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Understanding Battery Power

By: Dr Gareth Evans - Updated: 3 Sep 2012 | comments*Discuss
Understanding Battery Power

More than 95 per cent of off-grid renewable energy electricity supply systems use some form of lead-acid battery to store the power they generate – but despite their importance, the whole issue of batteries is generally little understood. However, getting to grips with the basics of battery power is not really very difficult.

Not All Batteries Are The Same

Car batteries are the most familiar sort of lead-acid battery and while many DIY renewable energy systems make use of them, they are not ideal. Although they are designed to supply a relatively high current, it only needs to flow long enough to start the engine after which the battery is recharged.

As a result, if this kind of battery is used in a renewable energy system, it can soon fail. By contrast “deep-cycle” batteries supply large currents over a far longer period; when it comes to renewable power supplies, the deep cycle battery is king!

Depth of discharge is a major factor in battery function and discharging a lead acid battery without recharging it fairly promptly can seriously affect its useful life. In addition, the process of discharging and recharging a battery – known as “cycling” – gradually reduces capacity; although a well-maintained battery can sustain thousands of cycles, the lifespan of a poorly-treated one will be much reduced.

It pays to make sure your batteries stay in top condition for as long as possible, so always follow the manufacturer’s advice about proper care, maintenance and topping up.

Battery Capacity

Battery capacity often appears on the side of the unit, written as a certain number followed by “Ah” – for amp-hours. In effect this tells you the how long the battery can supply a given current at its standard voltage (which is normally 12 volts) and calculating it is easy; simply divide the capacity in amp-hours (Ah) by the current in amps (A) and the result is how long the battery will last in hours.

However, the whole battery capacity issue can cause some confusion – at least until you realise that the capacity you see written on the battery is not necessarily what you’ll actually experience when it’s in use.

Functional capacity depends on a number of things, including the age of the battery, how hot it is, the number of discharge/recharge cycles it’s been through and the size of the current being supplied or drawn from the battery.

The maths involved in working it all out precisely can get to look a bit scary – which is why you’re always best off talking to professional installers if any renewable energy project is going to need battery storage! If you just remember that capacity, current, voltage and the battery’s state of charge are all linked and changing one can affect the others, you won’t go far wrong.

The bottom line is that in practice you may not get exactly the capacity that you were expecting from your simple “amp-hours divided by amps” calculation, but it should give you a pretty good general guide.

Picking the Right One

The cheapest and most commonly available batteries are open – or “vented” – lead acid ones and these are the kind, for example, which tend to get used for small solar photovoltaic (PV) systems. However, since they need regular attention and topping up, they are often not the best choice for larger stand-alone installations.

Although it’s certainly possible to do the research and work it all out for yourself, if your project calls for a battery to store its power, it’s well worth getting some expert advice to make sure you get the most suitable type.

Even if you’re attempting a largely DIY approach to your system, the equipment supplier or manufacturer should be prepared to help you decide what you need – so you don’t have to get involved in paying for expensive professional help.

For many renewable energy power supplies, battery storage is essential to the project’s success, but the batteries themselves can often end up playing second-fiddle to the generation technology and almost get overlooked.

Energy efficiency is as relevant to renewable energy sources as conventional ones – so in the long run, understanding even a little about the way batteries work can be a big benefit.

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