Batteries

To get the most bang per ounce - you want a battery that can put out a reasonable percentage of its capacity in that time.

How much current a battery can supply is measured in multiples of amp hours - referred to as 'C'.

A 700mah battery that can deliver a maximum of 700ma at once - would be rated 1C.

A 700mah battery that can deliver a maximum of 7 amps (7000ma) at once - would be rated 10C.

In a perfect world, a 10C / 700mah battery would be capable of delivering 7 amps for a total of 6 minutes (1 / 10th of an hour). In practice - batteries tend to get less efficient when you push them close to their limits.

A battery capable of 10C or better is probably a good candidate for use in a combat robot. You don't necessarily want to use all your capacity in a given match.

Lithium Polymer (LiPo)

They have several fold weight / performance advantage over NiMH or NiCad batteries. Since packs start at about $18 and chargers start around $35 - I recommend them even if you're just starting out.

Lithium Polymer batteries are capable of much faster discharge rates than Lithium Ion batteries are. Lithium Polymer batteries put out a large percentage of their capacity when discharged quickly - typically 85% or better. NiMH and NiCad batteries are not as efficient - and may be capable of less than 50% of their rated capacity when abused.

Typically LiPo's are good for at least 8C with some capable of up to 20C.

Lithium batteries have a voltage of 3.7V per cell. Usually 2-4 cells are used in a robot for a voltage between 7.4v and 14.8v.

A few words of caution: LiPo's do not like to be discharged completely. If you discharge a LiPo below 2v - damage to the battery may occur. This is specifically risky when using 4 cells since most electronics continue operating down to about 4v. It's fairly easy to keep driving around until the cells are discharged well past a healthy level.

Do not cross the wires! LiPo's do not like being shorted. If this happens (even for a second or so) they could "balloon" (puff up and lose all their capacity) or if you're lucky, catch on fire.

Lithium Polymer batteries tend to take longer to charge than NiMH's do. Typically they are rated for a 1 hour charge - but some can do it in 25 minutes or so. (I have been known to charge batteries rated at 1 hour in about 30 minutes - but I'm not suggesting it)

Do not try to charge a Lithium battery using a NiCad or NiMH charger. Things may smoke.

I have specifically had good luck with the Apogee batteries available from www.robotcombat.com.