Q:

Batteries for 12- to 24-V motors

I have some nice gear motors that run on 12 to 24 volts, and want to use them in a medium-sized mobile robot -- maybe 30 cm * 30 cm for the base.  I only expect it to run indoors, or possibly outside in nice weather.  If it had a half-hour run time, that'd be fine.  [If this is successful, though, I'd like to make a bigger robot that can withstand the out-of-doors, but probably only in nice weather].
I'm wondering what sort of batteries to use to power the motors.  These are the choices I can see in order from what I think is best to worst.

Use one or two 12 V (lead-acid) batteries.  This is sort of what I'm leaning towards, but I'm worried about how bulky and heavy it would be.  A car battery would be far too large and heavy.  
I already have a charger that claims to be "ideal for car, SUV, light truck, farm equipment, boat, deep-cycle, RV, motorcycle, ATV, snowmobile and lawn tractor batteries."  I imagine it would charge any 12-V lead-acid battery.  
Use some 9-V rechargeable batteries.  They appear to typically be a little less than 9V, but it appears that there are 9V and 9.6V varieties.  I would have to get a charger.  These would be light, compact, but they won't last long.  (230 mAH seems paltry compared to a 5 AH 50 AH car battery.)
I would need to have two in series, and, I suspect, have two or even three series in parallel.  That adds up to quite a few batteries and starts to negate the benefit of the small, compact batteries.
I could use 2 to 4 6-V batteries.  (I didn't realize you could get rechargeable 6V batteries).
AA batteries, and lots of them.  10 to 20 1.2V NiMH batteries, though, will take a fair bit of space and mass and be take a long time to charge.
Specialty batteries (like those used in RC vehicles).
Some combination of the above -- say, two 9V batteries and a 6V battery.
Any sort of battery and a DC-to-DC adapter.

The first two options seem like my best choices.  What would you recommend?

A:

I'm just going to say that 9V batteries are a bad idea, they generally have a very small amount of power stored in them. You're better off going with a bunch of AA batteries than with 9V batteries, if you're looking to get the best ratio. 9V are only good for the cases where you need the higher voltage of 9V, but have very low current requirements. A small SLA (sealed lead acid) battery is a good choice, I've seen these used before on medium sized robots. They are good for when you need a lot of power to drive motors and such, are very cheap compared to NiMh or LiPo rechargeable batteries, and you don't mind the extra weight. Make sure it's 'sealed', they cost a tiny bit more, and have slightly worse performance, but they are safer and don't require any maintenance.
If space & weight are at a premium, and you don't mind spending lots of $$$, then you can upgrade to LiPo (Lithium Polymer). For example, you could get four of these 3.6V 6Ah LiPo batteries, equivilent of a 14.8V 6Ah battery, and it would weigh less than 1lb, whereas a 12V 7.2Ah SLA battery would weigh around 6lbs. Of course, the LiPo's will run you about $80 versus $20 for the SLA. So as always, there's a tradeoff.
Also, your numbers above are wrong...a standard (huge) car battery is closer to 50 Ah than 5 Ah. But those weigh like 40 lbs, versus the smaller 6lb one I linked above.

A:

You can usually run motors at a lower voltage than their rating and they'll work fine, they just turn slower.  So if you find a lower-voltage battery that is otherwise perfect, but not 12V, try that.
On that note, I'd recommend looking into the battery packs used in R/C cars. 
For instance, here's a 9.6V NiMH 1500mAh battery pack for $3.99 (but currently out of stock).  
And here's a 11.1V LiPoly 1300mAh battery pack for $9.99.
If you want something right away and are in the US, you might have a Hobby People near you.

