Power outage can be a common occurrence during a severe rain storm. The sump system in a home typically relies on a heavy duty AC primary pump to discharge water entering the home through its drainage system. To compensate for the possibility of a power outage, the sump pit may include a battery-operated DC or AC driven pump in addition to the primary pump. How do these AC/DC pumps differ in function and performance?
A DC based back up system typically consists of a DC pump, a 12 volt deep cycle battery (2 batteries if a 24 volt system), and a charger. Most DC pumps used as part of a back-up system were initially designed to be bilge pumps for boats and are not heavy duty, but some work well. A characteristic of a DC system versus an AC system is that the rate that it discharges is related to the degree that the battery power source is charged. A fully charged 12 volt battery will power the DC pump to its maximum capability. As the charge of the battery is consumed, the pumping rate of the DC pump diminishes. For example, a given pump will discharge at the rate of 30 gallons per minute with a fully charged battery, but may only be pumping at 15 gallons per minute when half of the charge is still left with the battery. At the end when the battery is about to be depleted, the pump may be discharging at less than five gallons per minute. DC systems efficiently consume DC current and can run continuously for several hours (up to 10 hours continuously and intermittently for several days).
An AC based back-up system typically consists of an AC heavy duty pump, a 12 volt deep cycle battery (or two batteries if a 24 volt system), and an inverter with a battery charger). An inverter converts the DC power of the battery to its AC counterpart. Unlike a DC pump, an AC pump will run close to its maximum potential capability until the battery charge is depleted. For example, if the AC pump can pump at a rate of 50 gallons per minute, it will continue at close to this rate (with minimal loss over time) until the battery power source is depleted. It is important to select an AC pump with efficient amperage rating (3-7 amp hours), otherwise the system will not run long enough before the battery source is depleted and the system shuts down. An efficient AC pump can discharge as much as the best DC system because it is discharging at a greater rate than most DC systems even though it shuts down faster than a good DC pump system. The key advantage of going with an AC system is that it is easier to match it up with its primary pump counterpart. Since the AC system requires an inverter, it is typically more expensive than DC systems which do not require an expensive inverter for its operation.
The best way to compare different available systems is to compare the total amount of gallons discharged with a given battery (beginning with a fully charged battery recommended by the manufacturer) and that the data, ideally determined by an independent testing agency, represents the time and performance until the system shuts down). The values should have been generated at an 8-10 ft. head. The head is the distance that the pump discharges vertically (in a basement, it is usually 8-10 ft).
For the most part, A DC system will discharge at a rate less than the primary pump can discharge. Whether this is good enough is something that the homeowner and/or contractor must make a judgment call on. An AC system can be matched more effectively to the performance of the primary pump. The best scenario is that the homeowner has determined how effectively his primary pump has performed during a major storm and matches that performance with that of the back-up system, whether it be an AC or a DC system.
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