Leisure Battery Guide

Deep-Cycle Leisure Batteries are the heart of your electrical setup, distributing power to all of your equipment. Different from vehicle starter (or cranking) batteries that deliver a large amount of current over a short period of time, deep-cycle batteries (also known as leisure batteries) are designed to deliver smaller amounts of current over long periods.  Being such a critical component, it is useful to understand the different types of leisure batteries available so you can select the right one based on your intended usage, budget and application.

Starter Batteries vs Deep Cycle (Leisure) Batteries

When looking at batteries for powering domestic circuits in a motorhome, caravan or boat, it is often easy to think a cheap starter battery would be suitable, however, this is not the case as the two types of battery are designed for different jobs.

Starter batteries are designed to offer a large burst of power for a small period to start your engine, with power lost during this process being replenished by the alternator. The lead plates of starter batteries are designed in a way to favour high cranking capacity rather than recharge durability and, as a result, they aren't designed to withstand being deeply discharged then recharged, and doing so can significantly shorten their lifespan. A common term you will hear is Depth of Discharge (DoD) which is a reference to how much of the charge is depleted from the battery before recharging occurs. Quite simply, a 20% DoD means that 20% of the charge has been drawn from the battery. Starter batteries are typically subject to very low DoD as modern engines take little power to start.

Leisure batteries have a different lead plate design and favour delivering lower power over longer periods. They are much more tolerant of higher DoD (generally to around 50-60% for lead-acid technologies) and can be deeply discharged and recharged over multiple recharge cycles (this is where the term 'deep-cycle' originates because their charge/discharge cycles involve quite a deep DoD). This makes these types of battery perfect for use in motorhomes, campervans and boats where you are likely to be running multiple, low power items for longer periods and have recharging facilities available (typically alternator, mains or solar).   The number of cycles a leisure battery can accept will often be stated by the battery manufacturer in the documentation or on the battery itself. 

Battery Chemistry Types

There are 4 main types of chemistry used for deep cycle batteries. Below is a brief summary of each with the key advantages and disadvantages to be aware of.

1. 'Wet' Lead-Acid -  These batteries use lead-acid technology and contain lead plates housed in a liquid electrolyte which contains sulphuric acid.
2. AGM - These batteries use lead-acid technology and the liquid electrolyte is contained in Absorbent Glass Matts (AGM), which is how they derive their name. This provides greater protection against vibrations and offers a greater DoD.
3. Gel - These lead-acid technology batteries have a gel electrolyte which offers greater safety should the battery suffer damage. 
4. Lithium - These batteries use Lithium Iron Phosphate (LiFePO4) technology and, whilst still relatively expensive, offer a number of advantages over lead-acid technology.

Amp-Hours

Amp-hour (or Ah) ratings are often found printed on deep-cycle batteries and are a measurement of the battery’s energy storage capacity based on a continuous current delivered over a length of time before the battery is completely discharged. For example, if you had a battery rated at 1Ah then it could theoretically deliver 1 Amp for 1 hour before it became fully discharged. It could also theoretically deliver 2 Amps but only for ½ an hour before it became fully discharged.

However, use caution when determining how much capacity you need because batteries based on lead-acid technology do not like being discharged too deeply. As a rule of thumb, a lead-acid battery should not be discharged below 50% DoD or it risks becoming damaged, so the usable capacity in a lead-acid battery is only around half of its Ah rating, i.e. a 110Ah rated battery would have a usable capacity of around 55Ah.  It is very important to understand this characteristic of lead-acid batteries when sizing your battery bank to your application.  The usable Ah capacity of a lead-acid battery is also affected by other factors such as temperature, age/condition of the battery and the rate at which charge is drawn off (the faster you draw charge off, the lower the total usable capacity). 

The damage that can be caused to a lead-acid battery by over-discharge is one reason that a battery monitor can come in very useful because it can tell you the remaining capacity, allowing you to plan for recharging before damage occurs and maximising the life of your battery.  As a more simple alternative, you can use a battery protection device that cuts power to the circuits when it detects that the voltage is getting dangerously low.

