Battery Sizing:

As I learned, you could implement an electricity system with just one small battery; but it is likely that it would hold insufficient storage to do you any good. Also, at the other end of the scale, you could keep adding more batteries; the flaw here is that you may have too much power and never discharge the batteries to a sufficient degree to keep them in good condition. So the moral of the story is: size the system to suit your needs. You won't know what is the optimum size until you begin to draw power from it though. In depth of discharge (DOD) terms, the system should be designed to discharge up to 50% of your battery capacity, with occasional discharges allowed into the 60-80% range. This was the reason I opted to add another battery shortly after purchasing the base system; this would ensure my store was adequate but still have enough power though the wind/solar combination to recharge them.

Battery Cabling:

The bottom two connectors from the controller are used to connect the batteries. For this I used 16mm stranded panel wire cable. I bought appx 15 feet of blue and red, and terminated these with cable lugs at the controller end. You can run the cable 2 ways: either cable from the controller to the first battery, and then from the first to the second, and so on. Or alternatively, run one long cable and create smaller cable spurs to each battery. Both methods have their advantages and disadvantages. The problem with linking from one battery to the other is that you cannot isolate a battery without shutting of power to another battery, while using the spur system increases overall cable length, requires more joins and adds to complexity. 

Battery Connectors:

On the subject of terminal connections, I firstly purchased car battery connectors from my local Halfords. Oddly enough, when I tried one it did not fit - only after it broke following my attempts to force it onto the battery did I realise that the positive terminal is LARGER than the negative terminal - consequently the connectors are sized accordingly. How stupid. I had foolishly tried to fit a negative connector onto a positive terminal! The good news was, following this mini catastrophe, I found heavy duty battery clamps from Maplin. They are branded Ojop Quick Release Battery Clamps. They are colour coded (good for idiots like me), they snap on and off as required and give a very secure connection, and the cover protects the terminals from coming into accidental contact with foreign objects. They cost EUR15 for each set, but given the price of the batteries it's a small amount extra. Here's a picture of the battery bank, showing the clamp connections in a spur configuration.

Grounding The System:

Although just outside the picture, the far right connector has additional earth cables coming from it. I ran 2 short run cables from here to the 2 inverters (more on this later) and also connected up the main earth cable which I had run into the store when setting up the turbine cables. So I now have 3 earth cables connected to the negative of this battery. They are all going to the same earth rod outside.

Battery Sizing and Usage Chart:

In order to work out what you can really run with the batteries, you need to identify the load requirement and then check how long that load will be available from the battery store. Lets begin with the basics: one battery in the picture above is quoted as having 270amps. That means it, when full, it would give you 1 amp for 270 hours. Lets put it another way. It will also give you 12 watts for 270 hours. Why? because the battery is operating at 12V, and volts * amps = watts. Thus 12 x 1 = 12. Or to put it yet another way, watts / volts = amps. Thus 12 / 12 = 1. I am certainly no electrical expert, and I don't really have a concept of amps. But I am happy with watts, so we will stick to those for the following calculations. After all, everyone is familiar with the 60 watt light bulb.

Here's a chart I have put together to try to explain the different power requirements and the power availability:

The above chart is based on having 4 270amp batteries. That's where the 1080 figure in column A comes from. However, in order to maintain battery longevity, we must base the figures on depleting to batteries to a maximum of 50%, hence the values in column B. Now, in real world terms, it is unlikely that you would be starting every day with batteries at 100% capacity. So I have assumed a value of 80% in column C to average out the good and bad days. Now to the interesting part: the watts required (column D) is the total of all the watts for the set of appliances you intend to run. I have started with 2000watts and shown the different requirements as we go down the scale. Taking the voltage of the batteries in column E, this gives us the amps required in column F by dividing the watts required by the volts. Then in column G we calculate how long this amount of watts would be available to us by dividing the amps in the battery (column C) by the amps required (column F). The final column H shows the likely power available which is 80% of column G, to allow for the inverter inefficiency. Inverters, as we shall see later, require power themselves to convert the battery power into mains power.

So in theory, using the chart above, I could run a computer, rated at 150watts, for over 27 hours ! But if I were to boil a kettle continuously I would deplete to 50% after only 2hours, assuming the kettle was rated at 2000watts and I keep refilling and boiling.

Other Factors Affecting Battery Usage:

I don't want to over complicate the logic by introducing Peukert's law here; this particular law states that, in reality, current draw at quicker rates will actually deplete the battery quicker, and at lower rates will deplete more slowly, thus giving longer running times. For example, if we applied the law to the highest current draw of 2000watts on our chart above, the available amps could be reduced by a third, assuming continual current, such that column C is reduced from 432 to 332. However, it could also extend the available current at the bottom of the scale, where the amp draw is minimal, and increase the 432 value to give us longer running times at 150watt power requirements. Then if you account for other factors such as current depth of discharge, ongoing charging, temperature, age of battery etc, it becomes very difficult to predict accurately. 

If I have calculated wrongly in the above table then please feel free to tell me so. My aim here is to produce an estimate of what can be run, and for how long. Without this you will need to take into account the battery capacity and measure the depletion based on the wattage and time used. I will obviously be redoing these tests later so we can compare the 'live' results with my estimates above.