Thanks for the reference. Do you have any idea how many amp-hours your battery pack is?

NP. glad to help. i dont know off hand. will update when i get home...

sorry for the double post.

i looked at the manual,
Battery = 48 Volt, 10 A/hr sealed maintenance free

I guess it depends on where you are as to whats available, I just thought that cordless mowers would be a good source of parts.
I picked up 4 altogether so far, 1 was good sold to my friend the other 3. I was able to get 1 good 12v 17 amp/hr batt, 3 good motors, 1 24volt relay with a 100amp fuse, micro switches, and 2 hi amp switch, plus they all came with blades. These motors were made to turn a blade so you don't have to worry about that.
I don't think you can burn out the motor on these as the batteries can't deliver enough amps for a long enough period of time, maybe if the blade was jammed but then the fuse or cb would blow.

Getting back to the first post
For relay you might want to look at some 12volt RV relays. Maybe some auto relays, ex fuel pump relay I think there 30 or 40 amp. You could also use a standard 120volt household circuit breaker, 40, single.
For batteries what ever you can find, there are as I said before 12 volt sealed lawn tractor batteries, but $100. each that gets expensive for 2.
Scooter parts there are a lot of suppliers around for that stuff.
Hope this helps.

Keep use posted I'll be watching this tread, I'm sure to learn something.

20Ah batteries are only like $35. Unless you're looking for real long run times it shouldn't be that much for batteries.

Well, I've started collecting parts for the project.

Over the weekend I picked up a 300W 24V scooter motor for $30 shipped off ebay. I'm really hoping this is large enough to spin the blade and cut grass. I think it might be a little on the small side, but the price was right and I couldn't pass it up. If its too small I'm sure I can find another use for a 300W permanent magnet motor.

Last night I picked up the mower for the project too. Its my uncle's 20 year old Honda push mower. He recently replaced it with a reel mower since the engine was making odd sounds. Its a bit on the heavy side I think, but it should work. One downside is it doesn't have a side discharge. Its really only setup for bagging which I have no interest in using a bagger. So, I'll have to make something up for side discharge.

Pictures to come. :)

I'm still looking for suggestions too. I'd still like to keep the build price under $150 but exceeding that a little is okay too.

I'd just put a plate on the discharge and let it mulch. As long as you don't let the grass get too long, shouldn't be a problem.

Do you know the eBay motor's physical size? FYI, the motor in my B&D M3300 mower is ~3.5 inches diameter by about 5 inches.

I'm not sure of the exact size of the motor. It looks to be more like 4-5" in diameter and 3-4" long. I'll take measurements when it gets here.

I thought about the mulching option, but I would think it takes more power to mulch? I'd like to keep the power usage as low as possible.

True. All depends how long you let the grass grow & how wet it is.

That's the biggest problem with estimating run time on a mower: the grass is a huge variable.

As some of you know I have my own battery powered mower in the works. I have the motors mounted and have done some testing.
I am using motors form a Yardworks cordless mower, 20" cut, 24volt with 2- 12volt 17amp/hr batteries, that's what it was made with when new.

Tests> I wanted to know how many amps one of these motors draws with the blade on spinning free not cutting any grass.
Found that the motor at 24volts draws about 20amps, my amp meter is not the best. Picture shows the reading I got, the needle at start up goes higher then falls back to what you see.

Thought this might be useful as a point of reference.