Storage and Transfer
Understanding Leisure Batteries
Unless connected to a mains electricity supply, the 12V equipment in a caravan or motorhome relies on a leisure battery. Leisure battery technology differs from standard car battery technology as it is optimised to handle the loads and patterns of use that are common in caravans and motorhomes.
Standard Starter Batteries
A standard starter battery is designed to produce a high peak current to turn an engine over, typically over 1200rpm down to a temperature of -30°C. This is necessary to overcome the mechanical and thermal resistance of the engine, and depending on the engine design and standing temperature, requires cranking current up to 1000A. The vehicle alternator quickly replenishes lost battery capacity when the engine is running. Standard lead acid starter batteries are designed for standard non start-stop vehicles and are mostly based on calcium grid technology (low water loss technology). The design of these batteries focuses more on start-ability than durability, where plate count and plate surface area are optimised to provide high cranking capability, rather than cyclic durability. Poor quality low cost batteries in this market have been shown to last less than 3 cycles at 100% DOD (Depth of Discharge), i.e. fully discharged cycles, and typically less than20 cycles when compared to the leisure market requirement of 50% DOD. Mainstream automotive batteries tend to last 20-30 cycles at 100% DOD and more than 70cycles at 50% DOD. As a result, standard lead acid starter batteries are not suitable for the cyclic demands that are common to the leisure market.
In contrast to a standard starter battery, a leisure battery has to provide a steady flow of current over a prolonged period and seldom gets recharged until the user recognises that’s its Voltage level has substantially fallen. At this point, a recharge must be provided. If left for a long period in a discharged state, or left connected to a load which forces the battery into an over discharged state, a battery will not regain its former condition. These conditions can lead to the formation of soft shorts which will render the battery unserviceable.
It is general practice to market leisure batteries with some indication of the number of cycles that the battery can achieve at 50% state of charge. By design, increasing the number of cycles given by a battery (life and durability, rather than start-ability) involves the following:
• Adding more lead, generally using higher density active material during plate construction
Conventional lead acid leisure batteries are considered to meet the demands of entry level to mid-range applications. Subject to the application, a durability range of 70 to 360 cycles @ 50% DOD is common. Recent investment and developments in micro hybrid automotive technology, which bears similar requirements to the leisure market, has resulted in improved conventional leisure battery durability.
EFB Technology (Mid-Range)
Enhanced Flooded Battery (EFB) technology is a recent addition to the market, introduced primarily to aid vehicle manufacturers in meeting emissions control requirements in entry level start-stop vehicles. Typical vehicle manufacturer standards require that these batteries survive more than 200 cycles at 50% DOD which correlates to 50-60 cycles at 100% DOD. This makes EFB batteries suitable for mid-range leisure requirements where some degree of cyclic durability is required.
AGM (Mid-Range to Heavy Duty)
The automotive market has seen an increased volume of sales of high end start-stop vehicles that employ Absorbed Glass Mat (AGM) technology and this has resulted in benefits of scale and pricing for leisure market applications. Traditionally, this technology was confined to the industrial and high performance motorcycle battery market. Good quality AGM automotive batteries are also suited to heavier duty cyclic applications, such as frequently operated caravan movers.
Generally, AGM batteries are constructed with a pure glass highly compressed advanced mat, instead of traditional poly-ethylene separators. This is placed between each plate during the manufacture of battery cell packs. Higher pack compression improves the life and gas recombination efficiency, making the battery completely maintenance free, unlike conventional battery types.
The advent of this technology offered a further step in product durability allowing vehicle manufacturers with very high emissions to typically switch off the alternator for longer, thereby deeper discharging the batteries and also using the battery power to assist acceleration (power/passive boost technology). The batteries are better engineered to operate in lower states of charge, which reduces the effect of corrosion to the grids which is more prevalent in conventional batteries operating in low states of charge. Typical vehicle manufacturer standards require these batteries to achieve more than 360 cycles at 50% DOD, which correlates to 100-120 cycles at 100% DOD.
Selecting the Right Leisure Battery
These considerations lead to a decision to be made by the customer for either a low cost throw-away battery, which is unlikely to provide long-term reliability, or a more robust product that will provide multiple seasons of service. Furthermore, consideration must be given to the level of durability required, for example, a high durability option is best suited for a caravan mover, where low durability options are best suited to powering lighting alone.
A good leisure battery is constructed in a way that optimises the design for a life of repeated cycles and some limited over-discharge service abuse. Some of the limitations of basic lead acid battery chemistry cannot be overcome, but certain design rules and special additives are used to minimise the damaging effects of over-discharge in service. Others technologies with more severe duty cycles are sold in the traction and semi traction markets, but based on normal leisure usage, tend to be an expensive solution.
Semi-traction and traction type batteries typically use higher density thick plate or tubular technology designed for repeated deep discharge cycling. This technology tends to be expensive and out of the reasonable price range for the leisure market in general. It is common for this type of battery to achieve up to 1000 cycles at 50% DOD, but weight and cost are considered a serious limitation for high volume adoption in the current (non-specialist) areas of the leisure and caravan markets.