Batteries based on Lithium technology have very different discharge characteristics to lead-acid batteries.  They can typically be discharged between 90-100% (90-100% DoD) meaning that when you buy a 110Ah rated lithium battery, the usable capacity is much closer to that figure. In practical terms, this means that a single 110Ah lithium battery has a usable capacity nearly equivalent to that of 2 110Ah lead-acid batteries.  If you also factor in that a lithium battery is around half the weight of an equivalently rated lead-acid battery, you can see that lithium technology gives you an equivalent usable Ah capacity for a 1/4 of the weight of a lead-acid setup.

 Charge rates

All battery chemistries have limits on the rate at which they can be recharged. This is important to note because providing too high a rate of charge can lead to damage and shorten the lifespan of   your battery. With lead-based batteries, very slow charge rates can allow sulphur to build up on the lead plates which can reduce the battery's charge-holding capability.

For lead-acid batteries, a rule of thumb is that your charger should be rated at about 10-20% of your battery's rated capacity in Ah. So, for a 100Ah lead-acid battery, a charger with an output of 10-  20A would be appropriate. Lithium batteries can accept much higher charge rates without damage and so charging times can be significantly reduced should you require it.  Always refer to the  charge rate recommendations from your battery manufacturer as they will vary from battery to battery.

Charging Stages and Voltages  

The typical battery charging cycle consists of 3 stages: Bulk, Absorption & Float.

• Bulk - during this stage the charger attempts to deliver its maximum current and the voltage will drop gradually as the battery's internal resistance rises. When this stage is finished, the battery is about 80% charged.
• Absorption - during this stage the charger will delivery constant voltage and the current delivered to the battery will gradually reduce as the battery's internal resistance rises further. When this stage is finished the battery will be very close to fully charged.
• Float - during this stage, the charger will deliver a reduced voltage in order to bring the battery to 100% charge and then keep it from self-discharging.

​Some chargers will have more stages than this depending on the technology employed. For example, some can perform a 'reconditioning' stage to try and recover a damaged or dead battery, and some can go into standby mode, 'waking up' periodically to check whether the battery has discharged and then applying more charge as required.

Different battery chemistries will require different voltage settings in order to optimise the charging process. Always refer to the manufacturer's recommended charging voltage settings for the absorption and float stages and use these whenever possible.

Will I Have Enough Storage Capacity?

When choosing the best battery for your needs, as well as deciding what suits your application, budget & usage based on the advantages & disadvantages above, the main aspect you will need to consider is storage capacity.

You need to take into account how much storage capacity you need (in Ah) and how much you can replenish through charging whilst making sure (in the case of lead-acid batteries) that you don't discharge the battery so low that it causes damage. You will also need to take into account how often you will be able to re-charge and how much recharging power will be available (for example, mains or alternator recharging will typically provide much more power than solar). If you are planning on being off-grid and static for extended periods then you will find you need a much larger storage capacity than if you are have your engine running daily and have regular access to shore/mains power.

Below is a simple equation that will help you to work out approximately the battery capacity required:

• (Ah required per 24hrs - Ah recharged per 24hrs) x no. days away = Total usable Ah required

For example, if you used 70Ah every 24hr period,  you had access to 20Ah of recharging every 24hr period, and were away for 1 day, then the usable battery capacity you would need to support your electrical requirements would be:

• (70Ah - 20Ah) x 1 = 50Ah.

If using a lead-acid battery then something in the region of 110Ah rated capacity would provide enough energy without becoming over-discharged. If using a lithium battery then something in the region of a 55Ah battery would provide enough energy.

Repeating the scenario above, but this time if you had no access to recharging during each 24 hour period and you were away for 2 days, then the calculation would be:

• (70Ah-0Ah) x 2 = 140Ah

If using a lead-acid battery then something in the region of 300Ah rated capacity would provide enough energy without becoming over-discharged. If using a lithium battery then something in the region of a 150Ah battery would provide enough energy.

We hope this guide has helped but if you do have any further questions then please do not hesitate to get in touch.

Disclaimer - The information contained in these articles is provided in good faith and we do our best to ensure that it is accurate and up to date, however, we cannot be held responsible for any damage or loss arising from the use or mis-use of this information or from any errors or omissions. The installer is ultimately responsible for the safety of the system so if you are in any doubt, please consult a qualified electrician.