Part 1- getting the platform
This all started when we bought a house and it came with a bit of grass. And grass wanted to grow and had to be cut. Initially I paid a lawn service to come and cut it, but they kept cutting bits I did not want them to. So I looked at the annual amount I was paying them and decided that would be my budget for a mower. Being an electrical guy, I very much dislike all things gas powered, stinky and noisy. So the absolute requirement was for the mower to be electric. I looked around a bit, and my choices seemed to be a bunch of no-name large box store mowers running Lead Acid batteries or corded, a few exotic European market-only Lithium powered mowers or making my own. I picked neither
For a while, I’ve been looking around Craigslist and Ebay, looking for broken gas mowers to convert. Instead I’ve stumbled on a new old stock Canadian Yardworks cordless mower on ebay. Price was right, so a week later I had my first electric mower. That would save me the hassle of finding the correct motor and machining the bits to mount it on a gas mower deck. It was not clear how long it was sitting somewhere, so I did not have high hopes for the batteries.
Of course the first thing I did was take the cover off and see how it was made. Overall build quality wasn’t bad, with clean wiring and loctite on all screws.
It appears the deck is indeed a standard thing, used for both gas and electric mowers, they just bolted an appropriate DC motor in, driving the blade directly. The motor is powered by two 12V Lead Acid batteries in series. There is also an integrated charger on board, that uses an AC wallpack.
Overall design is pretty simple- two batteries are connected in series and power the motor through a pretty nice safety switch with a key interlock.
When the batteries are low, motor stops and you have to plug a wallpack to charge. I charged things up and it fired right away. For about one summer I’ve been using it as is and it performed reasonably well. Towards the end of the season though I started seen pretty significant voltage sag, and the run time kept falling. Finally one day one of the batteries vented- it blew the plastic cover off covering the cell vent. I took it as a sign it was time to update things to 21st century. An obvious choice was a Lithium ion pack, but that does come with a few considerations. It has to be able to handle rather high discharge currents, provide voltage in a safe range for the motor and stay overall safe. That lead me to the Iron Phosphate chemistry, ( or LiFePo4). The Iron Phosphate cells are relatively easy to keep happy- just avoid overcharging and over-discharging and keep the cells balanced in a pack and they will serve well for a a long time. I just had to size the cells correctly and use a proper BMS .The original pack was two 17AH 12V SLAs. My replacement was going to be an 8S pack of 10AH cylindrical cells.. The voltages would be approximately the same, plus I was banking on the LiFePo4 pack being able to sustain a much deeper depth of discharge and deliver its full capacity. SLAs typically can not do that as they provide less and less capacity at higher currents. This would also cut the weight of the pack quite a bit. My two old SLAs together weighted 24 Lbs. The new pack would be in a 6-7lbs range, making the whole thing significantly lighter. After lurking for quite a while on the Endless-Sphere ebike/battery forums, I’ve learned that I can get this kind of batteries ordered direct from China from a company called Headway.They were 38120 form factor- meaning 38mm in diameter and 120mm long. So I ordered the batteries, charger and BMS from them and prepared to wait.
Part 2- batteries arrive
The wait was about a month or so, but the batteries eventually showed up. Packed in pairs and loking pretty good:
The holders are a bit too tight, cracking if inserted all the way in. But the overall concept is very nice- you can build any pack configuration just by snapping blocks together and bolting the connecting bars to the threaded ends of the cells. I proceeded to build an 8S pack, charged each individual cell with a lab power supply and hooked it up to the mower through a Watt’s Up Meter to log current/voltage and charge. I waited for grass to grow a bit and did the first test run. It was a resounding success- I saw no voltage sag or motor slowdown no matter what I was cutting. Things just stayed pretty constant and perky all the way through my 0.25 acre plot. As the Watt’s up meter started approaching 8AHs sent to the motor, I decided it was time to stop. The concept seemed workable, now I needed a BMS and a charger. Incidentally, I ordered those with the cells, but they did not ship together. A bit more waiting and they finally showed up. The charger looked OK, though I did not care for the IEC style plug at the end of its output cord. Around here we use those for 120VAC, not 29VDC.
Part 3- custom BMS
Looking at the BMS that came in, I felt less than confident that i can trust it to protect my rather expensive batteries from real life perils. So I decided to make my own. I picked a Seiko S8209 chipset to work with as it provided all the features I needed in a very simple package- OV and UV are laser trimmed to a particular voltage, balance control is integrated and you just get two status lines upstream and two downstream. Armed with a few samples of the chip, I went to work. I’ve used ExpressPCB for the PCB design, so to stay cheap I picked their standard service with no silk or solder mask.
I ended up using an Infineon BTS555 power switch as a high side load disconnect. They are a bit pricey, but in all my designs I’ve done using them on military projects, I’ve never managed to damage or kill one. Built like I tank is a good description for them. I also use a smaller FET as a low side charge disconnect. Both are controlled by the Seiko chips. Being paranoid type I am, I added series fuses on all cell lines. Balancing resistors were supposed to be 2512 size resistors. Somehow I forgot to order those. So in go 6 1206 ones. If anything, I get better heat distribution as the spread around a larger area. I designed the board to fit into an extruded Hammond box, and put some lightpipes on top of the balancing LEDs. The connector in the middle is a Switchcraft sealed model, that’s my charge port. Two lugs go towards the motor and the Molex MiniFit Sr is the battery connection. I also wired the cells with individual sense lines and brought those to a Molex MiniFit Jr connector. The pack is wrapped in a heatshrink, that got a bit too shrunk. Woops.
It’s been over two years since I’ve completed this project. The mower is still going strong and the cells are well balanced. It is also very quiet, and emits no smell. Perfect for having other family members volunteer to cut the grass!
I do wish I’d get some sort of state of charge indication short of the motor stopping when I hit UV on one of the cells, but that may have to go on a wish list. If anybody is interested in more technical details about the conversion- let me know.