This is a self-build project by Bart PA7ELF, this information is intended as inspiration for designing and building your own 12 Volt battery box.

Every design starts with an idea and the idea for this battery box came bubbling up when I was fitting an automatic battery charger and a 12 Volt to 230 Volt inverter with PowerPole connection. “How handy would it be to have a battery box that I could plug these and other equipment into?” This idea soon crystallised into the following specification list.

The ready-made battery box

• Multiple 12 Volt PowerPole and banana connections.
• Ability to deliver high currents of up to 45 amps per individual output.
• All connections individually fused with a user-selectable fuse
• A consumption and charging current indicator
• And finally, everything as cheap and simple as possible

I deliberately choose only 12 Volt outputs, there are no USB connections at all on my battery box the idea behind this is; keep the design simple and modular. For charging a smartphone, I can easily use a separate 12 to 5 Volt stepdown converter with USB connection, which I plug into from the 12 Volt sockets.

• Being able to power the central heating boiler via my 12 Volt inverter, which comes in handy should the mains power ever fail for an extended period. I can then still min house heater, tap hot water and take hot showers
• Provide 12-volt power supply to transceivers when there is no mains connection available.
• As a power supply for 12 Volt hand tools when I'm doing odd jobs in the garden.

As a battery box is not a 'must have' for me, I would like to keep it as cheap and simple as possible, without compromising on safety.

Battery box with charger and inverter

It works!

Here is a list of all the components I used to build my battery box.

To power my solar garden lights, I had got some old 12 Volt car batteries from my garage. These batteries are no longer reliable enough to use for starting a car, but are still good enough for other purposes. The smallest battery I have has a capacity of 40 Ah. Sufficient capacity and still somewhat liftable in terms of weight.

Battery mounted on the bottom plate

In the attic, I still had an old plastic filing case for hanging files lying around. This case is tall and quite narrow and has a fold-out handle, a nice good portable form factor The 40 Ah battery I mention above turned out to fit well in this. A good and cheap start to my mobile battery case.

In my PowerPole mania, I had already fitted several transceivers and power supplies with PowerPole connectors. To build the PowerPole connectors into a cabinet, I use 3D contoured holders. I don't have a 3D printer myself, but a good friend of mine was immediately willing to print these holders for me.

PowerPole connection in holder

Besides PowerPole connectors, it is useful to have some banana plug connectors available as well. The 4 mm banana plug is still one the commonly used connections, not only for transceivers, but also for measuring equipment. By equipping the battery box with a number of banana plug connections, I can easily connect my Fluke DDM to monitor the battery voltage.

Speaking of measuring, it is also useful to measure the battery current. Mmmmm…. Since I would want to measure both battery load and charge current, a rotary coil meter with the zero position in the middle would be ideal. I once had one temporarily built into the charging system of solar garden lights. This meter turned out to be ideal for me battery box. Not too small and with a scale division of + and - 50 uA. Using a piece of copper wire as a measuring shunt and some experimentation, I made this meter suitable for measuring + and - 50 amps. This meter sits between the battery and the fuse holder and thus measures the total consumption or charging current.

For measuring consumption-charging current, I deliberately chose a rotary coil meter rather than a digital meter. A rotary coil meter has the great advantage that it does not consume energy at rest, while a digital meter does at rest. Choosing a rotary coil meter eliminates the need for an on/off switch.

When shopping for a cheap and solid fuse holder, I saw a fuse holder like the ones used in cars, which can hold six plug-in fuses. In addition, this fuse holder has a distribution option for the earth. This makes wiring with thick wire simple, safe and clear. With this fuse holder, I have access to six independently fused outputs, so can connect six separate connections to this fuse holder. As I have fitted many of my devices with PowerPole connectors, I have chosen four PowerPole connections and twice plus and minus banana plug connection.

To wire the PowerPole connectors and the battery connection, I useAWG 10 (5.26 mm3)silicon wire. This is about the thickest wire you can use in the 45 amp PowerPole terminals.

For the banana plug terminals, I use AWG 12 (3.31 mm3)silicon wire. This diamater is just barely solderable to the banana terminals used.

To connect the fuse holder, I use Faston crimp terminals.

The 45-amp PowerPole outputs are fused with 50-amp fuses, so the connections can handle peak loads. It takes some searching, but these can be bought online.

As I wired the banana plug connections with AWG 12, these are fused with 30 Ampere fuses. This is still more than enough for a 100-watt HF transceiver, whose peak current at full output power is around 20 Ampere.

To distribute the weight of the battery over the bottom of the plastic archive case, I made a bottom plate from an old piece of plywood, to which I secured the battery with beams. This plate rests on rubber tape that I taped to the bottom of the archive case. This prevents mechanical peak loads from the bottom plate on the plastic bottom of the archive case.

On the short sides of the battery, I mounted two bars with two metal eyes at the top where a rope is tied. These eyes are above the battery's centre of gravity so that the battery does not tip over when I lift it complete with bottom plate out of the archive case.

Battery on bottom plate, the tides are above the battery's centre of gravity

I mounted the fuse holder and all connectors in the lid, this allows the wiring to the connectors to be kept short, which still keeps the voltage drop in the wiring at high currents low. After drilling the necessary holes, mounting the connectors and gluing a board for the fuse holder, it was time for wiring. All connections are crimp connections except for the banana plug connectors. This simplified the wiring considerably, with the only drawback being that my fingers cramped a bit after wiring

Ready to go wiring

Minus wires of the PowerPole connectors mounted

Plus wires of the PowerPole connectors mounted

Wiring for the banana jacks mounted

Wiring for ammeter and battery wiring fitted

Wiring connected to the battery

I am very happy with the result and of course had to test it with charger and inverter and a 100-watt light bulb. With only the inverter, the battery supplies 10 amps to the inverter and as soon as I connect the charger, the current from the battery drops to about zero. Once the charger is connected, it takes care of the load current and the battery remains unloaded. With this, I have de facto built a modular UPS

Battery box inside

Outer battery box

It is important to make vent holes in a battery box if you use a non-hermetically sealed lead-acid battery. Through the overpressure valve of a standard car battery, hydrogen and oxygen can be released which is a flammable combination. I have drilled vent holes in my battery tray in two places. At the bottom at the back of the container and all around in the lid. Hydrogen is lighter than air and so can escape through the high-set holes in the lid, while fresh air is drawn into the bin through the low-set holes in the bottom.

My battery bin has no main switch simply because I do not have a switch available that is suitable for a current of 50 amps. A main switch is also not really necessary. The specification of PowerPole connectors states that the connection may be safely broken under the full load of 45 Ampere. The better track connector will certainly survive when breaking a current of 30 Ampere as well. This removes the need for a main switch. Remember that a battery in a car does not have one either.

All in all, I am very satisfied with this project. The result is solid and simple, without bells and whistles, has sufficient connections and via the rotary coil meter I can keep an eye on the consumption or charging current.