I would say the more ah the better :)

i use: 7.4v 3.3ah wall-e u-command 7.4v 4.0ah Omnibot 7.4v 5.5ah Omnibot 2000

If you do not need any extra device pair 12v, best is a 7.4 V battery, with no more temperature 12v heatsinks ez-b and 7.4 is an optimum voltage for ez-b And maximum amperage, 5000 or even 10000 milliamps, for my taste this is very appropriate and much lighter with more time autonymy www.leaderhobby.com/product.asp?ID=9394001227467

i thought i shouldn't go above 5amps since the ez-b has a 5amp fuse

is 6v (sealed lead acid) too less --- i have to stay in the ballpark of about 10-15 bucks (buying for someone else and he doesn't want to spend too much)

i can get a fan for the heatsink but is 6v ok

The 5A fuse will blow only means to avoid damaging ez-b when consumption exceeded 5A, but ez-b consumes less than 5A, you can assemble a battery of 20A or 200A and does not mean that consumption is of 5A over, but that the more amps the battery has, the longer you will have playing with the robot. Try not think about it now, but when you have your robot ready and need to recharge the battery every two hours of use come to think why not rode many amps a battery to avoid as many refills? Lead-acid batteries are heavy for the capacity they offer, but if your robot needs some weight in its base are perfect. LiPo batteries are lighter, but more delicate and dangerous and need a special charger for them. I hope I have helped.

thanks for the info R2D2 (really helpful)

maximum running time will probably about half an hour (tops) so lead acid battery seems like the ideal choice (it's for a mini wooden tank -- weight's not an issue)

just one last question, is 6v 5A too little for the servo and sensor needs or should i go with the 12v 5A link

if you use a 12 volt battery it will run much longer,depending on AH output servo's take up a lot of current ,mostly each one will use near 500 ma to 1 am,so with 7.4 battery at 5 AH you are looking at near 5 hours run time on one servo at 7 .4 volts and if 2 of them looking at 2 1/2 hours and so on ,i design my own servo's so if using 12 volts at 5AH with A DC to DC converter you are looking at may be 7 hours or more GEL CELLS are easy to charge on need a CV charger ,

witch i design and build to charge up to 100ah and up to 24 volts,on LI-POLY batteries are much ligher,dont last very l;ng and need a CV and CC charger

CC IS constant current ,CV is constant voltage,best idea the helps measure all current needed on your robot,motors ,servo's and boards and sensors and add up the current and how may hours run time ,plus 25% it will give about the size of the battery in AH (AMPS PER HOUR) ,then get the weight of the battery and see if your robot will carry it,or need to upgrade the motors or servo's

also DC to DC converters dont have much heat.,and can get adjustable types i am electrical engineer and design and build all types of circuits so you need any help it is FREE