How to choose a battery for off-grid applications
Unless you have a connection to the national grid, you will need to store the electricity produced by solar panels or wind turbines in batteries. This storage solution allows you to use the power harvested during the day whenever you wish.
So what type of batteries are best? Well, there are a number of types of rechargeable battery available. Lithium-ion or lithium-polymer batteries can store more energy for a given size of battery than any other type, and these are found in small, low-energy devices such as mobile phones. But the price would be prohibitive for storing large amounts of energy from renewable energy installations, and they require sophisticated chargers.
Nickel cadmium and nickel metal hydride batteries also have reasonably high energy density: small rechargeable AA, PP3 and other standard batteries normally use these chemistries. However, they are also relatively expensive if large battery banks are required. If you are lucky and find some surplus ones going cheap, it might be worthwhile to use them; buying them new is likely to be prohibitively expensive.
So that leaves us with the good, old-fashioned lead acid battery that we are all familiar with, and seeing as 99.9% of installations you will come across use some type of lead-acid battery, let's look at them in a bit more detail.
A cell in a lead-acid battery comprises a number of lead plates sitting in a bath of sulphuric acid. Each cell only produces around 2V. Most installations run at 12V, and use batteries which have 6 cells connected in series. Some systems run at 24 or 48V, and so require 2 or 4 12V batteries (or 12 or 24 individual cells) to be connected.
There are several variants of lead acid battery. The type of battery you find in your car has a large number of thin plates, with a high surface area. The high surface area allows a lot of current to be drawn from the battery quickly - which is useful for running a very power-hungry car starter motor. The disadvantage is that the batteries really don't like being deeply discharged: sulphate forms on the plates and the capacity quickly drops. They aren't suitable for use in renewable energy installations.
Traction batteries, such as those found in milk floats or golf carts, are far better at withstanding deep discharge. They have bigger, thicker lead plates - but are quite expensive. They are certainly the best to use if you can get hold of them.
'Leisure' batteries, found in caravans, narrowboats and yachts for powering domestic equipment, are a compromise between the above types. They can withstand a fair bit of deep discharge, but won't last as long as traction batteries. They are however perfectly good for most renewable energy applications, and are normally readily available from motor stores.
Two other variants of lead-acid batteries are worth a mention: 'gel' batteries, and 'absorbed glass mat', or AGM batteries. The idea behind both of these is to prevent gassing and loss of acid from the batteries during charging by using a gelling agent (in the case of gel batteries) or a binding of glass mat holding the acid around the lead plates in the case of AGM. Both these types are better at withstanding deep discharge compared to standard lead-acids, and have the advantage that they are maintenance free: there is no need to top the cells up with deionised water as is occasionally necessary with conventional batteries. They are more expensive however, and require a little more care in charging.
In most applications leisure batteries give the best balance between cost and performance, although if you want a system to last a long time, you might consider using beefier traction batteries. Gel batteries have some uses in remote power systems where maintenance has to be kept to an absolute minimum.
Adding carbon fibre
Some leisure batteries, such as the Elecsol range, incorporate carbon fibre in the lead plates. This draws acid deep into the plates, making the batteries more effective, and reducing the build up of sulphation. Such batteries will last far longer than conventional leisure batteries, and can have a comparable lifespan to gel or AGM batteries.