This guide is almost exactly like the one you will find on EBay for $20 (I kid you not, go to Ebay and search, `robomower batteries,' and someone is selling these!). I'll give you the information for 1/2 price!
A quick note about the batteries -- normally you can tell a battery needs to be replaced as it is no longer holding its charge. Over time, the length of time the mower mows will become less until it no longer does the job. As I found out recently, batteries can also *completely* fail without any warning!
This happened to me at the beginning of the summer and I thought the mower itself had died. I could have saved much heart ache had I had the batteries tested. Instead I bought another mower online (for cheap, yeah!). Now I have one for parts if needed.
Step 1 -- remove the top:
Put your fingers between the top and the sides, where the yellow arrows are. I suggest doing one side at a time. Simply pry the top out until the catch releases. Got that? Trivial.
This is the underside of the battery pack top. The tabs you see are the only thing holding this in place.
Step 2 -- Pop the front down
The front of the battery pack is connected at the bottom, and can be brought down by simply pulling outward at the yellow arrows.
A quick description of the pack, starting at the lower left:
Step 3 -- Remove the batteries
Simply remove the wires, and tilt the first battery out. Repeat for the second battery. If you do not know how to wire batteries in parallel, either note the positions of the wires before removing them, or take a picture.
The battery pack uses two 12V 18Ah sealed lead acid batteries. This is the main reason it is important to keep them charged at all times -- otherwise sulfation occurs seriously limiting the useful life of the battery. A well maintained battery will last 3 - 5 years.
The batteries are wired in parallel, yielding 24V.
I've found the local BatteriesPlus outlet as good a place as any to purchase the batteries. They run ~$60 each, or $120 for the pack.
When re-assembling the pack, the positive wire is smaller than the negative, so you must make sure the positive part of the battery is closest.
Now feel free to send me $9.99!