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John Deere JS40 E

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A little over a year ago I picked up a project out in Sac. A converted John Deere JS40 walk behind lawnmower. Self powered. Either Side, Rear or Mulch. Mulch is the only option right now. It was converted with a DC PM motor and a simple automotive solenoid. It is a 36 volt conversion and had 12 40 Thundersky batteries for its original set of batteries. It also had a simple BMS system installed for monitoring but not really balancing. It was used by its previous owner for many years but no longer was needed as there was no longer any lawn to mow. So now I have it and it works. It uses a simple little 36 volt lead acid battery charger. The system had an alarm built in so if the voltage went went below or above a specified voltage it would set off the alarm. That was the BMS. The problem was they connected the solenoid to only 4 of the cells so when it was being used it used a little more than the rest. Eventually over the years it lost its balance to the others and became a problem. I think 8 of the cells are still quite good but the others I think have been spent. It mows well but the battery life just sucked. So I decided to do an upgrade. Off come the Thundersky cells and BMS. My little setup now has a 36v DC DC converter and a smart brain for the charger. I had to find such a device because the lead acid charger would actually charge the cells too high and ruin them. I needed a way to shut off the charger a a specified voltage. As for the new batteries, I am now using 5 Nissan Leaf Modules in series and only charging them to 8.2 volts each. It is a little higher than a standard 36 volt system but it works. The DC DC converter now powers the solenoid so there is no imbalance drain on the main pack. I have a good 12v DC toggle switch to switch the solenoid for turning on and off the motor. I also picked up a 36v battery disconnect used on golf carts. So when I park the mower for the winter it will have NO devices draining the battery during the winter. Or I could leave it all connected to the charger and let the brain do the work. If it drains too a specified voltage it will turn on the charger again. There is no floating voltage any more for this charger. It is all controlled by the brain. Simple and elegant. The brain has an LED so I can see the voltage of the pack at all times. I need to now see if I can connect up the little alarm if the mower goes below a threshold while out mowing to alert me to a too low of voltage rather than just me.

Now that the pack is fully charged I will clean up the wiring and take it out for a spin around the property to see how long it takes to drain the pack. I need my mileage check now.

Below is an OLD video from last year that I did. New video will be coming soon.

I did find that what the brain shows and what the pack shows is not exactly the same. I set the end voltage to 41 volts. During the charge there was a difference of 4/10ths of a volt where the pack was at a higher voltage. The 4/10ths was pretty consistent throughout the charge cycle. The device shut off at 41.1 volts. My pack is at 41.2 volts after resting much of the night. So Im still in the safe zone of the modules.


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Little update on the E Deere. This evening I did some more with the mower. I had let it set for a week with the charger brains on and the dc dc on. I can turn off the dc dc and still leave the charger on. Seems like the dc dc and the charger device do suck a bit from the pack over a weeks time. I started with the pack charged to 41.1 and soon the pack was down to 40.8 then creeped down to 40.2 today. After doing a quick securing of the wires and contractor I took it out for a spin. Oooops. mower wanted to go in reverse. But the gearing would not let it actually drive in reverse. I flipped the pack over so I could change the polarity of the motor to the battery as one wire was too short to just swap the cables. Once that was done I wanted to see how far I could go mowing. So I started out at 40.2 volts and mowed our large side yard, large front yard and medium sized back yard. About a 1/4 acre total. Side yard had the tallest grass to mow. I did not stop until I was done and when I got done the pack was at 37.8 volts. Plenty to do the front and back again and maybe even the side. Maybe even more. So the mower is doing well and cutting well. Now for an update video of the beastie doing its job. Well pleased with the Leaf Module setup and the smart brain computer I purchased for allowing for controlled bulk charging. During charging Im sure I could go to 4.1.5 8.3 volts with this setup. It would take the module a bit higher than 8.3 volts each but still safe as the pack surface charge will settle down just about right.

So right now the mower is back on the charger with the same original settings I had set. No changes quite yet. The voltage did drop a little after I turned on the mower but the voltage sag was minimal. I'll double check the voltage sag next time out. Still have plenty to mow in the back 40.


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