choose the right batteries

Hi, I made heavy robot around 4-5kg. I use two 4.8v motors with gearboxes from electric screwdrivers. I use l298 ic and 10 x 1.2v Ni-Mh batteries(4x600mAh 2x1300mAh 2x2400mAh and 2x2700mAh). I am getting around 5v drop in l298 so the motors are using 6-8 volts. The robot has enough power to go forward an backward but the rotation(one motor forward the other backward) is not good.after few rotations the wheels don't have enough power to rotate the robot. When I change my 12-13v supply made from 10AA batteries with the 12V battery from my cordless drill the robot has enough power to rotate. Both batteries are 12-13 volts but the battery from my drill is giving more torque and the robot is rotating well. I thing that my 1.2 volt cells are not able to give me 4amps or at least for more then a few seconds If I buy 3000mAh cells ithis doesn't meas that I can take 3amps for one hour it means maybe I can take 300mA for 10 hours. so what batteries I can use that are relatively cheap and can provide me with 4amps. What to search for?I need something that can give me 4amps for an hour not 400mAh for 10 hours Thanks :)

The lower capacity (600mAh) cells in your stack are letting you down. See http://data.energizer.com/PDFs/nh15-2300.pdf for what to expect if you used new 2400mAh cells throughout.

LiPo batteries are probably best for your application, but are not cheap. I suggest you ditch the L298n for a modern mosfet-based driver with low voltage drop, then you can use a lower voltage battery, for example 7.2V LiPo.

Thanks again mate:) What h-bridge ic can I use that is based on MOSFET? Something I can find in the local shop?

It's unlikely that you will find a mosfet H-bridge in your local shop. You might consider something on this page http://www.pololu.com/catalog/category/11 or two of these chips http://uk.farnell.com/allegro-microsystems/a3959sb-t/motor-driver-full-bridge-pwm-smd/dp/1521714.