DIY Size & Build a Battery Power Backup Generator W/ 12V Deep Cycle Batteries

***NOTE: Be careful when working with batteries and electricity. Do not short batteries. Use insulated tools. Follow all safety rules when working with electricity.***

Be prepared before the next time the power goes out with a standby battery powered generator. Build your own battery backup system for your home or business. A battery backup system allows you to power your essentials when the grid is down. Using sealed AGM deep cycle batteries, this system is safe for indoor use; you can install this system in your closet, in the corner of your office, or make it portable by using a cart.

By building your own battery backup system, you can size it to your desired needs. We will go over how to choose the right size battery and inverter, and how to put the system together.

You will need:

-1 or more sealed deep cycle batteries

-1 DC to AC power inverter

-1 Smart Charger/Maintainer

-Inverter cables and battery link cables (if using more then one battery)

For this system I used the following:
-2 VMAX SLR155 12-Volt 155Ah AGM batteries connected in parallel (vmaxtanks.com)

-1 12V DC to AC 2000 Watt Inverter (online or from a hardware store)

-1 Vmaxtanks BC1220a 12V 20A 7-Stage smart charger

-1 Set of 2 gauge 6' 100% copper inverter cables (4Ga would have also worked, be sure to check the ratings of the inverter cables you buy)

-1 Pair of 4 Gauge 12" 100% copper link cables

Step 1: Choose a Power Inverter

When choosing an inverter, pick one with a wattage rating higher then what your devices use; add up the wattage of the devices you would like to power. Your appliances will usually have a label which indicate the input wattage or amps. Wattage is simply volts times amps. For example if your laptop charger uses 80 Watts, and your phone charger uses 20W, you need an inverter rated for at least 150W. Blenders typically use 300W, so to power your blender, laptop and charge your phone you would need a 500W inverter. It is always better to oversize your inverter. I chose to use a 2000W inverter.

Step 2: Choose a Battery

A deep cycle battery is recommended since deep cycle batteries can be cycled many times. If you use a flooded car battery it will be damaged by being deeply discharged. I chose to use AGM deep cycle batteries by VMAXTANKS, since they can be cycled many times and are sealed. AGM batteries are also maintenance free and safe for indoor use. By adding up the wattage of the devices you want to power, you can figure out what size battery bank you will need. Take the watts, ex. 400W, and multiply by how many hours you want to power the 400W load.

To power a 400W load for 5 hours:

400W x 5 hours = 2,000 Watt Hours

For 2,000WH, choose a battery bank which provides at least 4000WH(4kWH) to keep your batteries from going below 50% capacity (this will help your batteries achieve more cycles over time).

In my battery bank I used two VMAX SLR155 batteries, rated at 2.1kWH each, for a total of 4.2kWh, or 4,200 Watt Hours. Deep Cycle batteries can be cycled past 50%, but keeping your batteries above 50% will give you many more charge cycles. High quality batteries will give you more cycles, Vmaxtanks batteries have very high cycle counts and are military grade. Always fully recharge deep cycle batteries after every use. Below are several different vmaxtanks battery options:

SLR60: 0.8 kWH (800 Watt Hours)

SLR100: 1.35kWH (1,350WH)

SLR125: 1.7kWH (1,700WH)

SLR200: 2.66kWH (2,660WH)

XTR8D-350: 4.7kWH (4,700WH)

Step 3: Choose a Battery Charger

You will need a smart charger compatible with your batteries. For deep cycle batteries you will need a multistage "smart" charger/maintainer. The battery charger should be matched to fully charge batteries in ~15 hours or less.

I used a Vmaxtanks 12V 20A 7-Stage charger(BC1220a), which is capable of charging and maintaining my battery bank. Vmaxtanks chargers can be left on the batteries all the time, so your batteries will always stay charged and ready to go.

Step 4: Connecting It All Together; See Images for Steps

For my system I used the following:

-2 VMAX SLR155 12-Volt 155Ah AGM batteries connected in parallel (vmaxtanks.com)

-1 12V DC to AC 2000 Watt Inverter (online or from a hardware store)

-1 Set of 2 gauge 6' 100% copper inverter cables (4Ga would have also worked, be sure to check the ratings of the inverter cables you buy)

-1 Pair of 4 Gauge 12" 100% copper link cables

-1 Vmaxtanks BC1220a 12V 20A 7-Stage smart charger

Step 5: (OPTIONAL) Use a Charger/inverter With a Built in Auto Transfer Switch.

If you have a server, printer, computer station, or any other device you do not want to shut off when the power goes out, use an inverter which features a built in charger and an automatic transfer switch. The Charger/Inverter will plug into an AC outlet and keep the batteries charged. You can plug in your appliances into the inverter to draw AC power, when the power is out the inverter will automatically switch to battery power. This is very useful if you are running a server/printer/computers/medical equipment etc. and can not afford to have it unexpectedly shut down in the middle of a task.

A Charger/Inverter will also provide a cleaner looking setup since the charger and inverter will be combined into one unit.

Comments

[CyberHoff]

I'm somewhat confused about the 'optional' inverter with 'auto transfer switch'. I thought the whole purpose of this build was to create a battery backup that will act as a DIY UPS in case of a power failure. Is that not the case? I already have a charger and had planned to get an inverter.

[JUICER]

If I bought a high watt inverter can I plug it into a wall socket to power my plugs and lights? I do it sometimes with a generator. using a male to male adapter
Before starting the generator I shut off all the breakers.

