You will have to depress the key a second time to perform whatever function it was supposed to do. While any AA cells can be used in your unit it will work best with AA alkaline batteries. Four fresh batteries will generally produce slightly more than 6 volts of output and register full on the battery gauge that appears on the status screen. Battery life varies from one unit to another with newer units getting the best battery life. On older units the battery life can be as low as 8 hours while on the latest units using a battery save feature can reach a claimed life of 35 hours from a 4 cell unit and nearly 20 hours on a 2 cell unit.
Actual life varies a lot depending on conditions.
Many users prefer to use rechargeable batteries in their units. While they need to be replaced more often the total life of the batteries is much longer. For best battery performance you should replace rechargeables as soon as you receive the low warning indication. You will have to recharge any batteries external to the unit as there is no provision for recharging them while inserted in the case. In cold climates lithium batteries may offer the best performance.