Reef Tank Battery Backup Switch

Hi all!, I’m just enquiring about a little home job project I have been working on. I’m looking to create a battery backup switch for a battery back up I have for my aquarium. The battery backup is 12V and it needs to supply power when the power cuts off to four pumps which run at 24v and 1.5amps each so 24v and 6a at full load. Currently I have set an Anderson plug connection (to my battery box) with inline 10a fuse to DC-DC step up converter which is sadly rated only for max 2a (I didn’t check when purchasing). This is connected to a double pole double throw relay rated for 10A/24VDC/220VAC which is then connected to the input for the four pumps (24v 7a output power supply) and outputted to the four pumps. This is all connected with 7.5a rated power cable.

My problem then is what should I do about the step up converter given it will likely burn out if I try to run this setup?


Hi Zac,

Welcome to the forum!!
Awesome project!

There are quite a few factors in your project so I’ll point out what I think would have to be done

The converter
You will need one that can output 24V with a current rating of atleast 6A, you might be able to reduce that if you can run less pumps or cycle them slower (PWM control to slow them a bit, you’ll need a motor driver or MOSFET to handle the switching)

Wiring it all together
If you have both the battery and mains supply hooked up at the same time you will definitely run into issues with the battery. The mains supply will effectively be supplying 24V accross the battery at all times, charging it and causing a potentially catastrophic failure.

The fuse
While 10A would be perfect for the 24V system from the battery (12 V) you’ll actually be pulling closer to 13A (total power consumed = 4x pumps taking 12V @ 1.5A = 144 W; for a 12V input through a regulator 144W/12V = 12A assuming there is around an 80% efficiency that will add an extra amp or so)
Id be having a look for a higher rated fuse as when the motors start they’ll have in inrush current.

The battery
Depending on the size of the battery and its discharge characteristics you’ll only get so long before its discharged, you’ll need some smarts onboard to detect when its discharged too much to make sure the battery doesn’t go into failure (BMS would be what you are after)

A bit of guidance
In essence this is a UPS (uninterrupted power supply) I dont have any experience making on myself so if anyone else does or taking a look around on the web.
Using a microcontroller would make things a lot easier, baking safety into the system and keeping that powered as well brings up another problem though
I’d take a look at using a board with 4G to send a text when power goes out and another when the battery gets low.

There’s a lot to take in here but breaking them down into smaller parts and tackling them one at a time will be very rewarding.


Hi Zac,

Yes, attempting to draw more than the 2A max output from your converter (they’ve got a little wiggle room for factor of safety, but if all pumps were to operate at 1.5A with a total of 6A through it then it’ll almost certainly fail).

What I personally would suggest looking into is using either a larger converter which is suitable for the maximum load that you can put onto it, or more preferably, using two 12V batteries in series to eachother, I assume you’re using Sealed Lead Acid Batteries i.e. SLABs, in order to provide the 24V (with some window on either side of 24V depending on the charge level of the batteries and their output voltage vs charge level curve) in which case you wouldn’t need to use a DC-DC converter at all, although as Liam said, you’ll want some way to be able to detect undervoltage when the batteries are discharged for an extended period of time, such as with a BMS or similar undervoltage protection circuit.


Hi All
Being a simple person I usually look for the easy way out. This I think would be a combination of all suggestions.

24V Battery (12V X 2???) suitable for float charging. NOT an automotive type, this would be lucky to last 3 months. Run all the pumps from this.

Intelligent battery charger to suit battery chemistry.

Low voltage cut off to prevent probable battery damage.

Battery size??? Depends on how long you want to operate on battery alone. Expect $$$$

Charger size??? Depends on battery size and recommended charging rate. Don’t forget that after use the charger has to have enough grunt to effectively run the pumps AND charge the battery. Personally if going down this path I would look at Projecta. They have a range of nice chargers and some with settable maximum charging rates.

Second option.
12V battery and charger (instead of 24V) as above. 12VDC to 240VAC inverter then use your existing 240VAC to 24VDC power supply to operate pumps. This may be more attractive as 12V battery, charger etc may be more readily available than 24V.
Cheers Bob


Hi Liam! Thanks for the reply, in terms of what you have stated I’ll respond per paragraph.

Converter - the converter will definitely need a swap but I can seem to find one to suit, do you have any suggestions? The pumps are actually run through and app with adjustable flow so I’d be ideally running them at 20% (normal flow for me currently) each when power goes down. I just want the redundancy in case I want to max them out later.

Wiring - both the mains and battery are connected to a double throw double pole relay which switches the output from mains to battery depending if a voltage is received from the mains. Unless I’ve done my research wrong and that’s not what it’s use is. This means that the battery isn’t connected to the mains at all in this process. I have a trickle charger that supplies and maintains the battery and a battery monitor that sends alerts to my phone through an app.

Fuse - oh I didn’t look at it that way, I have a 15a fuse I can swap in as my fuse holder is rated for up to 30a and I have fuses from 5 to 30.

Battery - The battery is a 12v 140aH giant with a bms.

It’s a lot of fun that’s for sure haha

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