[dlhpdx]

If I'm not mistaken, this could back feeding the power grid unless you have a lockout switch Backfeeding the power grid is potentially deadly to any lineman that may be working on the power line to repair grid power. The easiest way to solve this is to get a meter connection from your power company. it is a meter with a plug on it to supply your home with an alternative power source with a built-in automatic isolator. check it out I am not an electrician,I hope this is not sounding like I am being not positive to your posting I just want to be sure everybody is safe. Best to you

[whoknowes64]

NO. this is a battery back up, not a whole house generator. Its designed for lights, a fridge, maybe a TV and probably not all at once. You do not want to back feed the service panel through a double ended male plug. You could blow circuit breakers or possible FRY a lineman working to restore power.

[ChrisB140]

Just an F.Y.I. from someone who has worked in the electronics field for 45 years, calling your system a "generator" is very misleading. It is much more correctly called a "Power Battery Bank", or similar names. It does NOT generate power. It stores it for later use when needed.
Don't get me wrong, it looks like a well thought out system, it just needs the addition of an actual generator of some kind to provide the power other than plugging it into a line socket. If worse comes to worse you could run cables from your car to provide regulated 13.8 VDC to keep those batteries from running down. Most automobiles use less than a half a gallon of gasoline per hour idling so you get nearly 24 hours of power if need be. Someone with a diesel powered vehicle will get considerably longer run times unless their car or pickup has a small tank. My pickup has a 35 gallon tank and only burns about 1/4 gallon an hour.
Or you could buy a small engine like a Predator from Harbor Freight or a Kholer or similar engine and use it to power an automobile generator. Best to get one of the one wire alternators and then you don't need a separate regulator. For about $450 you can get a 200+ AH alternator online but you'll need more than just a 3 horsepower engine for that one. Sorry, I don't remember the formula to convert horsepower to that needed to run said generator.
Something for you folks to look into.

[1topgun357]

Thank you for all of this great Information. I want to build a battery power system for my motorhome for a few electronics (TV, Computer, Camera System). I want to leave the power system plugged into one of the 110 outlets in the RV to charge it. The outlets in my RV don’t have power unless I am using my generator, or I am connected to a power source. Therefore, my plan is when I plug the motorhome up at home, or a RV Park, or run my generator, it would automatically charge my power system. Do you have any recommendations on a inverter charger, with a transfer switch? I don’t want to hardwire it, but I keep reading that pure sine power is better. Also, how do you feel about one large battery instead of using two small ones? Thank you.

[whoknowes64]

You're going to need something like a service interrupt breaker so you can swap back and forth between the generator and the battery system. Also a way to back feed your service panel from the batteries. Depending on the Amps you are going to supply, You should know all about the shoreline connector. You may even have one built in to the motor home already. You don't have HOUSE batteries to supply the motor home while on the road traveling? You may have, most do... Check with a repair shop to see if the batteries are just dead/disconnected. It would be odd for a motor home to NOT have house batteries.

[FreonS]

I don't see anything on the most important part: How do I connect it to my home wiring? It really isn't very useful without that information.

[whoknowes64]

YOU DONT. its an emergency back up... its for lights and a few small items. This is NOT for whole house back up power. You plug the item you want to keep powered into the inverter.

[joeculver]

If you notice the Step 5 (optional) is to use an inverter with transfer switch. In this case you plug in to AC and when AC drops out the inverter switches to battery.

[WayneFriesen]

This unit is meant to power devices that you plug into the inverter. You do not wire it into your home in any way. It is a stand-alone power supply during an outage.

[chefjake07]

But, to keep batteries charged and maintained 24/7 It would be connected to home grid power wouldn't it be?
The charger?

[marinebean]

Why have you not included batter isolators, which are important in a system that uses more than one battery?

[Lonestarr]

Learning about this stuff myself. Wouldn't using an isolator defeat the purpose of 2 batteries connected in parallel?

[kcknoeber]

I apologize if this is a stupid question. I do a lot of off road drivng. Reinflating my tires is always an ordeal and I don't want to shell out money for a small gas generator if I could build one of these that can power a compressor, and perhaps a few other items. A) is this a ridiculous idea? B) would the general guidelines here be the bulk of what I need to follow regarding capacity and sizing?

[ampmann]

I have a Superflow compressor in my Volvo. 30 amp fuse on it as built

[sp4rkn]

A good on-vehicle solution would be to add another battery with an isolator. That way your vehicle can charge the battery, but your starting battery won't drain while using the extra battery. If you use the battery up, you'll still be able to to start the vehicle. Also, it would be a good idea to carry a super capacitor jump starter. It has the unique ability to charge from a DEAD battery (unusable to your car but usable to the jump starter) and jump start the vehicle that has the dead battery (it can self jump a dead vehicle). The pack is very small and fits in less space than your tire inflator. It's piece of mind and can make you the hero when 4x4ing with buddies.

[rittersdad]

I don't know if kcknoeber solved this problem on his own, but it might be worth mentioning that are a lot of 12v automotive tire inflators on the market. If he already has an AC compressor, just use a 12v to 120v power converter connected to his vehicle and inflate away! Just be sure to match load to the ability of the converters' ability to deliver power.

[johnallen1952]

Hi, I like your idea of a Battery source for power with an inverter.
I'd like to use a Honda whisper quiet generator 2000watt to keep the batteries charged but would like the generator to start when batteries need to be charged and shut off when full?
do you have any ideas? how to make that work?

[jakecoffee1503]

Look at Power Converter/Chargers. Used mostly with solar. It does exactly what u want. Detects when to open or shut the flow of power depending on the charge level. How to incorporate that to the generator, I'm unsure. But it's a good place to start.