Leisure Batteries - What you need to know

There are that many different leisure batteries out there that selecting one can make your head spin. This guide is designed to help demystify some of the jargon and regulations for you and help you get a grasp of what you need to understand about leisure batteries. That said, this is by no means an exhaustive guide and should be used to supplement your own research.

Please note – the information we provide is guidance only, if you are in any doubt, please contact a qualified electrician.

Battery Anatomy

The most common design of leisure battery you will come across will be the lead-acid battery. There is a fair amount going on within the battery and a lot of components to make it work as advertised, but we will stick to the bits you really need to know about. As the name suggests, inside the case of this design of battery there is a series of lead plates with a liquid acid solution called an electrolyte between them. The other components to be aware of are the separators made up of porous, electrically insulated material and the grid plates that go between the lead plates.

The separators are in place to prevent a short circuit (also referred to as a short) between the plates. This happens when something bridges the gap between a positive plate and negative plate and allows the electrical current to take a shorter path than it's meant to. The electricity then goes directly from the negative plate to the positive plate instead of going around your circuit and can cause the battery to be damaged or even fail entirely depending on the severity of the short.

The grid plates are coated in reactive materials that enable the necessary chemical reactions to take place within the battery.

If you were to take apart your battery (don't try this at home), these components would be ordered and grouped something like the image below.

The components would obviously appear much closer together and there'd be a whole lot more of them, but this should give you an idea as to the very basics of the internals of the battery.

Whilst lead-acid is the most common, it is not the only design available nowadays. There are a range of designs and constructions available that improve upon the lead-acid design.

Battery Types & Constructions

Batteries can be sorted into 2 types; serviceable and non-serviceable. Serviceable batteries come with removeable caps to top up the electrolyte as needed and require routine maintenance to ensure performance and overall battery health is kept to a high standard. Non-serviceable or "maintenance free" (more on this term later) batteries have a very similar construction to the serviceable lead-acid batteries, the only difference being that there is largely no need to top up the electrolyte due to how they are built.

Within these types there are a range of different constructions for leisure batteries as well:

Serviceable:

• Open Lead-Acid – this is the least expensive and one of the most popular constructions of battery as they are simple and straightforward to maintain. They come with caps which you can remove should you need to top up the battery with electrolyte solution, which you likely will at some point. This is because water is produced as you use your battery and this escapes as vapour, so you need to top it up periodically. With this type of battery you need to keep the battery upright to prevent leakage.

Non-serviceable:

• Sealed Lead-Acid – the construction of this battery is very similar to the open lead-acid version, but there are no removeable caps and the electrolyte does not need to be topped up. This is because sealed batteries do not allow the water vapour to escape and so do not need to be topped up. This is a definite positive as it means you have one less thing to actively monitor with your battery.
• AGM (Absorbed Glass Mat) – the design of this battery is similar to the open and sealed lead-acid batteries, but instead of having a liquid electrolyte, the electrolyte is absorbed into a fibreglass mat, making the battery spill-proof. Other advantages with this type of battery are that they have rapid charge and discharge capabilities as they are valve regulated, meaning they can efficiently regulate pressure within the battery as gasses are created during use, allowing the battery to work more efficiently. They also can often be discharged to and even below half capacity, have high charge density, a long operating life and are better than most types of batteries at withstanding road vibrations. However, AGM batteries can be more expensive than standard lead acid batteries.
• Gel – Similar to AGM batteries in that they are spill-proof and valve regulated, the main difference is, as the name suggests, that this type of leisure battery uses a thick gel as its electrolyte rather than a glass mat. The main advantages with this type of battery is that it is impossible to spill electrolyte even if it breaks and that this type of battery can be discharged to a lower capacity than lead-acid batteries and even many AGM batteries. That said, the main downsides of this battery are that it needs to be charged at a slower rate to prevent excess gas from damaging the cells and that overcharging can cause voids or pockets of air to develop in the gel electrolyte which can cause your plates to short in extreme cases, and will inevitably shorten the battery's service life even in the best cases.
• Lead Crystal – this construction has a unique style of mat and electrolyte which means it doesn’t suffer from sulfation. This means that the battery can safely be discharge to zero volts and fully recharged without shortening its service life. As well as this it has a greater number of cycles (the number of times a leisure battery can be discharged to half capacity and recharged), and a longer lifespan than most other constructions of batteries. That said, this battery is rarely affordable for most people, as it can cost close to double that of an AGM battery of the same capacity.
• Lithium (LiFePO4) - The main advantages of lithium batteries are the long operating life, which is up to 10 times more than that of a lead-acid batteries, as well as the fact that they cope well in low temperatures, meaning you’ll keep near peak performance year round. As well as this, many lithium batteries also have electronic controllers to prevent overcharging. They also allow a greater depth of discharge than other types of battery. This means you can safely drain them more than other types of battery without shortening their life, so you actually get more usable electricity from the same size of battery. The main downsides to lithium batteries are that they are significantly more expensive than lead-acid batteries.

