Battery Guide

Leisure & Starter Batteries

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 and are not interchangeable.
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 - 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 a 50% total discharge of their rated capacity during normal operation.

Battery Types

  • 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
  • Gel – These batteries use a gel-based electrolyte. Like AGM batteries, this makes it non-spillable and therefore safer in the event of damage.
  • Lithium – High end batteries that offer increased capacity over lead acid, up to 50% higher capacity at the same size. Lithium batteries also have a longer life span than lead acid batteries, with up to 3000-5000 cycles for LiFePo 4 models.

Battery Ratings

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.

Choosing a Battery

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.

Parallel & Series Battery Systems

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)

2x 100Ah 12V batteries in parallel

2x 100Ah 12V batteries in series

  • 12V output
  • 200Ah
  • 2400Wh
  • 24V output
  • 100Ah
  • 2400Wh



Parallel connections

Two or more batteries in parallel will have their capacity (in Ah) added together while the system voltage will remain the same.

To wire a battery system in parallel, connect all the positive battery terminals together and all the negative battery terminals together, as seen in the diagram below. There is no limit to how many batteries can be wired in parallel, however accidental shorts can become increasingly dangerous with each added battery as the total available current also increases.

Series connections

Two or more batteries in series will have their voltage added together with the Ah capacity remaining the same. A higher voltage system can benefit from using thinner cables, for example a 24V fridge will use half the current of a 12V fridge with the same power rating.

A series battery setup is created by wiring the positive from one battery to the negative on the next. This will result in an increased voltage, an ideal setup if limiting voltage drop is essential, or for using thinner cables.
Two or more sets of batteries wired in series can also be wired in parallel to create a higher capacity 24V system.