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

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***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 witch. 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.

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    7 Discussions

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    kcknoeber

    Question 7 months ago on Step 1

    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?

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    LukaM5

    1 year ago

    you could switch the UPS power (outlet/battery backup) with a simple relay if you really need two-stage backup.

    1 reply
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    nic.bryan.73LukaM5

    Reply 10 months ago

    That's a very good point, and not particularly difficult. Wire the relay control coil to the mains-power side of the AC cable, and the NC side to the Inverter AC, NO side to the mains, CO to the UPS, with a second relay replacing the power switch on the inverter so it doesn't try to drain the batteries during charging.

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    NathanR56

    11 months ago

    Would I be able to use the same charger to fully charge 3x XRT8D-350 from ~50% in 17 hours or less?

    I plan on using the system as an UPS with 710-800 watt load during my peak hours for the utilities and would need it to hold the load for the seven hours I'm on peak and then charge back up once I'm off.

    1 reply
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    62390NathanR56

    Reply 11 months ago

    For 3 XTR8D batteries you need at least 45A in order to properly recharge them. I would recommend an inverter/charger unit with auto transfer. Many companies including tripp-lite and Renogy offer good choices.

    https://www.amazon.com/Renogy-1000Watt-Inverter-Charger-Outlets/dp/B01MUCN1WJ/ref=sr_1_7?ie=UTF8&qid=1514482284&sr=8-7&keywords=inverter%2Bcharger%2Bauto%2Btransfer&th=1

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    Bravodelta49

    1 year ago

    Don't forget the fuse! Mandatory for any battery backup system. Search on 'fuse for 12 volt battery backup' for references.

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    Swansong

    1 year ago

    Thanks for sharing :)