5W solar panel and charger sanity check - Page 1
Forum section: electronics / projects

author: fcb
author_rank: Super Contributor
timestamp: December 17, 2014, 12:53:51 pm
content: I've got to power a device from a solar panel.Device: <20mA @ 3.6v (averaged over a 24 hour period).Solar panel: 5W (BP Solar / Ameresco SX305) - 12V nominal (http://www.amerescosolar.com/sx305-bp-5w-0).Battery: 3.6v Li-Ion 1S3P pack (7.6Ah)My idea is to have the solar cell charge a fairly large capacitor, and then every-so-often drive a buck converter to charge the battery - probably under microcontroller control.So something like:1. Wait until cap voltage is at 12.00V2. Start the buck converter3. Switch off the buck converter when the cap hits 10.00V4. repeat until Vbatt=4.2VThe outside temperature will be from -20 to +45C, and I'll be able to monitor the battery temperature to make charging algorithm corrections. The reason it needs such a large battery is to allow for upto 14 days operation under NO SUN conditions.Comments/critique welcome - especially from those with experience in solar!!!

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: December 17, 2014, 01:19:28 pm
content: Check TI battery management and charger circuit page, a lot of information will help you to choose the right IC for that

author: fcb
author_rank: Super Contributor
timestamp: December 17, 2014, 02:11:42 pm
content: Thanks hamdi.tn,I'm considering the bq24650 from TI already as one of my fall-backs, the maximum power point tracking is attractive (although a little limited in the bq24650 - but probably better than what I was considering).

author: Seekonk
author_rank: Super Contributor
timestamp: December 17, 2014, 02:34:52 pm
content: I've used those cheap LM2596 modules to operate 12V solar panels at power point (17-18V). Operating at 12V-10V would be a big mistake because you would loose 30% of the possible power. I lift the #5 pin (ON/OFF) from common which is quite fast. A simple transistor zener switch will easily operate it at a manually set power point. For best operation make sure the panel capacitor is at least 1000uF.

author: splin
author_rank: Frequent Contributor
timestamp: December 17, 2014, 05:04:23 pm
content: Where in the world is this going to be deployed? If it's in Europe or Africa, there's a good tool to help you determine suitable panel and battery sizes based on historical solar insolation data:http://re.jrc.ec.europa.eu/pvgis/Select the location on one of the maps and then select the 'Stand-alone PV' tab. As an example, I tried Copenhagen and put in your values and selected battery discharge cutoff limit of 10% (ie. maximum discharge uses 90% of available capacity). The results estimate that on 4% of days in January and 28% of days in December, the battery becomes empty. I had to increase the battery to 12.8Ah to get a result where the battery never drops below 10% capacity.I don't know much about the way this tool works but I'd expect you'd need to consider the possibility that to cope with exceptionally bad years, then a rather larger solar panel or the battery will be needed - assuming that its important that your equipment never runs out of power. I wouldn't like to guess what that factor should be though.Also consider that the output of the panel will reduce gradually over the years (typical guarantees are less than 20% loss over 25 years). Any shading has a big effect on crystalline panels - the output current drops to that of the lowest illuminated cell in the panel (assuming it only has one string of cells) so even the shadow from an antenna for example could cut output by 50%.Dirt on the panel will also reduce output and bird poo might reduce your 5W panel to 1W or less - I'd experiment with a panel to see how much loss you need to allow for. Thin film panels, although less efficient, are much more tolerant to shading and might be a better choice. Alternatively, using several smaller panels in parallel may provide more resilience. Mounting the panels vertically or even slightly downwards facing might help significantly but will provide a bit less output (but might not be cleaned by rainfall as effectively). The tool allows you to specify the inclination so you can experiment.If this is required to be in service for many years then you also need to factor in the loss of capacity of the battery. In this application the battery will be near to or fully charged for the majority of the time. Li-ion cells lifetime is longest when kept at around 50% charge (3.92V). 100% discharge also reduces cell lifetime, though in this application discharges below 50% should be rare and 100% should never happen, but I'd still size the battery for a maximum discharge of 90%.The biggest problem of all though is that standard Li-ion cells should not be charged below 0C, so you'll need to find a type specified to be chargeable at -20C (if they exist) or use another type of battery - most Ni-mh and Ni-cd cells can be charged below 0C but the charge rate must be restricted to .1C or less which will probably require at least a 14Ah battery - 10 x 5W/3.6V. (Some of the 5W panel will be lost to inefficiencies but at -20C its output will be higher than 5W. At -20C there's also a chance that the area will be snow covered and thus a lot of reflected light will increase output). You'll need to research this though.Of course if this is in Greece or California then there's enough sun to make most of the above irrelevant.Good luck!