Battery Purpose

There are a few different designs of leisure battery based on what they are required to do:

• Standard Leisure Battery – also known as a deep cycling or auxiliary battery, this is used to power 12V equipment within the habitation area of caravans and motorhomes.
• Dual Purpose Battery – this type of battery can be used to power the 12V equipment as well as start the vehicle engine in a motorhome or campervan. This means that you don't need separate vehicle and leisure batteries. The downside of this dual-purpose feature is that, as a result, this type of battery can be discharged and recharged fewer times than a standard leisure battery so it might not last as long. That said, if you don't have space for a separate leisure battery, this type of battery may be the perfect fit for you.
• Semi-traction Battery - this is essentially a higher end version of the standard leisure battery. This type of battery provides high energy density, making it suitable for deep discharge, often to a lower capacity than a standard leisure battery, and offers a higher amount of recharge cycles than a standard leisure battery, though this type of battery will be a fair bit more expensive than your standard battery.

Capacity

Now we will have a look at leisure battery capacity and how to know what to look for. Leisure battery capacity is rated in Amp-hours (Ah), which is the number of Amps the battery can provide over a specified timeframe. Most leisure batteries will be rated based on using all of their power over 5 hours, 20 hours and 100 hours. The 20 hour figure is the one that the battery's capacity rating is taken from. If it’s easier to picture, think of it as Amps multiplied by hours, so if a battery is rated at 100Ah at its 20 hour rating, this means that the battery was able to maintain a current of 5 Amps for 20 hours (5 x 20 = 100Ah).

How do I figure out what capacity I need?

Before you start looking at leisure batteries, take the time to figure out exactly what you’re going to need. Work out which appliances the battery is going to be powering and how often you’ll be using the battery away from a mains connection to start with. We’ll work through an example as we go to keep everything clear.

Once you have determined which appliances you’ll be powering with the battery, you’ll need to work out how much power these will draw over the course of an average day. For each appliance, you’ll need to find out how much power they use in Amps, which is sometimes listed on the information plate of the appliance or in the user manual. If it's not, then all you need to do is find the wattage of the appliance and divide that by the voltage in the circuit (for leisure batteries this is usually 12V).

For our example, let’s say I’m powering a 12V Fridge, some LED lighting, and a water pump.

From here, take the value in Amps for each appliance and multiply it by the number of hours you are likely to be using each appliance per day – this will give you a figure for each appliance in Amp-hours (Ah).

^{*your fridge only actually stays on for around 2/3 of the day, this is where we've gotten this number from.}

Next you should add up the values in Amp-hours for your appliances to get your base power requirement per day. Then you should add 25% on top of this value as your battery’s capacity will decrease over time, so it’s better to be prepared now than have to potentially fork out for a new battery sooner than you should need to.

This is the amount of power your appliances will use in a single day, so obviously, if you're using your battery for more than 1 day you'll need to multiply your daily use by the number of days you'll use your battery between charges to get a battery with enough capacity for your needs.

Now that you've figured out your power requirements, we need to talk about an important figure you'll see on your battery: Depth of Discharge (DoD). With most batteries you can't actually use all the electricity that's in them. If you do you risk damaging the battery and making it so that it can't be recharged. Depth of Discharge is how much of your battery's capacity you can actually use before needing to recharge it. So for example, if you have a battery that has a DoD figure of 40%, that means you can only use 40% of your battery's stated capacity before it will need to be recharged. If you consistently use more than this amount you will drastically shorten the battery's life. The reason we mention it here is that you will need to be aware of it to make sure your battery is fit for your purpose.

Let's use the figure from the example above, 71.7Ah and assume you're only wanting to use the battery for one day before charging. You may look for a battery around 75Ah because that looks like it will power your load, right? Well if the DoD of this battery is only 50% you should only use half of the battery's stated capacity to avoid shortening its life. This means you'd actually need a 150Ah battery with a 50% DoD to give you the 75Ah usable capacity needed in this example.

Alternatively, you could get a battery which allows a higher DoD. For example, if you bought a lead crystal battery which allows a 100% Depth of Discharge you would only need a single 75Ahr battery. Or, if you bought an AGM battery which allows an 80% DoD, you would only need a 95Ahr battery to get a high enough capacity so that you can safely discharge the amount needed. These are just a few options, but you get the point; pay attention to the Depth of Discharge figure on your battery and plan accordingly.

