The batteries used for car racing are made up of cells, which are individually 
little smaller than standard C size cells. A battery pack consists of 4 to 7 cells, 
most common are 6 cell packs. You can build your own pack, or buy an already 
assembled one. If you purchased your car kit as a combo deal, the battery pack that 
came with it is probably going to be a 6-cell 'stick pack' (the cells are assembled 
nose-to-tail three in a row, in two rows side by side, sealed with shrink-wrap). 
This type of pre-assembled battery pack is a good starting point for beginners. 
Racers often assemble their own packs into a 'saddle pack' configuration (three 
cells side-by-side, in two groups connected by an electrical wire), or side-by-side 
configuration (all the cells side-by-side). The important point to note is that 
some car kits (especially high-end performance ones) can only accept certain 
battery configurations. This is often done for performance reasons - to allow fast 
battery changes, or to have a particular weight distribution. Again, check the 
manual that comes with the car.

An average battery pack will charge in about 15 to 30 minutes, and give you 5 to 15 
minutes of run time. For this reason people often get several packs. A good 
starting point is to get the same number of packs as the number of races you want 
to run in one day; this is especially important if you will run in sponsored races.

The batteries are rated in mAh: milliAmpere-hours. This is a measure of how long 
the battery will last before it needs recharging. To get an estimate of run time, 
take the rating and divide it by 300 to get a VERY approximate estimate of run time 
in minutes; mileage WILL vary! Currently on the market you can get batteries that 
range anywhere from 1000mAH to 3000mAH, which range anywhere from US$5 to US$60 per 
pack of six cells.
If you are going to be running at a sponsored race, batteries might be one of the 
restricted items. Check with the organizer of the race.

6.1. Battery care

An entire document could be written - actually has been - on this topic alone. The 
best and simplest advice that I have seen so far is from Tom Younger: "The people 
who have poor battery life are those who re-charge when their batteries are still 
hot, and who spend far too much time, money, and effort trying to discharge their 
batteries after using them."

I am not going to repeat what has already been covered very well. If you need to 
know more, check Dennis Clark's "The Care and Feeding of NiCd Batteries" page at 
<http://www.verinet.com/~dlc/battery.htm> for a discussion of battery care. At the 
end of that page you will find a link to "The R/C Battery Clinic" 
<http://www.rcbatteryclinic.com/>; this site has more info, more detail, and more 
stuff on batteries.
Also, do a search for past posts at rmrl on this topic. This is a very FAQ. Here 
are a few links, follow them to the threads:
<http://groups.google.com/groups?btnG=Google+Search&as_umsgid=Pine.SUN.3.96.1010526082947.109A-100000@valdes>
<as_umsgid=893ee903.0105271726.1222d70b@posting.google.com">http://groups.google.com/groups?btnG=Google+Search&as_umsgid=893ee903.0105271726.1222d70b@posting.google.com>
<as_umsgid=ql7g7.5720%24xb1.2523383@news1.rdc1.ne.home.com">http://groups.google.com/groups?btnG=Google+Search&as_umsgid=ql7g7.5720%24xb1.2523383@news1.rdc1.ne.home.com>
<rec.models.rc.land%2C%20uk.rec.models.radio-control.land&as_usubject=batteries">http://groups.google.com/groups?as_ugroup=rec.models.rc.land%2C%20uk.rec.models.radio-control.land&as_usubject=batteries>
<http://www.schumacher.clara.net/charging.htm>