author: fcb
author_rank: Super Contributor
timestamp: December 17, 2014, 07:37:02 pm
content: Thanks for the lengthy posting splin. Great link BTW - I'm been fiddling with it for the last half-hour.It will be used in the UK initially, I will probably have to add another panel or a larger panel if it makes it to the Nordic countries.I do have the option to increase the battery pack up to something like 11Ah using different cells (26650's from a different manufacturer). I'm also looking at doing more work on the power management during the night (when the unit has a lower chance of being active) - reducing the average power consumption somewhat.I'm still looking to roll my own charging circuit with MPPT (maximum power-point tracking), current thinking on this is now to use a high-efficiency (low Iq) buck converter with with a microcontroller setting the output current (charging the battery) and monitoring the solar panel output voltage. A simple algorithm will then increase/decrease the charge current to extract the maximum power from the panel.The peak current used by the device is in the 1-2A region for brief periods, will be interesting to see how the battery holds up at low temperatures.

author: Seekonk
author_rank: Super Contributor
timestamp: December 17, 2014, 09:36:17 pm
content: It was interesting when I read through the bq24650 data sheet that it doesn't really track (calculate maximum power from looking at current) it just approximates power point from looking at panel temp through external sensor. At my camp I have it at a fixed voltage since I am only there in one season. A charge controller is quite easy to build from a micro.

author: splin
author_rank: Frequent Contributor
timestamp: December 17, 2014, 11:16:06 pm
content: Glad that helped. I had a quick look at the PV tool's help - to allow for charger and battery losses it assumes that overall efficiency is 70% which seems reasonable for lead acid batteries, but perhaps a bit high for Li-ion. The latter have much better charge efficiency of 95% or more; along with an 87% MPPT charge controller that gives you 82% so 12% more than the tool estimates.I'd try and optimize the charger efficiency for those typical winter overcast days that can last for long periods here in the UK. On very dull days there will be b*gger-all charge anyway so losing half to losses won't make much difference. On sunny days the chances are that the battery will become fully charged so most of the panel output after that point will be unusable so the full load efficiency probably isn't so important.How are you going to deal with the problem of charging below 0C? You can get long spells of low temperature; although the temperature typically rises in the day you'll have to wait for the battery to warm up enough before you can start to charge it. Be careful measuring the battery temperature - surface temperature might not be a reliable indicator of temperature at its core due to thermal gradients/heat capacity when it's warming up.If the panel shades the enclosure the batteries won't warm up as quickly from direct sunlight (although that is an advantage on hot summer days as Li-ion cells don't like high temps either.Insulation probably won't help - if you had enough to keep the batteries 20C above ambient with your 72mW of load, then it will be 20C above ambient in summer as well - so up to 60C which is a bit high. If you insulate the battery alone then it simply slows the rate at which it warms up on a winter's day.I'd be very interested to know what temperature ranges are actually encountered (rather than specified in requirement specs) in small un-ventilated enclosures exposed to the sun - particularly in the UK. And without access to thermal modelling tools how much difference would 72mW of internal dissipation make?