The batteries used for powering electrics such as lighting and appliances inside a boat or campervan might appear the same as those used for starting the engine, both are 12V and are available in the same sizes, however they are designed for different purposes. Below we will look at the different types of batteries, the different ratings used to compare them and how they can be installed in an electrical system.

• Starter batteries

Used for starting the engine of the vehicle, these are designed to provide a very high current output over a short time while the starter motor is running. The design of these batteries means that while they are excellent at providing this high current, they are not able to withstand being deeply discharged without suffering damage.

• Leisure batteries

Leisure batteries (also known as house batteries or deep cycle batteries) are designed to provide a lower current output over a longer time, perfect for boats and campervans that need to run low power devices such as lighting. These batteries have a higher maximum discharge tolerance, with up to 50% total discharge of their rated capacity during normal operation.

• Flooded lead acid –This is the traditional battery technology used in most vehicle starter batteries, it uses lead plates and a liquid electrolyte (sulphuric acid).These are budget-friendly batteries and are available in both starter and leisure battery formats.
• AGM –Absorbent Glass Matt (AGM) batteries use a fibreglass matt to absorb the liquid electrolyte, making the battery non spillable. AGMs often have a higher resistance to vibration as well as a longer lifespan. Other advantages are: Faster charging and higher discharge rate and lower self-discharge and maintenance.
• Gel – Gel batteries use a silica gel to hold the electrolyte between the plates. This makes it non-spillable and therefore safer in the event of damage. It also means that the batteries can be stored in any position. Like AGM's they also have a longer shelf life, vibration resistance and deep discharge rate. They are a lot more heavier than the other technologies.
• Lithium – High end batteries that offer increased capacity, up to 50% higher capacity at the same size compared to other technologies. Lithium batteries also have a longer life span, with up to 3000-5000 cycles for LiFePo4 models, this is 10 times longer than a typical lead-acid battery. They are also light weight making them a lot more easier to manage and transport. Lithium batteries will charge 5 times faster than a typical lead-acid battery.

The power ratings are found printed on the top or sides of a battery, the two most popular ratings being Amp-hours (Ah) and cold cranking amps (CCA).

• Amp-hours (Ah) –This is the total current carry capacity of the battery, a rating of 80 Ah would mean the battery could provide a current of 80A for 1 hour (or 40A for 2 hours) before becoming completely discharged.
• Cold Cranking Amps (CCA) –The CCA is the maximum current output that the battery can supply at -18C for 30 seconds, while maintaining a voltage of at least 7.2V.
• Watt hours (Wh) – This is the total energy stored within the battery bank. To calculate, take the system voltage (often 12V or 24V) and multiply it by the total Ah capacity. For example, a 12V 345Ah battery bank will have 4140Wh capacity (12 x 345 = 4140).
  Watt hours can therefore be used to compare total energy capacity on systems with different voltages, as the voltage is considered when calculating this figure.

The main factor to consider when choosing a battery, as well as chemistry, is size. The size of the battery (or battery bank) should be based on how much power you are expected to use (in Ah) throughout the day, and how much power will be put back into the battery from sources such as solar panels or a battery charger.

Calculating battery size required

When selecting a batteries capacity for your electrical system you will need to calculate both the power used in a 24hr period (in Ah), and how many Ahs of charge will be put back into the battery in the same 24hr period.

From this example a lead acid battery will need to be a minimum of 100Ah to be suitable for this usage scenario, while a lithium battery will need to be sized around 55-60Ah.

Remember, lead acid leisure batteries can only be discharged to around 50% of their total capacity before they risk being damaged, so the total capacity must be double the capacity you require.

Multiple batteries can be wired up using parallel or series connections depending on user preferences and other devices that will be used in the system.

Whether a series or parallel system is used, the total available power remains the same.

For example, two 12v 100Ah batteries in parallel will result in a total of 2400Wh in available power (12 X 200 = 2400)

The same 12v 100Ah batteries in a series system will still result in 2400Wh in available power (24 x 100 = 2400)