off-grid solar and wind power specialists

Deep-Cycle Batteries for Off-Grid & Remote Power Systems

deep cycle solar batteries for remote & marine power

Deep Cycle Batteries - An Introduction

Deep cycle lead-acid batteries are used for off-grid solar & wind power systems, marine and industrial power applications. For remote power systems, traditional lead acid battery technology continues to dominate over other more modern battery technologies - mainly due to cost, predictable performance and high reliability.

'Solar' batteries are deep-cycle batteries which have been optimised by the manufacturers to be able to charge with very little current, and thus take maximum advantage of any available energy. Solar batteries also normally have a very high charge & discharge efficiency of around 90 to 95%.

The main difference between a true deep cycle battery and an engine starting battery is the number of times they can withstand a deep discharge. A typical starter battery will do between 50 & 150 deep discharges, whereas a high specification deep cycle battery will be able to complete approximately 1200 deep discharge cycles.

'Leisure' batteries and 'marine' batteries are generally a cross between a starting battery and a deep cycle battery. They are cheaper than true deep cycle batteries due to the fact that they use less lead in their construction. A typical 'marine / leisure' battery will be somewhere in-between, with around 200 to 300 deep discharge cycles.

True deep cycle batteries have far heavier and stronger lead plates inside, which enables them to better withstand the stress of a deep discharge. It also makes them more expensive as the worldwide price of lead (Pb) is very high.

6V MK gel solar battery

Types of Solar Batteries

There are two main types of deep-cycle solar battery - flooded (FLA - Flooded Lead Acid) batteries and sealed VRLA (Valve Regulated Lead Acid).

As the name suggests, an FLA battery has liquid electrolyte, which normally requires regular topping up. When charging, a FLA vented battery of this type produces hydrogen gas, which is highly explosive and needs to be vented to the outside. This is normally achieved through the use of a fibreglass or plastic battery box.

A sealed VRLA battery is maintenance free, and generally produces negligible gas when charging, which makes them easier & cheaper to install as they don't need a special battery box. Sealed batteries are also much easier to transport, as unlike wet 'flooded' batteries, they are not considered hazardous cargo.

Sealed VRLA batteries come in 3 sub-types, wet, AGM or Gel. The sealed wet type is primarily designed for the leisure and marine markets, some of which claim to be able to complete around 400 to 500 discharge cycles (to 80% depth of discharge). These batteries would seem to be a reasonable compromise for people on a very tight budget - especially if the quoted specifications prove to be correct.

AGM stands for Absorbed Glass Matt. This means that the liquid electrolyte has been absorbed into a sponge-like mat. Gel batteries have the electrolyte in a gel-type substance. Both AGM & Gel battery types offer more support to the lead plates and can be used in any orientation. In the event of a cracked case, gel & AGM batteries also don't leak acid all over the place.

Generally speaking, AGM batteries are less capable of full discharge cycles than a Gel equivalent. Gel batteries also perform slightly better & have a longer life than AGM in hot climates.

Types of Battery Plates

Two main types of plates are used in lead acid batteries.

- Flat Plate Batteries - these are normally 12V or 6V block batteries, similar to the type used in cars. They are the cheaper type of battery. High quality flat plate batteries can last for about 10 to 12 years.

- Tubular Plate Batteries - these are normally tall 2V cells, wired in series to provide the correct voltage. The tubular plate construction results in a much longer life than most flat plate batteries. Tubular plate batteries can last up to 20 or even 25 years, if well maintained.

A600 VRLA solar batteries with tubular plates

Charging Efficiency

Flooded or wet batteries generally have a charge efficiency of approximately 80%. Sealed Gel & AGM batteries normally have a higher charge efficiency of around 90 to 95%.

However, it is important to understand that charging efficiency varies depending on the state of charge of the battery bank. In some cases, flooded (FLA) batteries only achieve an efficiency of 50 to 60% at higher levels of charge, whereas a good VRLA gel battery can achieve a 90 to 95% charge efficiency.

