How To Build A Backup Battery System

The parts needed are a power inverter, a deep cell battery, cables to connect these two devices. Motomaster is the brand name of products available across Canada at Canadian Tire. If you are Canadian then you already know about this nationwide chain. If you are not, then when you come to Canada then you must find one and visit it. Quite possibly it is a national treasure! While Motomaster may not be available in your country, the parts you require for a unit like this certainly are. On Amazon a 1000 watt pure sine inverter can be found for about $175, a deep cell battery can have a range in price but one similar to this unit will be in the $200 range, the battery tender is going to be $45, a 100 amp circuit breaker about $25, and probably another $25 for the required cables: so all told you are looking at around $500 U.S. for a DIY unit like this. If you are Canadian, then you already know that you can add at least 35% more to this.  It really is that simple to set up a very basic power supply. We have added a trickle charger, a common household storage tub, and a piece of wood. Access to a full-scale battery charger and a generator or household power is required should you drain the deep cell battery right down. Or a solar panel if you intend to stay in an off-grid scenario for a longer time span.

pure sine
Pure Sine Wave inverters cost more to purchase, but the technology is better for your equipment than the inexpensive modified wave inverter.

The $50 price tag that you will see on some 1000 watt inverter that you will find may look like a great deal, but remember the old adage that you get what you pay for. That inexpensive unit will be a modified sine wave inverter and may damage some of your equipment. As a layperson, my description of the modified sine wave versus the pure sine wave is this. Modified inverters provide a electricity in a rather chunky form that bounces around. Possibly like waves created by 3 or 4 different sized motorboats – even though the boats are doing circles at nearly the same speed, the boats create different sized waves. The waves all land on the beach but not at all equally.  Modified wave inverters work but your equipment has to work a bit harder and, if it is sensitive like your portable hand tools, it could damage them. Pure Sine Wave inverters, on the other hand, have a nice smooth power wave. Soothing like being at the beach with gentle waves, naturally created, of equal size coming in over and over again. Talk to an engineer if you want a more detailed description about the differences between the two. Bottom line, if you run a lot of technology (or in our case medical equipment) then you will want to upgrade your power inverter to a pure sine wave technology versus modified wave technology. The extra cost of a pure sine wave inverter will probably be worth the investment in peace of mind as you know you can run anything without the worry of damage.

small battery
When light weight is more important than battery life, you can use a small battery that likely weighs a fifth of what the large deep cell battery weighs. This battery is about $50 and weighs just over 12 pounds.

The battery is the second thing to consider. How much power do you require? How portable are your needs? A small battery is going to be lighter and thus obviously more portable. The downside is that it may not provide power for a long enough duration. We have a small and thus lightweight battery that we could use for short-term scenarios. We have chosen to go with a Type 27 deep cell single battery for our purpose. Should we require even more ongoing power, we can add a second battery (or we can go extreme and add further pairs of batteries) that will cover our basic needs for several days. Make sure that the cables you use are a heavy enough gauge. We got pre-built 4 gauge wires that are more than enough for the short lengths we are using.

To facilitate moving about, we set everything up in a Rubbermaid container. There’s lots of room for the battery – a ton of room to add a second equivalent battery, in fact. Make sure that you get cables of the appropriate gauge – we used 4 gauge cables for the runs we have that are not even 16 inches long.


  1. Why didn’t you use a lithium ion Car battery for a light weight solution?
    Why not put everything on a movers dolly so you can wheel it around easier?

  2. Why not use an lithium ion car battery instead of the heavy deep cycle battery?
    Couldn’t you put the bin on top of a movers dolly to more easier roll the thing around instead of trying to carry it?

  3. Hi Davin x2,
    Great questions. The idea behind a deep cell battery is that it can handle the repeated draining and charging required in a setting like this. A Group 27 lithium ion battery, similar in capacity to the Group 27 battery used here, would cut the weight in half as they are just under 30 pounds. However, they cost in the neighbourhood of $1000 which is FIVE times the price of the battery used. Our choice was the best price to power ratio.

