No AC sump pump will work when the power is out or if the pump fails for any reason. 
Battery backup pumps protect against water when your power goes out or in the event of pump failure. Many times during the worst spring storms power can be lost, which normally will shut down your sump pump system. In these cases, a battery backup sump pump will automatically turn on and protect your basement until power is restored. For those of us that have experienced basement flooding a battery backup is not a luxury but an absolute necessity.
Many things can go wrong down in the sump pit to cause basement flooding. It can be a power outage, a failed sump pump, a stuck float switch, or a sump pump that cannot keep up with the inflow of water. By using a battery backup sump pump your basement will remain dry during any of these occurrences. Most backup systems fail because the battery is dead and you don’t know it. Quality systems are the only ones that monitor the battery, power conditions, and sump pump, then sound an alarm when maintenance is needed.
What does the Controller Monitor?
For added protection and peace of mind, install a battery backup sump pump system. The PHCC Pro Series backup sump pumps have a unique monitoring system that sounds an alarm when maintenance is needed, or when a problem arises. After all, a backup sump pump system with a dead battery, loose cables, or a clogged pump is like having no backup at all. The PHCC Pro Series backup sump pumps monitor all these functions and remind the homeowner when attention is needed.
As with AC sump pumps, DC battery backup units vary in performance and features. Once again gallons per hour and efficiency are the keys to picking the best system. If the system is not efficient, it will drain the battery quickly and put your basement at risk. Also, if your backup cannot keep up with the amount of water entering your sump basin, your basement will be at risk.
To make a battery backup work most efficiently it should be combined with a matching high capacity primary sump pump and an efficient sump pump basin. Sump basins or sump pits can greatly affect the performance and efficiency of a sump pump system. Basins with large capacities around the sump inlet are best suited to take advantage of modern backup sump pumps. The best of these new sump basins will have a sealed, child-proof lid to protect your family and restrict noise.
Backup sump pumps protect against:
The best backup sump pumps offer: