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Something which goes hand in hand with small renewable energy systems is battery storage .If you're using DIY pedal generators as an introduction to renewable energy, it's a good idea to find out how you can get the best out of your storage batteries and avoid damaging them.

There are all sorts of clever batteries available now such as NiMH and Lithium-ion but the higher tech a battery is, the more fragile it is and the more complicated its charging procedure. Lead Acid batteries may be old fashioned, heavy and crude but they're a lot tougher and easier to understand than any of the more modern ones, as well as being easier to recycle when they finally pack up.
Most people have come across the ordinary car battery, often rated as something like 12V 40AH or so. The Amp-Hour figure is the total capacity of the battery in Amps times Hours - it means you could get up to one Amp for forty hours out of it running a light, or twenty Amps for two hours running a sound system through an invertor. It's also the maximum amount of energy it can store; once you've got 40AH of energy in, any more will be wasted if you carry on trying to charge it.
As the power is at twelve volts, 40 AH is about 480 Watt-Hours or about half a unit on your electricity meter. However it's nowhere near as simple as that. If you actually took all the energy out of a car battery, it would be damaged, and if you didn't charge it up again immediately it would never work again!
Car batteries are designed for use in cars where they very rarely get run down but need to be able to supply enormous current surges of hundreds of Amps to start the engine. If a car battery is regularly discharged to less than 80% capacity, it will be damaged and its capacity will go down. For a renewable energy system using wind or solar power where the energy will be collected first and then used, a Leisure Battery is more suitable. They cost a bit more and are sold for use in caravans. Leisure batteries can stand being taken down to 50% of their capacity, so you still need one twice as big as your maximum energy demand. The only batteries which can stand anything like a full discharge are Motive Power batteries used in things like golf carts and wheelchairs. They are very expensive (generally £100 or so for 40AH) so it's probably only worth getting secondhand ones for an entry-level system. A disadvantage of deep cycle batteries is that their internal leakage is higher than other types so if you've got a festival type system  you will need to remember to charge them a couple of times while they're stored over the winter. All types of Lead Acid batteries will last longer if they are kept as close to fully charged as possible. Cycling them generally produces little improvement in capacity, unlike a NiCad or NiMH.
One of the nice things about Lead Acids is that it's dead easy to find out how much charge is left in them. Unlike NiCads which give out the same voltage for ages and then suddenly drop to nothing, the open-circuit voltage of a Lead Acid shows the state of charge. The textbooks say you should disconnect it and wait for the voltage to stabilise but in practice just measuring the voltage at the terminals is OK as long as you're not taking loads of current out at the time.

These readings are for ordinary Leisure batteries - the ones for Motive Power batteries are a bit higher, for instance 13.0V for full charge.
Charging Lead Acid batteries is quite straightforward. Feeding current directly into the batteries works quite well for low rates of charge - for instance a 10W solar panel could be left connected to a 30AH battery through a diode and wouldn't do any damage once the battery was fully charged. For higher rates of charge the voltage needs to be limited by a regulator otherwise a fully charged battery will start to dump excess energy as heat and convert the water in it into hydrogen and oxygen. This isn't such a great problem with 'wet' batteries you can top up (with distilled water only! Don't buy it - get it when you defrost a fridge) but isn't a good idea with sealed ones. For a situation where the battery is continually charged and discharged a voltage of 14.4V will give you maximum stored energy, but if the battery is going to be left on charge such as a backup for an alarm or a permanent installation it's better to aim for about 13.6V.
Regulators work in different ways. For a wind generator or a combined wind/solar setup you need a 'shunt' regulator which connects across the current supply and turns on above a certain voltage, wasting excess energy as heat. Limiting the voltage will prevent a wind generator from over-revving. Car alternators work by reducing their own field current so that they actually generate less power as they approach the regulator voltage which is why they work well in pedal generators but aren't suitable for wind power. Some solar regulators are connected in series with the battery and reduce the current by increasing their resistance, reducing the thermal stress on the panels but they shouldn't be used with a wind generator.

Although a Lead-Acid battery system generally operates at low voltage, so there is virtually no shock hazard, even quite small batteries can give a tremendous current if shorted. The terminals, particularly the positive should be covered to protect against dropped metal objects. Any circuit should incorporate a fuse of the right size to protect the gauge of wire used. The fuse should be as close as possible to the positive terminal and ideally the cable between the terminal and the fuse should be double sleeved with tough insulation as it's the only unprotected part of the circuit.
The main things which can go wrong with batteries are loss of capacity and leakage. As a battery gets older and does more charge/discharge cycles, its capacity gradually reduces. There isn't really a quick way to test capacity. Ideally you need to charge it up fully so that it reads 12.7 Volts or so and then apply a load quite a bit smaller than the capacity rating, say less than a quarter, monitoring the voltage regularly. When the voltage drops to 12V, you will have used up 50-60% of the charge and the time taken will give you an idea of the capacity. For instance if you've got a secondhand battery rated at 10AH and discharge it at 2 Amps, finding that it reaches 12V after 2 hours, it's probably nearer 8AH. For Motive batteries you could discharge them further; also their terminal voltage often starts higher, say 13.0V for full charge. One reason for losing capacity can be reduction of effective plate area due to loss of electrolyte so you should keep it topped up with distilled water.
A battery in good condition should stay charged for months; if it loses more than half its capacity in a month it's probably on its last legs. Once it starts to seriously leak charge there's not much you can do about it. Obviously once a battery actually fails you should take it to the battery compound of your local recycling facility. Even small sealed batteries should not be put in general rubbish.