Step 8: Select and Install the batteries
I got 2 car batteries (actually 1 marine deep cycle and 1 gel-cell car battery) both 12vdc. They together keep my lawnmower running strong for the duration of my front and back yard (I have about 1/2 acre of grass to cut and it is somewhat hilly). I slacked while trying to learn about batteries and just went with the biggest ones I could find for the price (the gel cell is actually used). I initially thought 12vdc would work, but the added weight of the mower deck made it travel so slowly at 12vdc, that it would not quite make it up some larger hills, so 24volts was necessary. The 2 batteries are connected in series with each other.
The microcontroller is also powered by these batteries. I have never had any problems with the electronics not getting enough power, so I didn't see the need to have a separate power supply.
The batteries (due to their weight) are mounted behind the rear wheels. This GREATLY improves control of the bot because it counters the weight of the mower deck in front. Zero-turns are very easy now.
I needed a place to hold the 2 big batteries that were going to power the lawnbot, so I measured the 2 batteries and welded a small 1" angle-iron frame to hold them. It is welded to the rear of the frame behind the drive axle to maintain even weight distribution.
You can use bolts and 1" angle-iron to make a battery holding cage that is bolted to the rear of the bot, or you can use smaller batteries and secure them to the top of the bot. 12v 20ah Sealed Lead Acid batteries can be found online for around $35-45 each. Any battery rack that you can whip up will likely be just fine, as long as it can support the weight of the batteries it is carrying. I used a welder to speed up the process.