Mega Power Bank

Here's a super-sized power bank, able to keep all your gadgets charged many times over. It offers both USB charging and a 12V accessory socket - with a suitable inverter you can even power small mains devices. It also features a digital voltmeter, to allow the state of charge to be judged.

The design here includes a rugged, waterproof case, with a storage compartment for leads and accessories. It's ideal for a camping trip, particularly when paired with a solar panel for recharging.

The main components for the build are:

• An (empty!) ammunition box. The one I used was a .30 (7.62mm) calibre ammo box, measuring 25 x 8 x 18cm approximately. These are readily available from Army Surplus stores or a number of eBay sellers.
• A 12V sealed lead-acid (SLA) or valve-regulated lead acid (VRLA) battery. This needs to fit in your ammo box with plenty of height to spare. I used one measuring 15cm x 6.5cm x 9.5cm approx, which is a common sized replacement for UPS units. Typically these have a 7Ah or 9Ah capacity rating.
• Panel-mounting 12V socket, USB charger, and DC voltmeter set. These can be found on Amazon searching for "12V three hole panel". The three sockets are attached to a plastic mounting plate, but can be removed if required. In this design they are attached directly to the front panel and the mounting plate is not needed.
• A chunky toggle switch - this can be bought from many electronics suppliers, or a car or caravan accessory shop.

You'll also need:

• Wood for the front panel - this should be a decent (10-12mm) thickness as it's a structural part of the whole design - and a number of other offcuts of thin sheet plywood or similar.
• A short length of aluminium angle.
• Wire, solder, and battery terminal connectors (these are often supplied with the panel socket set).
• Thin foam for battery mounting - the stuff I used was saved packaging from a laptop case.
• Nuts, bolts, wood screws, and double-sided adhesive tape.

The SLA battery will sit on the base of the ammo box, held in place on three sides by the walls of the box. I cut some thin foam sheet to size and stuck it with double-sided tape to the walls. The fourth side of the battery is held in place with a piece of aluminium angle, cut to fit the width of the box. Drill a 5mm hole in the centre of the angle, then fix another piece of foam to its other face. Put the battery in position, hold the angle up against it, and mark through its hole onto the base of the box.

Drilling the hole through the base of the box is fairly straightforward if you have a Dremel or similar small drill which will go in the box. If not, you may need to mark the position with a centre punch, then drill through from the other side. On my box the top lid was detachable, which made it easier to handle during assembly.

If all goes well you can attach the aluminium angle to the base with an M4 bolt and Nylok nut. Make sure it's good and tight as it has to restrain the heavy battery.

The front panel is a piece of softwood cut to fit inside the ammo box. The sides of the box are slightly tapered, and the corners are rounded, so I found it easiest to cut out a template from cardboard first and trim it for the best fit, then draw round it on to the wood. I rough-cut it using a power saw then shaped it, first with a surform and finally a power sander.

The final width of the panel is a balance between keeping a comfortable spacing between the sockets, and leaving a good width for the storage compartment. On my box it came out as 175mm, as measured along the centre line.

Next, cut holes for the three 12V sockets and the switch. I used a 30mm hole saw and a 12mm wood bit, respectively. The spacing between each of the hole centres came out at 42mm. I also needed to chisel out a rebate on the reverse of the panel to allow the switch to poke through sufficiently.

When the wood is cut, it can be finished as you fancy. A medium wood dye and a couple of coats of glossy yacht varnish will give it a rather nice 'vintage car dashboard' look.

When the woodwork is ready, the power sockets can be mounted on the panel, then wired up. I decided to leave the 12V accessory socket unswitched (connected directly to the battery), and the USB charger and voltmeter connected via the switch. They have a small (40-50mA) power drain when on, so will discharge the battery gradually if not switched off.

When finished, the wiring looked as shown in the photo. I put heat-shrink sleeving on the finished joints to provide mechanical support and avoid exposing any 'live' conductors should any stray bits of metal find their way inside. You can briefly test the wiring by connecting up the battery, before further assembly,

The heavy battery needs to be held securely in place to prevent vertical movement. This is done with a wooden 'channel' which sits on the top of the battery and is held down by the front panel. The channel is made with a small piece of 6mm ply, cut to the width of the battery, and two wooden offcuts (25 x 12 x 150mm approx) for the sides. These sides need to be tall enough to allow clearance for the front panel sockets and their wiring.

The sides have a step cut in one end to leave a gap for the battery terminals. They are glued to the ply, and held in place with a couple of panel pins. (I made sure the heads of the pins were punched below the surface so they didn't scratch into the top of the battery).

When this is done, the channel piece can be placed on top of the battery, and the front panel is then put in place above it.

The front panel is held in place with four screws, two on each side. You'll need to drill holes in the ammo box at the right position to hold the front panel in place - I found this easiest to do by measuring the distance from the top edge to the face of the panel (see photo), then adding on half the thickness of the wood. Position the mounting holes so that the screws avoid the the DC sockets when fully inserted.

Finally, you need to cut a piece of thin ply to form the side of the storage compartment. It can be held on to the right-hand edge of the front panel with a couple of veneer pins. When this is done, the panel can be pushed into place and the mounting screws put in.

Lead-acid batteries are fairly forgiving (compared to other battery types) of electrical abuse, but their performance and lifetime will be improved if treated properly. I'm using a proper microprocessor-controlled charger to recharge the power bank - this has a multi-stage charging cycle, finishing with a 'float' stage which holds the battery fully-charged indefinitely without damaging it,

The voltmeter can be used to judge the state of charge of the battery. During discharge the voltage will drop from 12.5V or so to under 11V - it's not recommended to discharge below about 10.5V as a deep discharge can permanently damage the cells.

You can use a solar panel to recharge the battery, but note this will usually be a 'top-up' rather than a full charge - a 5W panel will charge at 0.3 amps or so in full sunshine, which will need 25 to 30 hours to deliver a full charge. Don't leave a solar panel permanently connected if the voltage is capable of exceeding 13.8V or so.