one pack. With two packs, and 50 amps avalible before a fuse blew, it worked good, but did still tend to bog down on thicker stuff.

This summer, I will run 6s 15 ah lipo. Very light, Perfect blade speed, easy to charge since it's just a big 6s pack. Just pop it on the charger and back to the mower with no rearanging stuff. No thick grass yet, but I don't think it's going to have voltage sag now! 8) With 15 ah, I will never need to worry about having the capacity to finish the yard. I will likely be using just about 6 ah per cut. The extra lipo is worth it, to never have to watch a voltmeter while mowing.

I did run 8s just a bit last summer, and would see doing that if you had a lot of very thick grass to cut. 7 s would be good too I'd expect, a bit fast on the blade, but not too fast. 8s is really too fast a blade speed for safety unless you are cutting really thick grass continuously.

I don't know about making a lipo pack truly foolproof. But a largish 6s pack and a cheap charger wouuld be pretty close, along with the usual beep and light flash lvc warning device. All he'll have to do is pop it on the charger. Hopefully dad can learn to punch the buttons on the charger to get it on balance charge, and then just leave it on that setting.

Lipo should work better, giving the mowers ability to send your battery a vicious voltage spike in thick grass. A larger ping would work great too, but would cost. Might be a 24v 30 ah ping would work good for Auraslip, since he'd want it to run a few hours.

 

[dogman dan]

Now that I had that second cup of coffe,,,, Maybe a 10 ah 24v A123 pack from cellman would be ideal.

 

[mr.electric]

I have been setting up a similar pack for a gored esr. I have two 8ahr 6s lipo packs from hobby king. The packs are 30c!!! And only $100 each so two packs gets you a 22v 16 ahr 60 c pack for just $200. I see your concern about giving your father a lipo pack but it could be an option for your mower. I have two cell logs but no interface circuit to accomplish an automatic lvc. I'm using an rc charger so hvc is not an issue. I will be interested to see what ends up working to accomplish lvc on the lawn mower. For some reason the cell log alarm signal will not work directly with the brake signal on the go ped controller even when I change the settings from n.c. To n.o. Alarm signal. I assume I need an interface circuit as outlined in this post.

 

[reagle]

On my Yardworks (Canadian version of Homelite methinks-got it used on ebay) the charger was in the unit and an external supply was just a regular supply. After I've converted to LiFePO4, I am running 8S1P of Headways and it typically is sufficient for my 1/4 acre yard. Originally the unit had two 18AH 12V SLAs in series.
I use my own BMS with over/under voltage cutoffs and passive balancing at the end of charge. Charger is from Headway.

 

[x88x]

I don't know how the homelite mowers are built. On my B&D one the sla charger is part of the mower. But it took a 24v power supply to run the charger.

Looking at the 'charger' online, I think that's how these are too. At least, the 'charger' looks like just a 24V brick. We'll know once the brick he bought gets in. Also, I'm gonna be at their place next weekend, so I'll be able to tell more about them then. I'm saving ~1/8 acre of my backyard with nice thick grass that I haven't mowed yet this season to test mine on, both with the stock battery and with 6s LiPo.

They actually had a B&D at the ReStore as well, also for $50, also with no power supply...if I had been there I would have gotten it too, picked up a power supply cheap, tested the battery, and sold it at a profit. From my research online, though, the Homelites looked like a bit better build quality, though, so I recommended he get those. There was also a Torx when he first saw them on Friday, but it was gone when he went back on Saturday...though, they were all $100 each on Friday, so it's just as well he didn't pick any up then.

I did think of cell_man cells, but that would be ~$200+shipping. Not that they're not worth it, and I would definitely do that before a Ping, but it's still more than I want to spend.

 

[auraslip]

I did think of cell_man cells, but that would be ~$200+shipping. Not that they're not worth it, and I would definitely do that before a Ping, but it's still more than I want to spend.

I tend to agree with you. No need to spend big $$$ on a homelite lawn mower. I wish it wasn't, but I was under the impression that they're like the lowest rung of lawn care tools. I wish there was actually decent electric lawn care tools. It's like stock e-bikes: underpowered, under built, and on craigslist for cheap once the battery goes!

 

[x88x]

I wish it wasn't, but I was under the impression that they're like the lowest rung of lawn care tools. I wish there was actually decent electric lawn care tools.

IDK, I didn't get that impression when I was researching them online. I'm definitely no expert on lawn tools (hell, I couldn't even tell you which brands are considered "high quality" or not), but they looked to be around mid-range from the info I could find. Granted I haven't seen either one in person yet, but the current models by Homelite, B&D, and Worx that compared to the refurbed ones he found all retail for ~$300-400, which yeah, isn't crazy expensive, but it's not cheap either. Whether that price translates into quality, I don't know yet since I haven't seen them yet. There do definitely look to be quality electric lawn tools available though, from a number of quality companies. You just aren't going to see, say, a Honda or Toro (since as far as I can tell neither makes any electric lawn tools at all...well, Toro makes a small corded electric snowblower but that's it).

