For any stand-alone renewable energy system, (where electrical input is dependant on slightly random variables e.g. wind/sun/water etc), the large bank of storage batteries MUST be genuine deep-cycle cells. This does NOT include cheaper car or 'leisure' batteries which simply will not last very long.
The problem is that with a stand-alone system the battery bank will be drained (or cycled) down to a low state of charge regularly - thus it is essential that the battery will not be damaged by this cycling effect. Deep-cycle batteries are specially designed to cope with this, where-as car and 'leisure' batteries are not (fork-lift batteries are also suitable).
The result is that you will have to replace the battery FAR sooner if deep-cycle cells are not used, and the overall long term expense will be much greater (this doesn't mean that you can't experiment and have fun with second hand batteries, but don't expect a lifetime out of them).
Most storage batteries are of a Lead-Acid content - they have thick lead electrolytic plates surrounded by sulphuric acid. These cells come in all shapes and sizes - usually individual 2 volt cells or 6 volt triple cells. These cells progressively deteriorate both with age and more importantly with regular deep discharging. However it is necessary to discharge the cells in order to use them as storage, the point is to minimise the effect that this causes. This involves buying quality batteries and looking after them appropriately (see below).
We currently use Trojan and Hawker cells, as we believe they are high quality, tested cells available at a reasonable price (Trojan are the number one deep-cycle battery in the US, used extensively by the renewables industry, and Hawker have been making quality fork-lift cells for decades). The storage battery is an expensive part of the installation and although better cells are available, the added expense of these is not really justified in most cases.
In order to look after your batteries properly you need to be constantly aware of their state of charge (SOC) - just like checking how much petrol is in your car. An accurate SOC 'fuel' gauge is available (£160) which monitors exactly how much charge goes in and comes out of the batteries and displays exactly what your SOC is (they can also sound an alarm or switch on a generator at a programmed SOC - e.g. 50%).
A simple glance at the SOC guage once a day will let you know whether you have plenty of spare power (washing day!) or whether you are low on power (reduce your load). Often charge controllers
and inverters use green and red LED lights to show the state of the battery but these are inaccurate guages and no substitute - they simply look at the voltage not the SOC.
Inevitably batteries will get deeply discharged to 30 or 40% SOC from time to time (when you use extra power or if its not very windy or sunny for several days). Some of the damage that this causes can be rectified by regular equalisation charging of the cells - this is when you slightly over-charge them by raising the voltage above normal, causeing them to 'gas' and bubble, which has the effect of cleaning the built up sulphur off the plates. Normally you should perform an equalisation charge once per month for two hours, and most inverters will do this either automatically or at a push of a button.
