Are you still lookign for a power solution for your robot cart? I designed you a VERY SIMPLE(4 compenents) circuit that wil met your needs. Its costs $28.1(plus parts shiping) and when used with a lawn tractor battery(about $20 from home depot) will give you more power than you will ever need.
Here is what you will need along with part numbers from
http://allelectronics.com
TX-12625 $20 power transformer
FWB-352 $2.50 bridge rectifier
EC-648 $0.65ea( you need 2 of them, so $1.30) 680uF capacitors
LCAC-129 $3.00 7.5' power cord
ACS-3 $0.65 plug for power cord
Note: Don't use this with SLA batteries(like the robot battery). The charge curent is way too high and it will signifiganly shorten their lifespan.
Tell me if you(or anyone else) are/is interested and i will post a schematic.