12v Battery Backup (UPS)

About: Part software developer, part maker.

I’ve recently purchased a wireless alarm system for my home that uses 9v batteries for the sensors. However while building the house, I’ve already installed the wiring for the wired alarm so I decided to centralize the power for the alarm, and power the sensors from there.

This way I will not need to replace the batteries every few months and the entire system can be powered for quite some time in an event that power is cut to the house.

Step 1: Find the Battery

The battery that I used is a 12v lead acid battery, that is specifically designed for such applications. It can be trickle charged to a specific voltage of around 13v without losing too much capacity over time. Mine is 7 AHrs so in theory it can power the system for more than 48 hours. Depending on your system you can choose to go up or down depending on your requirements.

Step 2: Build the Circuit

The circuit is very simple and it only consists of few components. For the regulated output to the sensors we have the LM317 adjustable voltage regulator, we have two 1N4007 diodes to prevent any reverse current flow in the event of a power loss, a 1k Ohm resistor to limit the current output from and to the battery and 2 more resistors to set the correct voltage output to 9v.

In order to calculate the values for the resistors I used this handy calculator from Circuit Digest for which you can find the link below. You can play around with the values of R2 and R3 to find what works for you.

Additionally, there are 4 screw terminals on which all of the components attach to: J1 is for the input power source J2 is where the 12v battery is connected J3 is the 12v output for the central alarm unit and J4 is the 9v regulated output

Step 3: Prepare the Enclosure

Once I had the schematic ready. I’ve built it up on a perfboard, made sure to test everything on the bench and then proceeded to install it in the wall mounted box for the enclosure. There all of the sensor cables are converging so I connected everything up and made sure to isolate all of the connections as a safety measure. To power the entire system, I’m using a 12v LED power supply that’s been voltage adjusted to output 13.8v.

Step 4: Enojy

I’ve been running the circuit for over a few months now and it has run without issues. It is easily adjustable to work for a lot more voltages and you can add indicator LEDs or additional regulated power outputs if you choose so.

If you have any suggestions on how to improve the circuit then feel free to leave them down in the comments and if you liked this Instructable be sure to follow me.

Additionally, you can also Subscribe to my channel on YouTube to view other similar projects.


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    5 Discussions


    4 months ago

    You asked for improvement suggestions . I would toss the lead acid mess and use LFP or LiFePo4 battery . If you can't afford one (they are getting a lot cheaper) then a 4S RC battery would work as well but it would just not last as long as the above because most of the small packed ones can be got with 18650 cells and a bms inside..
    Go here and join to learn

    2 replies

    Reply 4 months ago

    I haven't worked with LFP or LiFePo4 batteries so far but I'll take a look. How are they being charged? Do they need a special charger?


    Reply 4 months ago

    Short answer Yes .
    Users get a standard solar controller , specially for Lithium if you can but old Bulk Absorb Float lead acid chargers are being used too very often . That will slowly change . It does not matter you see because you put a $20 BMS on the negative terminal and if anything is not right it switches all power off. LFP wants a voltage above where the battery is with a current flowing which the BMS restricts if it gets too high . Same on the discharge side.


    7 months ago on Step 4

    So.. umm.. you went through all this trouble.. to wire the wireless sensors from your wireless alarm system into a central power supply.... wouldn't it have been a lot simpler and cheaper and frankly more reliable to just INSTALL A WIRED ALARM SYSTEM IN THE FIRST PLACE?

    1 reply

    Reply 7 months ago

    Not really. All of the wired alarm system I could source were made using an older technology and were A LOT more expensive than their wireless Chinese variants. For example, just the GSM module for the wired alarm, costs more than the entire setup I have.