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12 Volt Backup Relay Switch

One of our members has kindly offered to show us how to make a 12 volt backup relay switch for the backyard aquaculture and aquaponics. Yet another fantastic way to protect your fish from power losses.  This 12 volt backup relay switch is easy to put together without the need for an electrician.

Trevor continues: This is my 12 volt backup relay switch box for my Aquaponics system. All the internal components are 12volt DC (No electrician needed). I have included a Battery Discharge Protector in my switch box, the Battery Discharge Protector is adjustable from 10 to 13 volts and it will cut of the power to your backup air pump or water pump when the battery volts drop below the set level (this prevents the total discharge of the battery, thus extending its life).

12 volt backup relay switch

I have 2 outlet cables; currently I only use one (500 gpm bilge pump, drawing 2.5amps). If you decide to run a large air or water pump (50 watt or higher), then I would suggest that you use heavier cable then I have used (I’ve used 3mm twin core wire).

Backup Relay Switch Box for Aquaponics


Jaycar Parts List
Part Description Part No Cost
Sealed ABS Box- 171 x 121 x 55mm HB6128 $17.95
Panel Mount Fuse Holder (Accepts 3AG Fuse’s) SZ2020 $1.95
Plug Pack Charger (12v DC 500mA Output) MP3011 $19.95
12v Relay SPDT SY4070 $5.95
Battery Discharge Protector AA0262 $39.95
Additional Parts- Grommets. Electrical Connectors (Spade & Eyelet), 12v Electrical Plug Connector, Electrical Wire, Small Cuphead bolts.


Backup Relay Switch Box for Aquaponics

Assembly Instructions

  1.  Cut plug of end of Plug Pack Charger & attach 2 Spade connectors. Connect spade connectors to terminal 85 & 86 on the 12v relay.
  2.  The positive from the battery run through the fuse (I use a 5amp fuse), then connects to terminal 30 on the 12v relay.
  3.  The negative from the battery connects to the “Battery Negative” terminal on the Battery Discharge Protector.
  4. Terminal 87A connects to “Battery Positive” terminal on the Battery Discharge Protector.
  5. Connect the “Load Positive & Negative” terminal on the Battery Discharge Protector to your backup air or water pump.

If you don’t wish to use the Battery Discharge Protector then you would connect terminal 87A on the 12v Relay to the positive wire on your backup air or water pump and the negative wire from the battery would connect to the negative wire on your backup air or water pump.

A note from Paul:
The battery discharge protector is a good idea if you want to protect your battery from completely discharging.  Discharging the battery will shorten its usable life.  Be sure to allow the sizing of your battery bank to allow for the length of time you may need backup power.  Keep in mind the discharge protector will keep some energy charge in the battery which will shorten the run time of your backup equipment. 


  1. I like the whole “off the grid” idea battery/solar combo in which case you would not need this item, but useful info thanks

  2. Nice work. Is there some sort of formula to work out what will be a sufficient air supply for backup? Understand different fish/different requirements however lets look at a 2 k FT with say 60 trout (as trout are air hogs ) 🙂

  3. I agree with chainsaw, recently we had 6 days without power and I would have gladly killed the battery to keep my fish alive.

  4. Good to see the 12V design getting out and about. I question the requirement for a battery discharge protector, I would sooner my battery run flat and keep the fish alive than cutout just to save the battery.

  5. great directions and information,Trevor. Thank you Sir.

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