Links to all my notes (includes mini-projects and source code etc)
(+) 0001 How do I change the Pi System partition size ?
(+) 0100 Device tree settings
(+) 0201 Power over Ethernet - (PoE)
(-) 0202 Solar Power - (with WiFi)
Using Solar power
The problem we have is exactly the same as that confronting the Solar Power industry. Solar will run the kit just fine when it's generating power during daylight, however we also need to power the kit at night (especially if it's for a CCTV application) :-) The only solution is to install at least 'double the required capacity' of Solar and use the 'second' half to charge up batteries during the daylight hours so the Pi can run from battery power at night. So what's required ? The Model A Pi uses 'at least' 500mA, however running at 90% CPU capacity (and adding the Pi camera and a WiFi 'dongle') a 'reasonable' assumption is that we need 1A. The supply voltage will need to be 5.2v (if we want the USB WiFi dongle to work reliably). The battery If we assume 14 hrs of darkness, and need to deliver 1A, then the battery capacity required is 14 * 1 = 14 Ampere-hours, which is way beyond 'normal' NiMH AA/C/D cell capacities (which are in the 1-3 AHr region). If we use a low-drop-out (LDO) regulator, it should be possible to deliver 5.2v from a standard 6v 'lead acid' rechargeable battery - and a sealed 6v, 12 AHr 'lead acid' type can be had for less than £20 from CPC. An alternative is the LiION based 'USB recharger pack'. These are now very competitively priced and can deliver capacities up to 10AHr (10,000 mA-Hr) at well under 1/4 the cost of 'lead acid' battery. The solar panel The Solar Panel will need to deliver approx 2.5A (1A to run the Pi, 1.5A to fully charge the battery in 10 Hrs, assuming no losses). Panels are rated in 'watts', so 6v * 2.5A = 15w. Unfortunately, this will cost rather more than the battery. A 15w unit is approx £88 from CPC, and, at, 3ft x 1ft, it's not exactly 'inconspicuous'. To use this we will need to add a rather clever battery charging 'control' unit (the solar panel o/p voltage is 17v, and we can't just 'throw away' the unwanted volts since that would also throw away 66% of the watts). It should be possible to 'adapt' a DIY DC-DC 'constant current' battery charging regulator circuit, however we should allow at least £5 for that. The cost Solar power is, of course, hugely expensive compared to almost anything else. Compared to PoE, a Solar solution will cost you at least £100, whilst PoE comes in at under £20. However Solar does have two advantages over PoE. First the built-in battery pack means you don't need to worry about mains power loss (for PoE you would have to add another £10 or so for a separate 'UPS' circuit) and second you can site a solar powered Pi camera in places where cables can't reach. One disadvantage of Solar + WiFi is that the WiFi will limit the distance more than PoE - only if you are very lucky (direct 'line of sight' to the Router with no obstructions - such as people) will you get anything like 30m from WiFi, whilst a DIY PoE cable will reach at least 50m.
This note last modified: 29th May 2018 14:36.