Cost of Ownership

Total cost of ownership needs to be taken into account when buying a battery. The following factors need to be considered.
  • Price - A cheap battery will always be attractive. However, if this results in lower quality or battery life, the costs over time will be higher as the battery will need frequent replacement.
  • Capacity - The capacity of the battery dictates how much energy it can store. Ensure when comparing battery capacity that the same discharge rate is used (often C20 or C120).
  • Cyclic Life - Examine the manufacturer's cyclic life graphs carefully. Not all marketing figures are completely transparent, as many manufacturers give the cyclic life at 80% depth of discharge. Others give the figures for 100% discharge, which is far more stressful on the battery. For maximum life, no lead-acid battery wants to be regularly taken below 50% discharge (see Sizing Battery Banks - below).
  • Quality - Build quality of batteries can vary hugely. Always try to select a manufacturer with a good reputation, as the better manufacturers use highly advanced materials and plate design, ensuring a longer life and higher charge efficiency.

Batteries for Solar, Wind or Off-Grid Power Systems

Solar deep cycle batteries are available with storage capacities from 7Ah to 4600Ah in a single cell. These can be combined to provide battery banks with many thousands of amp hours of storage.

We supply the following types of batteries, all suitable for charging from solar pv panels or a wind turbine. Please visit the links below for further information such as prices, data sheets and performance graphs.

Sonnenschein gel batteries Sonnenschein Dryfit Solar Block Batteries - Gel, 6V & 12V blocks, maintenance free, 60Ah to 330Ah.

Sonnenschein gel batteries MK Solar Batteries - Gel, 6V & 12V blocks, maintenance free, 36Ah to 265Ah.

Sonnenschein A600 gel batteries Sonnenschein Dryfit Solar A600 OPzV - Gel, 2V cells, maintenance free, long life, 240Ah to 3500Ah.

Fiamm gel batteries Fiamm OPzV SMG/S Solar Batteries - Gel, 2V cells, maintenance free, long life, 265Ah to 3900Ah. Exclusive 5yr warranty.

Exide classic wet batteries Exide OPzS Classic Solar Batteries  - Wet, 2V long life cells, 190Ah to 4600 Ah.

Exide powerfit agm batteries Exide Powerfit 12v AGM Batteries - Absorbed Glass Matt, 12V Blocks, 7Ah to 40Ah.

deep cycle solar battery manufacturers

Sizing Battery Banks for Remote Power Systems

The amount of energy a battery can store is rated in Ah (Amp hours). The Ah capacity varies depending on how quickly the power is taken out (see Peukert), however most deep-cycle batteries are rated at the 20hr or 100hr rate. This means that a battery rated at '100Ah @ C20', is able to produce 100Ah when discharged over 20 hours, i.e. a constant load of 5 amps.

Off-grid power systems generally have a battery bank which is sized relative to the anticipated daily loads, and for optimum life should rarely be taken below 50% depth of discharge. For a solar or wind power system, this can mean that the battery bank has anywhere between 3 and 15 days reserve, which also allows the system to take advantage of more sporadic periods of sun or wind.

The life of a battery is expressed as 'design life' or 'cyclic life'. If a battery is not cycled very often, it will normally last for the design life. If a battery is cycled regularly, its life will depend on the depth of the discharge (DoD) and the number of cycles it has to do. This data is often shown plotted on a graph by the manufacturers and is the key to a good deep cycle battery.

Long-life deep cycle industrial batteries are produced in 2 volt cells, which are joined together on-site to provide the correct voltage (i.e. 12 x 2V cells = 24V battery bank). This enables the assembly of massive battery banks, which otherwise would be very heavy and virtually unmovable as a single unit.

Trojan Batteries provide an online battery bank sizing calculator.

Fiamm solar 2V OPzV gel cells - up to 3900Ah in a single cell

Looking After Your Batteries

A car starter battery will quickly fail when used on a power system which discharges the battery deeply. This is due to starting batteries having lots of thin, low density plates, which in a deep-discharge situation will sulphate or loose some of the plate material, both of which will cause the battery to fail prematurely.

Lead acid batteries must not be over-charged or over-discharged. In a solar or wind energy system, the charge controller takes care of the batteries, ensuring that they give an optimal life. All well-designed battery systems also have some sort of automatic Low Voltage Disconnect (LVD) device which shuts off the loads to prevent the batteries from being over-discharged.

Sealed (VRLA) Gel or AGM batteries require little or no user maintenance. Wet 'vented' batteries need occasional topping up with distilled water. Unsealed 'wet' batteries will produce hydrogen gas when charging, and therefore need to be housed in an enclosure which is vented to the outside and kept away from all sources of ignition.

Extreme caution is always required when working with batteries. It's very easy to drop an un-insulated tool onto the battery terminals, which can cause major damage, perhaps even an explosion.

Related Links / Useful Information

Servers powered by renewable energy