Battery backup for DC pumps with fuse and alarm

[salty joe]

I don't like the idea of converting DC to AC then back to DC when I have DC pumps. Every conversion is a power sink and when counting on a battery, well, I want to use all the battery for running pumps.

So, I came up with this. For maybe $15, not counting the battery or Battery Tender Jr., you could build two of them. If you don't know how to solder, it's not that hard-the main thing is to never have a cold joint.

If the power supply comes unplugged or a GFI trips, the alarm sounds and battery kicks in. If the power supply fails, the alarm sounds and battery kicks in. If the fuse blows, the alarm sounds.

The pics show a circuit for four pumps using two power supplies. I added a 3mm blue LED to each 24V power supply to show power and a 3mm red LED to each buzzer so I could quickly tell which pump has a problem. I also added four push button switches to turn of the annoying buzzer. Who wants to listen to that while fixing a problem...

Here are the parts I used. Maybe there are better choices of parts...I am by no means an electronics expert. It's been running a few days and everything is cool to the touch and every time I unplug a power supply, the alarm goes off and the battery kicks in.


https://www.ebay.com/itm/5Pcs-5-Pins-24V-SRD-24VDC-SL-C-SRD-24VDC-SL-C-PCB-DC-Coil-Power-Relay/232430290313?hash=item361decc589:g:8ioAAOSwbmdZfuyb


https://www.ebay.com/itm/10pcs-16-ohm-2KHz-Passive-Buzzer-Electromagnetic-Alarm-3V-5V-12V/391722950573?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649


https://www.ebay.com/itm/2-x-Prototyping-PCB-Circuit-Board-142x74mm/250887390914?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649


https://www.ebay.com/itm/10pcs-x-1n5408-1000V-3A-Diode-US-Seller/332180622836?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649


https://www.ebay.com/itm/10Pcs-BOURNS-Multifuse-1812-SMD-SMT-PTC-Resettable-Fuse-1-5A-24V-MF-MSMF150-24X/171593556363?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649


https://www.ebay.com/itm/30Pcs-2-Wa...200560&hash=item2a7f635507:g:vXIAAOSw3utY7NQZ

 

[nautical_nathaniel]

Thanks for sharing hopefully it works out long term for you

 

[salty joe]

Thanks, it's working as planned. I like having a fuse on the pumps in case something gets stuck in a pump.
Also, the buzzer linked is passive and not what I used. Ooops. I used a regular buzzer like this.

https://www.ebay.com/itm/DC-3-12V-1...039987&hash=item1c9f4e5445:g:sDUAAOSw8vNaaVVc

You can also get a 'wake the dead' loud buzzer for just a little more but I assume they draw more power.

 

[doughboy]

you need to monitor the battery voltage so it is not overdischarged past the point of no return.
like you need to disconnect the battery once it has discharged to 20v from full charge.

 

[salty joe]

Not sure what you mean...The battery is 12V and is maintained by the Battery Tender. The instructions with the Battery Tender said it was fine to leave it plugged in and connected to the battery, so I don't think I need to worry about overcharging the battery.

If I'm not around and the battery gets drained running pumps and is damaged , well I will wish the battery was still good but it did it's job.

I feel like I'm missing something...

 

[doughboy]

when power is out, your battery tender has no output.
you don't want to completely drain your battery. There is a point of no return, if it drains below that voltage, the battery cannot be charged anymore. It's like if your car alternator dies, and you drain the battery, you end up with a dead battery. just google battery management. transfer switch, solar charge controller, etc. they all work on the same principle. I don't think it is right nor a good idea to run your battery to the ground on every use.

 

[salty joe]

I know what you're saying-I just replaced the battery in my daughter's car after she left the dome light on for a few days.

Do most battery backups stop working when the battery gets to a certain point of discharge?

 

[Joshua Kerstetter]

I work around VRLA 12V DC batteries, and all of power our plants have a low battery disconnect. Telecom equipment runs at 48-52v. So we typically use 4 12v batteries together in a single 48v string. Our equipment will simply stop working around 46 v so the rectifier plant disconnects them from the DC bus because things like cabinet fans, and lights will continue to run well after that 46v plant is reached. It comes down the the chemistry of the battery, once a batter goes significantly below 1.7-1.8v per 2v cell, 6 of which are in a 12v battery it chemically doesn't respond very well to taking a charge. You can have issues like thermal runaway, hydrogen gas discharge, all types of really bad things. Now this doesn't happen much in the normal UPS or car battery world. But with telecom equipment, it does. But applying this knowledge to aquarium setups, yes, its great you could rundown a battery with your dc pumps and led dc lights down to nothing, but remember, its really not good for our equipment to run on sub optimal power past its reasonable point, and if you get reasonable time out of the battery, and have it drop before its junk, when your AC comes back up, you won't have lost a battery as well. I've seen.. very...very bad things, when a battery is totally discharged down to the 7-9V range, and then when the AC comes back on a rectifier tries to charge it.

 

[salty joe]

Never knew there was such a thing-thanks guys. How low could the shutoff point safely be on a typical 12V deep cycle battery? Looks like my choices are 10V, 10.7V, 11.4V and 12.1V.


This do the trick?
https://www.ebay.com/itm/Low-Voltag...&itm=202273308734&_trksid=p5411.c100167.m2940

 

[salty joe]

I used to assume a deep cycle battery could be run way down without a problem. I also used to assume a 12V battery run down to 6V was half discharged. I very much appreciate you guys enlightening me about batteries. I will probably set the low voltage disconnect at 11.4V with the intention of getting my generator fired up ASAP. Next up is figuring out how to get a text alert when power is lost. I gotta love this hobby for everything it teaches me!

I found this.

"A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to some point not lower than 20%, and then charging back up to 100%. Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to only 50% each cycle, it will last about twice as long as if it is cycled to 20%. Running the battery down totally flat will have a very negative effect on the lifespan of the battery. See the table below for voltages as related to depth of discharge. This chart is designed to be used when monitoring a battery under load. This chart is a little more useful to the average RVer, as we are most interested in monitoring the state of charge of our battery bank while it is actually in use. See the Testing your batteries section below for information on determining open circuit state of charge using either a volt meter or a specific gravity tester.

Voltage State of Charge
12.6+ 100%
12.5 90%
12.42 80%
12.32 70%
12.20 60%
12.06 50%
11.9 40%
11.75 30%
11.58 20%
11.31 10%
10.5 0%

Avoid discharging the battery below the 40% level whenever possible."