A few years ago, I (mistakenly) purchased the Homelite 24V Cordless lawn mower from Home Depot. I wanted something that was appropriately sized for my small lawn and relatively low maintenance. I wasn’t particularly interested in a gas powered mower just because I didn’t want to deal with the fuel and oil mess.
Enter the Homelite. It seemed like a pretty decent mower and it was the smaller of the electric mowers available at The Home Depot. Unfortunately for me, it turned out to be a pretty lame product. In retrospect, I should have read more reviews before buying this one. This mower turned out to be a complete pain in the butt because if it’s battery and charger.
The back story…
I think I had the mower for about 2mo. before I realized that I had made a mistake. I got in the habit of using the mower to cut the front and back lawns then hooking it up the the charger so that it would be ready to go the next time I needed it. When I first purchased the mower, I was pretty excited about having a beautifully trim lawn so I was probably out using it every weekend. As my home projects picked up, I scaled back to about every other weekend. After about two months, of this (maybe 6-8 uses), I started to notice a significant loss of endurance from my battery. I was suddenly finishing my lawn right as the battery pooped out it’s last amp. It seemed like the battery’s was life deteriorating fast.
I talked to my neighbor down the street, he actually had the same mower. Apparently, he was having the same issue but he had the warranty (I got cheap at the register). Looking back at the reviews, it seems that everyone struggled with the crappy battery. After a few more weeks, I noticed it was getting much, much worse. I remember one particular day that I was taking a look at the battery before mowing the lawn and noticed that it was particularly warm. Beyond that, it also appeared to be expanding (a very bad thing for batteries). This battery was toast!
So, with a dead mower in hand, I did what any dedicated home depot patron would do: I returned the mower, dead battery and all. Then, since I obviously had no common sense, I bought the same lawn mower again! And, like the first one, it worked great, right out of the box. This time however, I had a plan to extend the battery life. This time, after every use, I would just park it in the garage. I’d continue using it until the battery was low. Then, 4 or 6 hrs before the next use, I’d plug into the charger and charge up the battery. This seemed to work pretty well and I got a lot more mileage out of the mower when I payed more attention to the time it was spending on the charger. This lead me to what I believe was the source of the mower problem: the cheap-o-charger.
I think the problem I was having wasn’t really related to the battery but the charger. By leaving it plugged in all the time, I was continually cycling the battery without realizing it. The battery would charge, the charging circuit in the charger would switch off then the internal resistance of the charger and mower would slowly start draining the battery. Once the voltage got low enough, the charger circuit would switch back on and the process would repeat. This would eventually wear down and ruin the battery. By removing the charger between uses, it allowed me to extend the useful life of the battery to a few years.
That brings us up to now. Its been a few years (about 2) and my battery is once again, toast. It won’t hold a change and it’ll barely cut grass. This time, I can’t simply take it back to Home Depot. I did find a new (I think it was new) battery online but they wanted ~$80 for it and it was back ordered. I can’t, in good consciousness, go back to another Homelite product so I thought it would be fun to find/ build something better. So, I decided to disassemble my Homelite battery and see what I was working with. Here’s what was inside…
Two 12V lead acid batteries, hooked in series. Luckily for me though, these batteries were nicely packed in a container. All I had to do, is cut the wire leads to the battery enclosure’s connector and I have a handy little box for the new battery module. Plus, from the outside it would look just like the original battery did.
For the new battery, I chose Lithium Polymers (Li-Po). Lipo batteries are light weight, compact and have incredible energy density. To drive the mower’s electric motor, I’ll also need to source a lot of current. Some Lipos can source as much as 50x their capacity so lipos were a clear choice for my new battery.
I spec’ed out and bought two relatively cheap Zippy lipo batteries from Hobby King.
These batteries are 6S (6-Cell) so they have a nominal voltage or about 22.2V, pretty close to the 24V of the original batteries. I opted for the 5000mAh capacity. That should give me a decent run-time but just to be on the safe side, I bought two. If need be, I’ll hook them in parallel for the addition capacity. They are also rated for 20C, which means the maximum continuous current draw is 20x the capacity or 100A. For those who don’t work with electricity, 100 amps is a lot of current and it should be enough to get the mower’s electric motor up to speed and carry it through the high grass.
