New to Electric RC world.

I was wondering what people do to improve their experience when using their RC's with the 8.4v NiMH batteries. We have two XL5 Rustler trucks, and I bought an additional NiMH battery to swap out and charge while we are running the trucks, to reduce downtime.

We are really enjoying our trucks, and usually run them pretty fast in a grass oval track in the back yard, and run them out before the next battery is ready. The batteries get pretty warm when we pull them from the truck. They get to cool down a bit while we wait for the next battery to finish charging, but they do not get even close to fully cooled. When the next battery finished charging, we slap it in the truck, and put the first battery back on the charger.

This seems to work well enough, trucks run very fast, but keeps the batteries warm all the time.

How can I tell when this use/abuse has reduced the usefulness of the batteries to the point where they should be replaced? I can check the maximum resting voltage and see it drop, or do we just monitor runtime, and when it gets too short, realize the capacity has been reduced?

* Which goes first, the Voltage (speed) or the Capacity (run time)?
* Will the batteries not get as hot if we don't discharge them so fully, and just pull them when we see the truck has lost power (and just drive it to us)? I read here that _fully_ discharging them until they will not move the truck creates a lot of unwanted heat.
* Will I see a loss of performance when it gets up to 110* here in Texas during the summer?
* Is there a need to purchase some sort of thermo sensor to track the temperature of the batteries or motor?