Batteries are an important component in Off-Grid and Backup power supply systems. The batteries are charged when energy is available (such as during daylight hours for photovoltaic/solar energy systems), and energy is used as required at a later time.

 Many types of batteries exist today, and all have certain features that make them the obvious choice in certain applications - be it starting of cars or portable devices like cameras or mp3 players, or in backup power systems. The batteries often used in Off-Grid and backup power systems are lead acid batteries as they are reliable, cost efficient, and lots of manufacturers offer chargers and accessories for these types of batteries.

The lead acid batteries used for Off-Grid power systems differ from those often found in cars. Car type batteries are designed to provide very high current (hundreds of amps) for a very short time (seconds), whereas the batteries for Off-Grid use are designed to provide a long life (many discharge-recharge cycles) at deep discharges (discharging the battery 50 to 80%). The lead acid batteries used in Off-Grid and backup power systems are therefore called "Deep Cycle" batteries, because they are discharged much deeper than car starting batteries, and they can do so and still offer good lifetimes (hundreds or thousands of cycles).

Deep cycle batteries have had many advancements over the years, and so quite a few variants exist today. The main idea has been to reduce the amount of battery maintenance required over the lifetime of the battery. Some common variants include:

  • Flooded: this is the "old style" battery with vented caps and liquid electrolyte (sulphuric acid). This style needs occasional topping up with distilled water, regular checking of specific gravity, and must be situated in a vented place (due to the emission of flammable hydrogen gas during charging). Trojan Batteries have developed a special water distribution system called Hydrolink - so topping up of water can take as little as 30 seconds!
    This battery is usually the cheapest to purchase.

  • Valve Regulated AGM: This type has the electrolyte absorbed onto a mat that is situated between the electrodes (AGM stands for Absorbed Glass Mat). Generally the electrolyte can not be topped up and is also often referred to as "maintenance free".

  • Gel Electrolyte: This type of battery has a gel electrolyte instead of liquid, so it is impossible to spill any acid. The batteries can therefore generally be mounted at any angle, and release no gasses. Gel type batteries are often used in indoor applications such as alarm systems and computer backup UPS systems. These batteries are much more expensive than the flooded type.

The type of battery you choose depends on your personal preference, and whether you're willing to pay extra for less maintenance. Regardless of the type of battery, every battery should be stored in a cool place, preferably vented, and will require at least some regular maintenance (checking terminals are clean and tight, and inspecting for abnormal conditions such as high temperatures).

One more point to note is that the standard lead acid battery Cell has a name plate voltage of 2V. During discharge that voltage can be down to around 1.85V, and during charging it can be up to around 2.5V (depending on the type). Batteries are made up by a number of cells in series. So a 6V battery has 3 cells in series, an 8V battery has 4 cells, and a 12V battery contains 6 cells. So when you are planning your battery bank, just add the number of batteries in series as required (eg: to make up 24V battery bank, use 4 of 6V batteries in series, or 3 of 8V batteries or 2 of 12V batteries).