One thing that I changed on this battery was the connector. I’m not real fond of the bullet style connectors that came installed on these batteries so I switched them out for the XT-60s. I just prefer this style connector.
I also installed the mating connector on the interior of the original battery enclosure. If I choose to use two batteries instead of just one, I’ll either make or buy a “Y” cable to go inside the battery enclosure. That would allow me to hook the batteries in parallel if I needed the extra capacity.
One thing to be aware of while working with lipos (or any other kind of battery for that matter) is that they will always have a charge. If you decide to change out connectors, give extra care to making sure that the wire leads (the positive and negative wires) never touch. This battery can source 100 amps and the last thing you want is your finger to complete the circuit. Just be careful.
Another thing to be aware of is that lipos have a very specific operating range. If drained to low, it could damaged the battery or even cause them to combust. The same can also happen if they are over charged so it’s really important that there is some safeguard to prevent that from happening. After all, I want to trim my lawn, not set it on fire.
Since the lipos will operate in a slightly different range than the original lead acids, the color charge indicator built into the motor handle won’t really help. but it will still provide a voltage indication. I’ll just need to re-mark it. But, what if I’m not paying attention to battery indicator or I forget to watch it? The easiest, low cost solution I could think of was a battery monitor. This little guy provides an easy to read battery voltage as well as a (very loud) alarm should the battery voltage ever get too low (plus it’s real cheap ~$3, so I got 2). It plugs into the batteries balance port so it won’t get in the way if I use it while running the motor.
Since the lipos are obviously very different from the original batteries, I needed a new charger too. I’m using a simple ~200W charger from Amazon although I may get a nicer, higher watt, 4-way charger later.
I got it all hooked up and tested this setup just last weekend. It worked like a dream!! My lawn was in desperate need of a trim. I think it’s been over a month since my mower crapped out and it took a little while to get all of the components for this build. By the time I got everything together, the grass was plenty long. The longer grass did bog down the motor a bit but I was still able to get my entire lawn mowed on a single 5000mAh battery. The new battery pack is about 1/8 of the size and maybe 1/10th of the weight. All in all, it seems to be far superior to the original battery.
Per request, I’ve created a schematic. It’s a crude drawing but it shows how everything connects. It should help if my original post didn’t make sense.
I’ve also checked the consumption. It was very unscientific but it was easy. I got about 12mins on the first battery before I finished mowing. A separate battery checker gave the capacity as 13%. I’m not sure how the battery checker actually calculates the capacity (or that it actually works) but it must be giving a capacity reading relative to the theoretical minimum operating voltage.
Lipos cannot be fully discharged. The lowest safe operating voltage for a Lipo is around 3V per cell. For this 6 cell battery, that means I can use it until the battery voltage drops to about 18V. So, I suspect that 13% capacity really means that the voltage is sitting around 19V (13% of the operating range of a 6 cell battery). If I assume that the discharge is linear (which it isn’t) then I’m getting about 2 mins of run-time for every volt I discharge through the motor. If that’s approximately true, then I’ll get about 14mins of run-time before the voltage alarm goes off and I’ve reached the 18V minimum.
It would probably make sense to do some additional testing to determine actual run times and current consumption but it’s a lot of extra work. If someone is really interested, I can make some additional measurements with the fluke meter.
One of the other things on my list was to package up batteries a little better. I want the new lipo battery modification to look as original as possible. So, I drew up a model of what a new enclosure would look like.
This part, which is modeled after the original, was designed to be large enough to hold three 5000mAh batteries and is also compatible with the original battery enclosure lid. It’ll take a while to print on the 3D printer but once I’ve got finished and assembled, I’ll add more photos.
-As of October 2016, this modified mower is still running strong AND on the original battery!
I did complete the 3D printed enclosure, I’ll add a pic soon.
I’ve made additions too. Since the new lipo batteries do not operate in exactly the same range as the original SLA batteries, the mower’s built in charge display is incorrect. To prevent over discharging my batteries, I added an external charge monitor. It’s kinda slick so I’ll add this build as a separate post when I get around to it.
-Make a video -> Check it out here!
-Add a link for the “dream” charger setup. I actually use the the Hitec x4 but the new Turnigy charger at Hobby King is probably a better deal
– Get a copy of the battery enclosure here.