Larry Lunsford

If you’re like many, after a few years of Koi keeping you have amassed a fairly valuable collection and you are very fond of your wet pets. You may want to consider building a battery backup system to help care for your pond in the event of a power outage. There are many commercially available backup products, but they have two main drawbacks: large expense and small reserve capacity. The battery backup described here is relatively inexpensive and you can use whatever size battery you need to meet your capacity requirements.

The backup consists of the following: switching relays, battery, inverter, charger, and optionally power fail indicator. The switching relays control the flow of power. While power is being supplied by the electric company grid, the relay coils are energized, the pond equipment is being powered by the grid, and the battery is being charged. When the power fails, the relays switch and the battery is now connected to the inverter, and the pond equipment is powered from the inverter.

The relays used are commonly available 120VAC coil, DPDT. What that means is that the coil (the electro-magnet that causes the switches to change) requires 120 volts of alternating current (normal house voltage) to activate the relay. DPDT indicates that the relay is double pole, double throw. Double pole means that there are two switches. Double throw means that each switch has two positions: when power to the coil is off a connection is made between the common connections (C1 and C2) and their normally closed connections (N.C.1 and N.C.2 resp.). When power is applied to the coil, the common connections become connected to the normally open connections (N.O. 1 and N.O. 2).

Relay R1 controls where 120VAC is taken from to supply power to the pond equipment. When grid power is on, the relay is on, C1 and C2 are connected to NO1 and NO2, and the pond equipment is powered by the grid. When the grid fails, C1 and C2 are connected to NC1 and NC2, and the pond equipment gets power from the inverter.

Relay R2 controls how the battery is connected. When the grid is on, C1 and C2 connect to NO1 and NO2, and the battery gets charged. When the grid fails, C1 and C2 connect to NC1 and NC2, and the battery becomes connected to the inverter. Inverters require large amounts of battery amperage - make sure your relay can handle the load.

Relay R3, the lights, and the switch are optional. They are used to indicate if power has failed. While the grid is on, briefly pressing the switch allows power to flow to the coil. Once the relay is on, its own contacts are used to switch power to the coil. If the power ever fails, the relay will switch back to its off (normally closed) position. If there has not been an interruption in the grid, the relay will stay on and the Power Good light will be on. If the grid fails once or more, then the Power Fail light will be on when grid power returns. While the grid power is off, both lights will be off.

The battery used is an ordinary 12 volt car battery. You can buy whatever size battery you feel necessary. You may want to consider using a marine duty battery which will give your more capacity and will hold up better to deep discharging. Battery capacity is measured in amp hours. A battery rated at 10 amp hours can supply 1 amp for 10 hours, or 10 amps for 1 hour, or 2 amps for 5 hours, etc. To determine approximately how long you can run your pond equipment on the battery, divide the amp hour rating of the battery by the amps required by the inverter - the result is your run time in hours. If you need more battery capacity, you can put multiple batteries in parallel. If you put batteries in parallel, don't mix and match - only use new and identical batteries.

The inverter is used to convert the 12VDC from the battery into 120VAC for use by your pond equipment during power failures. Inverters are available with outputs of just a few watts to thousands of watts. To get the longest backup duration, you should put the minimum amount of equipment on the battery backup system as possible. You should also select an inverter that is able to meet the demand of this equipment, but which is not excessively over sized. An oversized inverter will not be as efficient. One drawback of inexpensive inverters is that they output a square wave instead of a sine (smooth) wave like you get from the grid. Many pumps do not operate as well (and some not at all) with a square wave. Make sure your pumps work ok with your inverter. Most pumps will also have a much larger startup current than running current. Your inverter will need to be sized to supply enough power to start your pumps. Even though the relay kicks in as soon as power from the grid goes off, it takes a couple of seconds for the inverter to come up to full power - long enough for your pumps to slow down quite a bit. Many pumps are also highly inductive which means they will require higher current from the inverter. To determine the size of inverter required: add up the maximum currents of all your pond equipment that is to be run on battery backup; multiply current by 120; the result is watts required. Allow an extra 10% to 20% inverter capacity to prevent over loading. Place your inverter where it will get good air flow to prevent over heating. Be sure to test your system: make sure the inverter can handle the startup loads; make sure your inverter is not going to over heat (let it run for about an hour); if your battery is not sealed, make sure the fluid in the cells is kept topped off.

The trickle charger will keep your battery charged, but will not recharge it after its been discharged in a power outage. To charge your battery, you will need a larger charger. Get a charger that's intended for the size and type of battery you are using (regular auto or deep cycle marine). After power outages or at least once a year you should disconnect your battery and charge it with the large charger.

The parts used are all commonly available from many sources. Boat & RV stores are good sources for marine duty batteries, chargers and power inverters.

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