Battery cart for outdoor gigs and emergency power

I call it the Cart of Theseus – a battery cart for mobile PA (camping, busking, etc)

I built this battery cart out of an old IKEA storage box. It had been sitting around by the trash waiting to either get used or thrown out for ages and it turned out to be roughly the right dimensions for what I wanted to do.

The other band members and audience members and casual passersby are impressed and thrilled about it.

And if you’re looking for ways to get outdoor gigs and put musicians together especially in the times of Coronavirus, this puts you right at the heart of the action.

You could also use it for emergency backup power for your house or host your own dance party on a camping trip. 

IKEA item used:

• Some kind of black storage box on wheels from 10 years ago. (Either bedroom or office organization.) Most likely SNACK.

Other materials and tools:

• 2 6-volt 236-amphr golf cart batteries
• 2′ AWS #2 cables, two sets
• One rapid charger
• One battery tender charger for downtime
• 1500-watt 3k peak pure sine wave inverter with 15 amp 120v output
• Table saw
• Sawzall
• Electric drill
• Work surface with clamps 
• Set of (4) 255lb capacity casters
• 16 5/8 3″ carriage screws
• 2×4 wood, 1×1 wood
• Gorilla tape
• Hinges
• Hardware, various

Building a battery cart

First, decide how you want to mount the inverter in the box to go alongside the batteries.

Inverter

I chose horizontal mounting and cut the box to accommodate. Your mobile battery cart may look totally different. Mine has changed a couple of times, hence the name. 

Use Sawzall to remove sections of box not needed. Save the edge brackets to re-use on newly placed corners. 

Reinforce newly shaped box edges with screws and saved end-brackets from cuttings.

First fitting, without lid

Build 2×4 / 1×4 frame inside IKEA box to withstand 150 lbs of battery and an extra 75 lbs of gear to roll on top for one-trip mobile PA juice and equipment from car or home to location.  

Reinforce the interior of the box with 1×4 and 2×4 pieces to hold the weight and not fall apart. Measure to insure batteries will fit snugly but not be cramped.

Change out wheels

I initially tried using the casters that came with it, but they fell apart 200 ft out of the car. They were probably rated for about 25 lbs of weight and this behemoth came to roughly 2200-225 with the batteries, inverter, and music gear on top.

You’ll have to use a bunch of wood screws and a table saw to cut the right pieces for the frame.

With closed top

Also, be sure to place the frame in such way that you can screw into it with the carriage screws and secure the big casters. This is something I may need to revisit later as not all the screwpoints are seated properly. 

Build the shelf for the inverter

I used a couple pieces from the cut-away outer edge of the box to make the shelf. 

Mount the inverter. 

Put the batteries in. 

Wire the two  6-volt golf cart batteries in series to bring the voltage up to the required 12 volts for the 12-volt inverter to bump to 120v. Positive terminal of one battery to the negative terminal of the other battery.

Open, with inverter in place

Then when placed in the cart, negative terminal of the first battery to the negative terminal of the inverter. Positive terminal of opposite battery to the positive terminal of the inverter.

This way, the two 6-volt batteries are operating as 1 single 12-volt battery into the inverter and they have 236 amp hours of energy to offer ~ 4 hours at 1kw output!!

How long did it take to hack this?

Initial time spent on this hack was one evening. However, there were a fair number of learnings along the way as I needed to replace the casters and extend the inverter shelf further out when one of the caster screw holes popped through the bottom of the 2×4 and I had to tape/cushion it from the battery. All in all, probably 8-12 hours. 

What do you like most about this hack? What’s the hardest part?

What I liked most about the hack is the fact that although most of the box is still there, it’s doing very little apart from providing the initial framework for sizing: ie- there’s almost zero structural integrity provided by the box itself.

Although the box is still present, it’s really just a starting point. 

Hardest part: Doing the calculations to be sure I could provide enough power to service the amps in my band. 

What to pay special attention to?

Pay special attention to the calculations. Consult electrical professionals. There is a battery store somewhere in a strip mall not too far from you. Explain what you want to if your wattage and consumption needs are different from mine.

Every time you work with batteries, wear gloves and safety goggles and have some baking soda at hand in case you find yourself at some point down the line dealing with a sulfuric acid leak. Baking soda is a base and will neutralize it.

Also pay special attention to the type of inverter you buy. It has to provide enough regular and peak wattage to service your battery load and be the right voltage to and from.

Special note on the inverter

Also, your inverter should be a pure sine wave inverter. If not, you’ll risk having inconsistent power to your amps. Modified and other sine wave type inverters are fine for power tools and things, but this is for sensitive musical equipment and people will be listening.

Be sure to add up the wattage consumption of the amps / PA you want to power. That’s the first step. Remember that the wattage consumption is different from the wattage delivered. The wattage delivered is the wattage your amp / PA is advertised. The consumption will be listed on the back of the unit near the power plug. Unless you are Lemmy and Motörhead, you probably won’t max out that number.

Also, be sure to grab a battery hoisting strap which they generally give you when they sell you the batteries and carry a crescent wrench to connect and disconnect the cables from the batteries because when you transport the box in a car or something. You’ll need to disconnect / connect or else you won’t be able to lift the cart into / out of your car. I think that’s about it.  

What would you have done differently?

Looking back, I might have just done something cheaper and lower maintenance in terms of the cart and dealt with the consumer guilt of throwing the box away. I could have just grabbed the dimensions from the box and tried making it out of a plastic bin; but it’s fine this way.  

~ by Seth Schoenfeld

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