What size battery?
Basically I aimed to be able to run off batteries, assuming no solar input, for 3 days. No particular reason but I imagined that three days of really bad weather, parking in the shade or whatever would be the most likely where I am going.
Basically the first thing you need to know is that you must not run your batteries down under 50% of charge. This essentially shortens their useful life or, if you run them right down, can destroy their ability to hold charge.
The size of the battery you need essentially depends on what you will be running off it. To work out how much power you will need you just find out out the Watts for each appliance and how long you will use it for.
A simple example: LED lighting array = 5Watts Hours used = 6 hours per day
Watt hours = 5 x 6 = 30
To convert this in to Amp Hours (which the battery is specified in) just divide it by the voltage of your system; which will probably be 12V.
Therefore; 30/12 = 2.5 Amp hours.
If you had a 100 Amp Hour battery, based on using no more than 50% of charge:
(100 x 0.5) / 2.5 = 20 days.
If all you were running were these LEDs you could go 20 days without charging the battery. However you will be powering more than just some LEDs. Here is the calculations I used.
Hours of a day
LED Strip 3m
12v Cool Box
533 Watt Hours / 12V = 44.4 Amp Hours will be used per day
So if I want to be able to run for 3 days without any solar power coming in, I would need to have 44.4 x 3 = 133 Amp Hours.
Based on going no lower than 50% of charge I just multiply this by two. On this basis I will need 266 Amp Hours of battery.
I found a good deal on 140Amp Hour leisure batteries – so I got two of them. This gave me 280 Amp Hours in total.
What type of battery?
I went for two Sealed Leisure Batteries. Leisure batteries are have a longer discharge cycle to normal car batteries. Yes, they tend to be a bit more expensive but they are the right batteries for the job.
Personally, I would not bother considering non-sealed batteries. Lithium Ion batteries are super expensive – but also pretty cool if you have lots of cash to burn.
Keeping check on your batteries
Here is a useful table to tell you how charged your battery is based on it’s voltage.
Note that the voltage that displays on your voltmeter (or remote solar monitor) tends to be inaccurate in the following circumstances;
§ Your solar panel is pumping power into it (it will sometimes show as high as 14V in this case)
§ You are drawing power to lights or other things
§ You have been doing the above in the last 20minutes or so
It basically takes a little while for the battery to ‘settle’ so you can get an accurate idea of the charge.
Wiring the battery
Two ways to wire up your batteries. Series and parallel.
§ Parallel means you will use your two batteries at 12 Volts.
§ Series will double the voltage and the system will operate at 24 Volts.
Parallel battery wiring
I would generally suggest keeping to a 12V system. You can easily buy 12V lights adaptors and appliances.
To wire the battery to the solar charge controller, and other charging it is better to run across the two batteries not just one.
So drawing power, or inputting charge connect to the batteries as follows;
This means that the charge is added more evenly across the two batteries prolonging their combined life-time. Apparently this works!