I am not sure how important run time is to some members of the
group, or how much additional run time would be sufficient (other
than infinity), but here is some additional information. As for me,
I have two zones (soon to be three) and I just do one zone per day
and have had no problems. However, when the batteries do die, I plan
to replace them with the bigger ones (see below).

I took my battery box apart to get information on the battery. I
have included picture in the photo section of the disassembly. Sorry
about the focus of some of the pictures.

Here is the obligatory disclaimer – If you are not comfortable in
opening the battery pack, or are not a do-it-yourself person, do not
proceed. I assume no responsibility for damage or consequences to
you, your robomower, any nearby people or animals, your
neighborhood, your city, your state, or anything else as a result of
following or attempting to follow these instructions. Note: Do not
attempt to open the actual batteries, only the battery housing. Lead
acid batteries are NOT environmentally friendly (nor people
friendly) and should be disposed of appropriately. I use the Home
Depot near us to dispose of all of my old batteries for recycling.
Proceed at your own risk.

The batteries in my unit are two 12 volt 17AH sealed lead acid
batteries. I have identified a replacement battery with 20AH of
power. This is a gain of over 15%. If your mower currently has a run
time 3 hours 45 minutes, this would result an addition 33 minutes of
run time, putting you at about 4 hours, 18 minutes.

An exact replacement part would be Panasonic part number LC-RD1217P
(12v 17 ah). The higher power battery is Panasonic part number LC-
X1220P (12v 20ah). Amazingly, they are exactly the same physical
size. The two batteries are the same technology and close enough in
power so that changes in the charger would not be required.

The battery supplier, part number, and price is:
Supplier part number price (each)
Digikey LC-RD1217P $36.23
LC-X1220P $41.13

MDS LC-RD1217P $58.00
LC-X1220P $65.00

BatteryWeb LC-X1220P $84.95

The charging current on my unit is about 575ma, reducing to a
trickle charge of about 115ma. At that rate, it will take (worst
case) about 29.5 hours to charge the original batteries. Since the
batteries are never drained down to zero, the charging time would
actually be less. The higher capacity battery would add (worst case)
5 hours to the charging time.

Disassembly guidance: After the battery case has been removed from
the robomower, there are little tabs on both sides in the middle
top, immediately under the cover. Behind these two tabs are two
other tabs attached to the cover that have hooks on them. To remove
the cover, both the tabs on the lower case and the tabs attached to
the cover must be pried out simultaneously while lifting the cover.
Repeat the process on the other side and the cover should be free. I
have had mine apart several times and the case appears to be very
rugged. (I am very impressed with the design and construction of the
mower.) After the cover is removed, the side (back) has two small
flaps that can be opened and the side folded down. NOTE THE POLARITY
OF THE CONNECTIONS TO THE BATTERIES AND WRITE THIS DOWN PRIOR TO
DISCONNECTING THE BATTERIES. IMPROPER POLARITY CONNECTION DURING
REASSEMBLE WILL DAMAGE THE ROBOMOWER. The battery closest to the
open side is removed first, after removing the pressed on connector
tabs. The inner battery can be removed by disconnecting it, moving
it to the center of the battery housing, and lifting the case off
the battery through the opening in the bottom of the case. Reverse
the process to replace the batteries and reassemble.

If your batteries are beginning to fail, you could replace them with
the less expensive batteries. If you want the additional run time,
you could replace them with the higher capacity and more expensive
batteries.

http://www.batteryweb.com/panasonic.cfm?sort=lbs

http://www.digikey.com/

http://www.mdsbattery.com/

Good Luck
Terry