The battery manual states an optimum charging temperature of 32-104F (0-40C). It also states the battery protection circuit will shut the pack down if the internal temperature exceeds 158F (70C).
Since summer heat in Phoenix is pretty extreme, about all you can do is try it out and see. Maybe someone else in a similar climate can pop in and share their experiences? Maybe check out the reviews of the mower at HD and look for hot climate feedback:
http://www.homedepot.com/p/EGO-20-in-56-Volt-Lithium-ion-3-in-1-Cordless-Lawn-Mower-LM2001/204746234I would suggest storing and charging the battery indoors in an air conditioned space, especially in the hotter summer months. The cooler the battery is when starting out the more heat it will be able to generate/absorb without overheating.
The mower comes with a 4.0Ah battery, but I can use the small 2.0Ah battery in the mower to cut my lawn and even though it goes from a full charge to fully depleted and shut down in less than 25 minutes, the pack is not even warm to the touch. The phase change cell wraps in these packs do a remarkable job of managing heat in the packs, though our weather here in the summer rarely exceeds 90F (32C).
