For a while I’ve been keen to build a solar powered charger for my phone and iPad, mainly because I wanted to have a play with solar power – something I haven’t done before. It certainly wasn’t to save money. I think I calculated that it would take several lifetimes of daily device charging to pay off the investment in parts. Anyway, I recently completed the project and thought I might showcase it here. The internals aren’t that interesting, being all off-the-shelf parts, but I was particularly satisfied with the way the external design worked out. It’s a style I have dubbed “atomic punk” – a sort of steam punk variant with a 1950-60s technical aesthetic. Let’s go straight to a photo of the finished charger in action.
The coiled black lightning cable is an essential part of the aesthetic.
The case is made from plywood scraps, and is finished with brushed on “Hammerite” hammered silver finish paint to give it that 50’s technical equipment look (note that before painting with Hammerite I sealed the plywood with a coat of water based varnish). The top panel and sloping instrument panel are removable to allow access to the battery and wiring. The instrument panel has two analog meters, one indicating battery voltage, and one indicating the current being drawn by externally connected loads. It also has a 5A fuse (connected between the battery and the load), a power switch and indicator lamp (filament of course). I really wanted a nice 60’s era coloured glass-bezel panel lamp, but I couldn’t find one. The switch switches the load to the battery, and the lamp indicates when load power is available.
The power conversion and USB interface circuitry necessary for charging phones and iDevices is provided by two off-the-shelf 10W Belkin USB car chargers. These are plugged in to a twin car accessory socket mounted to the top panel. I actually started out by building my own USB interface with a small buck converter to provide the step down from 12V to 5V, and circuitry to provide the appropriate voltages on the USB data lines as required when charging Apple devices (2.7V on both data lines lines – as measured on a recent 10W Apple charger). It worked, but I wasn’t sure how to neatly integrate the USB sockets, and my circuitry was large and unweildy compared with the neat and compact off-the-shelf car chargers. So in the end I gave up and went with that solution.
On the inside, the box contains a 9Ah, 12V sealed lead-acid (SLA) battery connected to a Genasun GV-5 MPPT charge controller. Also connected to the controller is a lead running to a 20W monocrystalline solar panel (max. power at 17.2V / 1.17A). The panel is actually mounted flat on a carport roof, but seems to easily provide enough energy to keep up with daily phone and ipad top-ups (at least in summer – we’ll have to see what happens in winter).
The photo above shows the meter readings when charging an iPad at maximum rate. The ammeter was reading about 50mA high at this time, and the light bulb consumes about 70mA, so we are seeing an energy draw by the USB charger of about 0.88A x 13.5V = 11.9W. I haven’t measured the actual current draw on the 5V side of the charger, but the Belkin units are rated at 2.1A at 5V, or 10.5W, so that’s not a bad conversion efficiency. With an iPhone and iPad plugged in, the maximum 12V side current draw is about 1.5A. If the two devices are topped up daily from say 75% battery, this level of current draw is required for maybe up to an hour, after which it tails off to zero as the device batteries reach full capacity. Looking at the discharge rate data for the SLA battery (see graph below), at a 1.5A discharge rate the battery will be flat in about 5 hours, assuming no input from the solar panel (e.g. charging at night). However, we don’t want to completely drain the battery, ideally we only want to take it down to about 70% capacity. This would happen after about 1.5 hours of discharging at 1.5A, which is just about perfect for its intended duty.
So that’s it, the “Fusionmaster Deluxe” nuclear fusion powered device charger. I hope you enjoyed reading about it.