Author Topic: Hi guys! Need help choosing the right battery type for my solar powered project.  (Read 1512 times)

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Offline laban93

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It's my first post, sorry if it' s in the wrong place.
I've started a project where i'm powering a screen which draws around 260mA. I'm trying to make it solar powered, and therefore i need some batteries to power it during the night. It's supposed to be on 24/7.
I'm trying to place the batteries inside a 2 inch pole prefereably.
I'm quite new to this, but thinking  lithium ion 18560 batteries would be a possibilety.
Does anyone have some suggestions for me about which ones would fit my project?
-Simon

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Online splin

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The required battery capacity will depend on a) load, b) solar panel output and c) amount of solar insolation - in the north of the UK for example, the solar panel may, in exceptional circumstances, provide almost no energy for 6 consecutive days or more. An excellent resource to help estimate solar insolation at a given site, if you are located in Europe or Africa, is: http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php#

Select your location then select the 'Stand-alone PV' tab. Then you can try various combinations of panel and battery sizes. It is geared to large systems so multiply all your numbers by 1000. Assuming your load is 260mA at 3.3V, enter daily consumption as 20592Wh (260 x 3.3 x 24). Enter 0 for battery discharge cutoff limit (meaning 100% maximum discharge).

Now play with the numbers; I chose London for the location, 20000W (ie. 20W) for panel size, 3.6V, 8000Ah battery (ie. 8Ah) and 68 degrees for 'module inclination', 0 degrees for orientation (south facing). Clicking 'calculate' gives the result that the battery will become flat on 5% of days in January and December, thus a bigger battery and/or panel will be required. I chose 68 degrees for inclination because it gives the highest output in the winter months at that latitude. 90 degrees (verticle) isn't much worse and may stay cleaner. Increasing the battery to 11.5Ah with the same 20W panel results in the battery never becoming 100% discharged.

Select 'Show graphs' and 'Show horizons' for more information. You can provide your own horizon data for better results - anything which cast shadows on the panel, such as trees, hills, buildings etc. are generally a much bigger problem in the winter when the sun is low.

A bigger panel means you can get away with a smaller battery. The daily consumption should be increased to allow for battery charging and power supply inefficiencies - say by 25%. You also need to allow for ageing of the panel and dirt reducing its output by, say, 25%? And the battery capacity needs to be increased to allow for loss of capacity with age and temperature changes. I'd suggest that the battery capacity should be twice the calculated value. Remember that the tool is based on historic insolation data and the future may be rather different!

The biggest problem you might have is if it needs to operate at low temperatures - Lithium based cells must *not* be charged if the temperature drops below 0C which is usually in winter when you most need the battery. Ni-mh have the same problem. Ni-cd and lead acid are OK but their charge efficiency (especially lead acid) is much worse than lithium cells. Ni-cd are very rugged though and may well provide the longest service life if they are not overcharged or over-discharged.

If it is a critical application it might be a good idea to use 2 panels as one could get hit with bird poo/leafs/litter etc. - panels generally lose most of their output if only part of it is shaded.

Of course if it is a hobby project such that 100% availability is not necessary then it becomes rather easier.


 


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