NCC Category

Now that you know what capacity you need, let’s look quickly at something you’ll see on most leisure batteries; NCC Category. The NCC (National Caravan Council) created a verified leisure battery scheme to help consumers understand what batteries are suitable for their caravan or motorhome based on how they’ll be using it. This is split into 3 categories:

• Category A - higher capacity for people who frequently use their touring caravan/motorhome away from a mains connection point.
• Category B – aimed at those who frequently use sites with mains connections but require higher capacity for devices such as motor movers.
• Category C – batteries for users that require lower capacity to cover basic operation of their habitation equipment for short periods away from a mains connection point.

You’ll see these classifications written on the information label on top of the battery.

Location & Storage

If you're just replacing an old battery in an existing battery compartment with a new battery of the same size, you just need to consider the dimensions, terminal layout and terminal type of the new battery to make sure it fits in your compartment and connects to your existing leads. However, it's worth reading through the information below to make sure you existing battery compartment is safe and upto spec.

If you're fitting a new battery setup in a campervan conversion, or increasing the number or capacity of batteries in your existing set-up, there's a bit more to think about. Generally speaking, if your leisure battery is not installed in the engine compartment, it should be placed in a compartment which is designed to protect it from mechanical damage, allow "easy access" for maintenance and/or removal and be secured to stop the battery from moving around while your vehicle is moving.

Thanks to the term “easy”, there is a fair amount of wiggle room in this in terms of the location of the battery compartment, as people’s definition of "easy to access" can vary wildly. The key thing is making sure that your battery compartment is easy for you to access should you need to check anything in it, though there are some stipulations that you need to be aware of regarding the storage of the battery.

The battery itself must be:

• Securely mounted.
• In the case of batteries with liquid electrolyte, they need to be placed upright to avoid potential spillage of said electrolyte.
• Have an acid-resistant, liquid-tight tray capable of holding at least 20% of the electrolyte capacity of the battery in case of spillage or leakage.

With regards to the battery compartment, you need to make sure the interior is well ventilated and protected against the corrosive effects of acid-laden gases that batteries produce. This can be achieved a few different ways:

• Installing a sealed battery with an external ventilating kit so the battery vents to the exterior of the van or caravan.
• Installing the battery in an enclosed battery compartment which is sealed from the habitation area, protected internally against corrosion, and ventilated to the exterior of the vehicle by means of tubing.
• Installing the battery in a sealed compartment, accessible from outside the vehicle with ventilation from the top of the battery compartment directly to the exterior of the vehicle.

While we're talking about battery compartments, it is worth mentioning that in the instance where you are fitting multiple leisure batteries in an older or lightweight caravan, you may need to install a second battery compartment. Some lightweight caravans can become unbalanced if you had multiple leisure batteries on the same side of the vehicle, so you should install these in such a way that the weight of the batteries is distributed evenly

Depending on the location of your battery compartment, there are a few more things to note. When the battery compartment door is located inside the vehicle, it needs to have an airtight seal to prevent gases getting into the habitation area and also have ventilation to open air through a vent tube.

When the battery compartment door is located outside the vehicle, but the box itself extends into the habitation area, you have to make sure there are no entry holes, cracks or fissures present that would allow gasses to enter the habitation area. That said, sometimes you can’t help having some holes drilled for cable access, so in these cases, you need to provide gas-tight passage between the battery and the connected equipment, most commonly by using gas-tight glands that manufacturers of battery compartments and even motor movers are starting to supply.

Another point we need to mention is that no fuses or other overcurrent protective devices should be placed in the battery compartment based on the current guidelines laid out in EN 1648-1 from the European Standards organisation which state that, “overcurrent protective devices shall not be fitted… in the compartment for housing an auxiliary battery whose electrolyte can flow in case of malfunction”. Now you may be wondering whether this still applies to batteries like the AGM construction where the electrolyte is not a liquid. In short, yes it does. This is because, even though AGM batteries don’t have a liquid electrolyte, they still produce gas with excessive discharging which could pose a risk of explosion in the event of a fuse sparking in the compartment, so best practice would be to have your overcurrent protective devices as close as you can to the battery, but in a separate container.

With regards to underslung batteries, the only somewhat clear guidance we get is from Directive 2007/46/EC, Annex II of the European Parliament. This states that for caravans or motorhomes (special purpose M category vehicles) weighing less than 2 tonnes, the minimum ground clearance under either axle must be at least 180mm and 200mm between the axles. This then increases to 250mm under each axle and 300mm between the axles for caravans and motorhomes weighing between 2 and 12 tonnes.