    A car battery is designed to provide a huge burst of power for a short time and then be recharged by the alternator. Drain it frequently and it’s life will be shortened. A deep cell battery is designed to provide steady current over a long period of time; drained right down without shortening their lifespan. It will not work well to start your engine as it doesn’t have the cranking power.

    The Group 27 is a large and obviously heavy battery and was specifically chosen for the lifespan and amount of ongoing power it would provide at the expense of weight. We knew it wouldn’t be portable. As mentioned in the article on page 3, we will be getting some castors for the case so that it can be rolled about more efficiently.

  4. Hi there,

    You mentioned solar panels. How would one go about hooking this up to solar? My son had SMA and we’d like to take him places next summer. This with solar would be perfect.

  5. Hi Mike, I don’t know a lot about solar power but it appears to me that many solar panels have the ability to charge batteries. I will be looking into this more eventually. Sorry I cannot give you more substantial answers but I just haven’t set this up myself.

  6. Quick crazy Question..
    could i adapt this as a in car solution to not use the existing car battery? how would i charge? i guess plug in the charger to either an alternator or in to a lighter socket? Is this possible?

    1. Hi Jeff, first sorry that I’m over a year late in responding. Second this is not a good idea as it would drain your car battery quickly thus leaving you stranded. There are low power inverter systems you can get that plug into your car lighter socket that would be a better solution – but they will drain the batter if the car is not running.

  7. 2.5 years later.

    Lemme think. A typical larger 12v lead acid deep cycle battery is 100ah (or 1200 wh).

    I think that lead acid batteries can use less of their total rating than a lithium battery (50% vs. 90%? something like that).

    A lithium backup battery setup complete with built-in charging stuff and inverter is something about $1 / wh, so for $500-$600 you could have roughly the same setup but much more portable,etc.

    I’m just not feeling the love here.

      1. Not in the case of lithium all-in-one emergency batteries.

        They’ve been at about $1 / wh all this time. It’s the sort of thing I keep track of living in a state that has regular power shutdowns in order to avoid fires. Gas generators are kind of a pain to listen to.

        I’m actually kind of surprised there haven’t been big price drops, perhaps the technology is currently at a sticking point with these sizes.

        Now, a really brilliant product would be to take an electric start generator, hang a good sized battery on the front of it, and cycle the generator to keep the battery full. That same battery could be used to start the gas motor when needed. All downstream power would be through the battery (and an inverter, a thing that better generators already have). There’s no point in having a gasoline engine roaring the whole time.

      2. as an aside, the best part of plumbing a battery between a gas generator and a power consumer is that the generator could be sized to average load rather than peak.

      3. Just as a data point, I bought a Polaroid PS600 (578wh) for $329 plus tax in October of 2019, and an Audew 220wh unit for $154 in Dec of 2018. Prices tend to jump up and down on these sorts of products.

        If you can build something like that out of a cobbled together lead-acid setup, that would be cool. One advantage of homemade is that the individual pieces are replaceable if something fails plus, if you go big on components, the system is expandable. Pretty soon you are building a whole-house battery backup which is a whole ‘nother thing.

  8. Lots of great ideas, William. For comparison, a PS600 in Canada is $1000 – if you can find one. My hope for the article was to provide some lower cost options for people who needed a short term solution. Sounds like you’ve got ideas for ongoing and long-term solutions! And I’m guessing that if you haven’t gone the whole house route, that this isn’t too far away!

  9. Your design does not include an auto-on for your inverter. It is possible for your inverter to be turned off (unintentionally perhaps) meaning no backup if your system is unattended.

    That remote switch on the Eliminator Inverter most likely has unused pins. Mine has. Two of those non-populated wire on the RJ-11 connector (black wire and white wire can host a fix SPST switch. I build a circuit that monitors the hydro and turns on the inverter is power has gone off. The circuit uses an electronic switch instead. I added a relay to switch the load from main to inverter power with hydro goes off. It returns to original when hydro comes back on. I like your choice of AGM batteries and multi-stage charger.

    1. Hi Ray, that is more complex than I needed but certainly extends the functionality. In my case, we are always present when running the inverter. And as it’s an RV that is usually ‘boondocking’, there’s no electrical connection to shore power when the inverter is in use. Thanks for making me think about possible expansion routes!

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