You can preset a battery level in your account so when the phone battery is lower than the preset level, you'll receive an email or SMS alert (depending on your alert preference setting).

To set the battery alert level for a unit, just login to your AccuTracking account, then click "Manage Units", from the list of units, click the "Battery Alert" column to change the alert level: "Don't alert" or "1/3 full, 1/2 full, or 2/3 full".

Note: Not all phones support battery level alerts. See here.