Batteries: introduction

What are batteries?

Batteries are units containing various combinations of metals and chemicals, between which occur reversible chemical reactions to store and then provide electrical power. Some of the metals and chemicals can be quite nasty and toxic (e.g. cadmium). We will focus on lead-acid batteries, which form the heart of many off-grid, or hybrid renewable energy systems.

A single 2-volt, lead-acid traction cell

A single 2-volt, lead-acid traction cell, as used in forklifts (a 24-volt forklift would use 12 of these, and a 48-volt forklift, 24).

Although other chemical reactions can provide more electricity, the lead-acid battery is proving to be the most successful, because of its affordability, availability, and the fact that it is so robust. A basic lead-acid battery comprises 3 elements – a collection of positive plates, a collection of negative plates and a liquid or gel electrolyte. In a lead-acid battery, the active material of the plates is lead oxide, and the electrolyte is dilute sulphuric acid.

In a discharge situation (when giving out energy), elements of the electrolyte react chemically with elements of the plates to release energy in the form of electrons. The flow of electrons through a conductor is electricity. When being charged, the reaction is reversed and the battery absorbs electrons.

testing the specific gravity of the electrolyte in a cell with a hydrometer

Testing the specific gravity of the electrolyte in a cell with a hydrometer.

Alessandro Volta (who gave his name to the volt) invented the first electric battery in 1800. His plates were of copper and zinc, and the electrolyte was brine. The technology has been constantly improving since then, and now there is a range of different kinds of battery, notably nickel-cadmium and lithium-ion. But lead-acid are still the most popular, because of their bulk manufacture and second-hand availability.

There are 3 kinds of lead-acid battery:

  • gel – the electrolyte is in a gel form. They are used mainly for emergency standby, and are no good in a cycling (charge/discharge) situation.
  • glass mat – the electrolyte is held within a soaked glass mat between the plates. They are better than gel batteries for cycling, but over time, anomalies will develop between the individual cells as regards voltage and state of charge. They are used in situations where a liquid electrolyte might spill.
  • flooded battery – contains liquid electrolyte that can spill; this is the best type for a renewable energy system. They need to be topped up with distilled water from time to time.

The best types for renewable energy systems are leisure/deep-cycle batteries (for caravans, boats or mobile homes) or traction batteries (forklifts), as they can be repeatedly deeply discharged and recharged. Car batteries are starter batteries, providing short bursts to start an engine, and are unsuitable for home power systems.

A collection of 600 amp-hour, 2-volt cells wired in series (part of a 48-volt battery pack);

A collection of 600 amp-hour, 2-volt cells wired in series (part of a 48-volt battery pack); 600 x 48 = 29kWh, and so the available power is 50% of this, or 14.5kWh.

What are the benefits of batteries?

The main benefit of batteries is for people who are off-grid. If you’re building a renewable electricity system, you’re not looking to use electricity as it’s generated – the sun won’t be shining all the time, and the wind won’t be blowing all the time (although if you’ve got micro-hydro, your stream is probably going to be flowing all the time), and you’re going to want more electricity at some times than others. So you’ll need to store your electricity.

If you are not on the grid, you’re going to need batteries. There is a strong environmental case for saying that if you are in an area where you can have a grid connection, then you should have one, and use the grid like an enormous battery, as the infrastructure is already there. Also, batteries contain lots of noxious metals and chemicals – so the fewer of them the better.

A second benefit, however, is not environmental, it’s about energy independence. Many people want to be sure that they are in control of their own energy supply all the time. They don’t want to involve themselves with giant electricity companies, or risk everything going down if there is a power cut, especially as they have generated their portion of the grid electricity themselves. There is also a case for saying that with loving care, renewable electricity enthusiasts can be responsible for extending the life of second-hand batteries that would otherwise have died in scrap yards.

The third benefit of batteries is transportability – rechargeable leisure batteries allow the use of renewables in boats and vehicles.

testing the voltage of batteries with a voltmeter

Testing the voltage of batteries with a voltmeter.

What can I do?

First, do your sums – work out how much storage you need. Battery capacity is based on amp-hours and volts. Here’s an example that will (hopefully) make this a bit clearer: a 12-volt bulb drawing 1 amp from a 12-volt battery will draw 1 amp-hour per hour. So if you have a 50 amp-hour battery, then theoretically you should be able to run the bulb for 50 hours before the battery is completely flat. But – you should not discharge a battery below 50% of its capacity, or you will create serious imbalances within the battery over a relatively short period of time. So, in this case – a 12-volt bulb drawing 1 amp – you can safely run for 25 hours with a fully-charged battery.

If you want to run something bigger – say a fridge – you have to think about wattage. Let’s say the fridge takes 150 watts to run, and the time it is actually running over the course of a day is 3 hours, then it’s using 450 watt-hours (150×3) per day. If you disregard the extra current needed to start the fridge, and the inefficiencies of your inverter, then just to run the fridge, you will need almost 40 amp-hours per day (450/12 – because watts = volts x amps). But when you take extra current and inefficiency into consideration, it will be more like 70 amp-hours. And then, bearing in mind that batteries are only around 80% efficient, you’ll need to put around 80 amp-hours into your battery on average every 24 hours to run the fridge. You can extrapolate from this for the other appliances you want to run.

a cupboard containing 4 leisure batteries and an inverter on a vehicle with 12 x 80-watt pv panels on the roof

Cupboard containing 4 leisure batteries and an inverter on a vehicle with 12 x 80-watt pv panels on the roof, plus a 350-watt wind turbine.

Next get your batteries – from industrial battery manufacturers (Google), or forklift/traction battery suppliers and recyclers. Traction batteries are designed to be cycled, and are tougher than leisure batteries. You can get them from scrap yards, but only if you really know what you’re doing – you need time and perseverance. The price will vary with the scrap price of lead. The more you understand batteries, the cheaper you can get them. How many? Well, if we say an average house requires about 3.5kW-hours a day, this equates to 150 amp-hours a day from a 24-volt battery bank. So to give enough power for 2 days without discharging the battery below 50%, you would need 600 amp-hours. This represents 2 battery packs of 6 cells per pack. That’s 12 cells, each weighing around 25kg, so you’ll need a bit of space. Batteries can be dangerous, so make sure their box/housing is kept locked and vents to the outdoors. NB: to reliably run a system using 3.5 kWh per day the generation side needs to be large enough, say something no less than 1.5 kW of installed capacity.

Exposing the elements of a lead-acid battery

Exposing the elements of a lead-acid battery.

You’ll need a charge controller in your system to make sure you don’t overcharge and damage your batteries, and to run standard 240-volt appliances from a 12, 24 or 48-volt battery bank, you’ll need an inverter. It is possible to buy 12-volt appliances, but they are a lot more expensive, plus mainstream 240-volt appliances are being made much more efficient all the time – so it may be as well to focus on getting a really good inverter.

So, looking after batteries is a bit of a dark art, and can be quite complicated. Here’s an article about battery installation, and here’s one about monitoring. Batteries are the recalcitrant teenager of your renewables system. Keep an eye on them and they’ll be alright, but ignore them and they’ll cause problems.

Thanks to Andy Reynolds of the Ecolodge for information.


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Andy Reynolds is a carpenter / joiner and woodsman who has tutored courses and authored books with us. He has lived on a smallholing in Lincolnshire since the early 80s, renovated a house, built a holiday cottage and got off-grid. He records his adventures with educational videos on his YouTube channel.

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