Deep Cycle Batteries - An IntroductionDeep 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.
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.
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.
Batteries for Solar, Wind or Off-Grid Power SystemsSolar 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 Dryfit Solar Block Batteries - Gel, 6V & 12V blocks, maintenance free, 60Ah to 330Ah.
MK Solar Batteries - Gel, 6V & 12V blocks, maintenance free, 36Ah to 265Ah.
Solar A600 OPzV - Gel, 2V cells, maintenance free, long
life, 240Ah to 3500Ah.
SMG/S Solar Batteries - Gel, 2V cells, maintenance free,
long life, 265Ah to 3900Ah. Exclusive 5yr warranty.
Classic Solar Batteries - Wet, 2V long life cells,
190Ah to 4600 Ah.
Exide Powerfit 12v AGM Batteries - Absorbed Glass Matt, 12V Blocks, 7Ah to 40Ah.
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