 

[auraslip]

Sorry. Just because it's "consumer level" equipment doesn't mean it's not quality. There are lots of good tools sold for non-professionals.
Look at me being a snob about lawn care equipment! :lol: :lol: :lol:
My experience working in the garden center of wal-mart has scared me away from anything wal-mart and the cheaper stuff the big box stores sell. We had soooo many returns of weedeaters and mowers. We contracted out repairs to a local company and at the peak of the season we had probably ten refurbished mowers for sell. That and all the dead mowers I see while mowing lawns around town. It could be that people just don't have experience taking care of the equipment though.

I guess what I mean, is that for my uses I wouldn't want to convert an electric mower made by a company that doesn't make products for commercial users. Check out the gear the professionals use! Hell, my Echo weed eater was $230 and that was for their bottom line model! That's as much as the Toro mower I use! (that's now having some trouble after only three seasons.) You don't see mexican work crews using electric tools because Echo, Sthil, and Honda don't make them. Electric tools are relegated too consumer level products - although that probably has more to do with the practical aspects of being able to run them for 8 hours with no where to charge a battery!

I was playing with the idea of converting standard electric lawn care equipment into something workable for professionals. It's really a big gaping hole in the market that's just waiting to be filled!

Don't be discouraged though. Your usage will be very different from what mine is. I expect to be able to use my Echo 4 hours a day, 5 days a week, all summer. I wouldn't expect that from a consumer grade tool - I'm already saving up to repair or replace my mower. For casual usage the homelite will be fine. Just don't over volt it, push it through wet grass up to your knees, and expect it to be fine! Perhaps install a watt-meter on it and throw a temp sensor in the motor, so you can push it to it's limits?

 

[x88x]

Haha, ok, fair enough. I suppose I should say, from my research it looks like these Homelites are mid-range consumer units. I can definitely see where using one for your living you would want something rock-solid, and yeah, I agree, these probably wouldn't be up to the task. Hell, I probably won't be using it more than 4 hours a month.

It could be that people just don't have experience taking care of the equipment though.

I'm guessing that's a huge part of it. I've never used any really shit quality mowers, but I have used middling quality ones (probably worse quality than a lot of the ones that you would get a lot of returns on) that as long as you do proper maintenance (ie, clean the air filter and mowing deck, top off the oil when it gets low...basic stuff) they last a really long time. IMO, though, a much better route is to buy used/refurb and get a high-quality unit for low-to-mid-range money. But that's the philosophy I take with most things.

 

[auraslip]

I'm guessing that's a huge part of it. I've never used any really shit quality mowers, but I have used middling quality ones (probably worse quality than a lot of the ones that you would get a lot of returns on) that as long as you do proper maintenance (ie, clean the air filter and mowing deck, top off the oil when it gets low...basic stuff) they last a really long time. IMO, though, a much better route is to buy used/refurb and get a high-quality unit for low-to-mid-range money. But that's the philosophy I take with most things.

What do you mean I'm supposed to drain the gas at the end of the summer?


:lol:

 

[auraslip]

89. For example, Tecumseh reported to the EPA that its LV195 lawn mower engine
produces 3.67 horsepower, yet Tecumseh represented to the consuming public, including to
Plaintiff, on labels and elsewhere, that the LV195 engine produces 6.75 horsepower. This is an
overstatement of approximately 84%.

I own this mower. Yeah, that is pretty damn shitty.
On the upside it makes building an electric lawn mower much easier! Why? Because I was thinking I would need a HUGE 6.5hp electric motor, when In fact I only need a 3.7hp motor.
Well actually, ICE motors are rated at peak capacity whereas electric motors are rated at nominal capacity or something? I read it should be a 3 to 1 ratio or something.
So really I would need an electric motor rated at 1.3 hp?
This motor is 1.5hp and a drop in replacement for a gas motor.

 

[x88x]

I'm guessing the electric/ICE power difference is much less pronounced in an application like this where the engine runs at a constant RPM, ie, a constant power level. Whether that means a 3.7HP engine runs at 3.7HP all the time or if they run them at a lower power, idk.

 

[x88x]

Well, as it turns out he only picked up one of them...communication disconnect there..oh well. I have it up at my house now though, and I'll be running it through its paces tomorrow on both the original batteries (24V20Ah SLA) and 6S LiPo. ...that battery pack is crazy heavy! Seriously, it weighs more than the rest of the mower combined!

Concerning build quality, is seems to be pretty well built. The power switch lever seems pretty cheaply made, but it's definitely the odd part out in that respect. The motor is a little brushed inrunner with the brushed armature spinning in the center and permanent magnets around it (ceramic though...) and the electronics are beautifully simple. Basically just three wires; battery- to motor-, battery+ to switch, switch to motor+. With the charger just two more wires, tapping into the battery- and battery+ posts.

 

[auraslip]

Sounds cool - you shoot hook a watts up meter to it and see how it fairs.
I'm excited to see how the 6s of lipo works for you!

I found another conversion with the temuche engine http://jeanegan.com/2007/06/01/lawn-mower-conversion/