24V electric lawnmower upgraded to LiFePO4

[theGoPedal]

Recently I upgraded a 24V cordless electric lawnmower to LiFePO4 for better power and increased range.

I found the mower on craigslist for $30. The reason it was so cheap was the seller couldn't find the 'key', which was really just a 40A DC circuit breaker that plugs into the front panel. Once that was replaced, it worked. The mower came with a 15ah SLA battery that was only good for about 20 minutes of mowing when I got it.

I purchased a 20ah (8s2p) Headway pack, assembled with BMS and charger, direct from Headway. Price of the battery plus shipping to USA was $360. I increased the size of the battery for a couple of reasons. First, I wanted to considerably increase the range so I could mow my whole yard on one charge. Second, the size of the 20ah Headway pack fit the dimensions of the original SLA battery within a couple of mm.

This particular mower was a self-propelled model so it had a secondary motor for the wheels. This motor ran with a high-pitched whine (much different from the blade motor) that really bugged me, so I removed it. It was so loud it kinda defeated one of the main advantages of the electric mower - it's quietness. I don't miss it at all since the mower is much lighter now without the lead.

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I added an inexpensive watt-meter to the top of the case so I can monitor voltage and watts while mowing. A piece of sticky-back velcro attached the meter to the case. I mounted the charging plug to the front of the case using a dremel to cut into the plastic.



Yesterday I did my first full mow and I'm very happy with the results. On the driveway it uses about 8A and in medium grass it goes up to about 12A. On a few on the thickest spots I saw the amps jump up to about 20. When I was finished, the total ah used was a little under 8. Total mowing time was about 50 minutes. Since I'll probably never see 2000 cycles on this battery (I'd be mowing 'till I was 100!) my goal is to extend the calendar life of the battery as much as possible. So I'll take the battery off of the mower at the end of the season and store it in the basement for winter.

I really had fun with this project and learned quite a bit (thanks ES!). Definitely want to try a bike conversion next.

 

[TylerDurden]

Nice!

Make a 24v bike and the pack can work for both...

 

[dogman dan]

Nice. I did a similar thing with my mower. Ditching the heavy lead really helped. The mower is light as a feather with 15 ah of lipo instead.

 

[gogo]

Deluxe! Another winter use of your battery pack can be for an electric heater (with timer) in your car. Nothing like having a warm seat when you get to the car after work and while you wait for the engine to warm up. Here in Iowa the battery pack needs to be insulated with its own heater and thermostat to keep it from freezing.

 

[auraslip]

It might be worth your time to take a look at the wattage rating on the motor. I say this because a) they know run times with the SLA pack will only be half an hour and b) 8s lifepo4 is not only a slightly higher voltage, but it will sag a lot less. This means you're putting more power (lifepo4 26.4v * 20a = 528w vs. sla 20v * 20a = 400w) into the motor, so the possibility of overheating is quite real. Dogman found this out with his weedeater. Maybe even a temp sensor on the motor, if you're feeling really fun.

Other than that, NICE JOB!! Looks great! I'm very jealous!

 

[theGoPedal]

Thanks, guys. This power electronics stuff is fascinating to me, I'm really getting into it. I'll definitely try to think of something else to do with the pack for winter.

Auraslip, I appreciate the tip about motor temp. Will keep an eye on that.

 

[dogman dan]

Fortunately my B&D mower has an over temp breaker. And it will pop when I've mowed a short time in thick enough grass. Like always, stalling electric motors makes heat.

For those not following the other thread, on my trimmer a couple very long sessions on the 24v trimmer melted down a cheezy bearing. Could be just chinese quality controll on the bearing. But even so, 3 hrs non stop use was a bit much for it.

By now, with the drought continuing, I could cut my lawn with a pair of eyebrow scissors.