[Al Skierkiewicz]

A few of the things that kill batteries as pointed out above.
Holding by the leads is like lifting a person by their ears. The terminals will break apart inside the battery leaving you with a battery that reads 12 volts with a meter but can supply no power.
Over charging by using a car charger instead of the supplied charger. charging at greater than 6 amps raises the temperature of the battery causing the battery to vent to atmosphere. The electrolyte eventually dries out or causes an internal short.
Dropping the battery causes the individual plates to break off inside the battery. This causes one or more cells to have reduced capacity. The battery may run the robot for a minute or so and then suddenly drop by two volts. If you drop hard coming off the bump this could be the effect.
Extreme current demand can cause the internal plates to buckle and twist causing internal damage. If you have sticky wheels and tank style drive and push the limits of the drive system constantly, then you are stressing the battery.
I suspect you may have other problems. You might try giving details as to what conditions the battery died under.