Chargers

Another figure you'll see listed on leisure batteries is the number of "cycles" they can do. This figure indicates the number of times the battery can be discharged to half capacity and recharged before failing. The number of cycles can range from 70 to up to and above 400, so it's a number that is worth paying attention to.

One way to make the most of the number of cycles you have is with a good quality charger.

When selecting a charger, you need to make sure its charge profile matches that of your battery because not all batteries use the same voltages and current when charging. Charge profile refers to the set of instructions for how best to charge your battery based on the manufacturer’s maximum voltage specification and can usually be found in the user manual you got with the battery.

You need to be picky about this and make sure you select correctly; trying to charge a flooded lead-acid battery on an AGM charge profile or vice versa can cause gassing and severe damage to your battery. That said, some chargers can accommodate for various charge profiles, though they have to state this explicitly in their user manual, so make sure you check this is the case if a charger claims to do this.

Modern chargers are quite clever, most of the process is automated and there’s certain safety measures built into them so you don’t have to constantly monitor them, and some can even power leisure equipment when the vehicle is connected to mains (typically provides around 25A/300W). That said, if your chosen battery emits combustible gas when on charge, make sure the charger output is limited to 10% of the battery’s rated discharge rate in Ah at 20 hours to minimise the risk of an explosion of any kind.

How do leisure battery chargers work?

Leisure batteries typically charge in 3 stages: Bulk, Absorption and Float.

Bulk Charge - At this stage, the battery’s current sits at the maximum safe rate until the internal voltage rises to round 80-90% capacity. Voltages often range from 10.5V to 15.5V at this stage.

Absorption Charge - Now the battery’s voltage stays consistent and then gradually tapers off as the internal resistance within the battery increases. Voltages at this stage usually range around 14.2V to 15.5V.

Float Charge - At this stage the battery has reached full charge, so the voltage from the charger will drop to around 12.8V to 13.8V to reduce gassing and prolong battery life. You may see this referred to as trickle charge as it is designed to keep the charged battery from discharging.

How do batteries fail?

A battery fails when the active material in the plates inside the battery can no longer sustain a discharge current. This can happen for a number of reasons, which we will dive into now:

• The most common way batteries fail is due to something called positive grid corrosion; this is a natural process that happens with most batteries eventually, and occurs because the electrodes within the battery are always reactive, and so the positive grid will degrade and inevitably fail over time. This in turn causes a short between the plates of the battery.
• The active material of the plates will also shed or flake off during charge and discharge cycles which causes a loss of plate capacity as well as a brown sediment called “sludge” to build up in the bottom of the battery case. As this sludge is made up of reactive material, when it builds up too much, it can cause a short circuit between plates and thus cause the battery to fail.
• Sulphation is one of the main ways leisure batteries fail if not looked after. This is when hard lead sulphate crystals coat the plates of the battery when it has been left in a discharged state for an extended period of time. You can potentially recharge and salvage a sulphated battery, though it depends on whether or not the sulphation has caused permanent damage to the battery or not.
• Another potentially serious battery fault is called a dendrite short. Dendrites are metallic microstructures that look almost like tree branches that are made up of charged particles on the negative plate as a result of repeated charging and discharging or leaving a battery in a discharged state for a long period and then recharging it. If these particles cannot be absorbed in time, they build up and form a dendrite, which can then grown enough to the point where it bridges the electrolyte and creates a short between the battery’s plates. This may not sound that bad, but it can be very serious if left unchecked and in fact was the main cause of the Samsung Galaxy Note 7 catching on fire when charged.
• Batteries also don’t do well in excessive heat. Heat can cause a range of problems including positive grid growth, positive grid corrosion, negative grid shrinkage, buckling plates and loss of water (which in sealed batteries cannot be replaced), all of which can cause your battery to fail.
• Improper charging practices are the final one for this list. Discharging your battery too deeply, fast charging and overcharging your battery can accelerate the aging process. Always stick to the Depth of Discharge (DoD) figure listed on your battery, charge your battery over a long period of time (usually from 10 hours up to a full day), and make sure to take steps to avoid overcharging by investing in a good quality, modern charger with a built in overcharge protection feature or actively monitoring your battery throughout the charging process.

How to keep your battery healthy and functional

Limit the depth of discharge – Remember this from the "Capacity" section? Pay close attention to it, this will help your battery last as long as it can. To refresh your memory, Depth of Discharge (DoD) is the percentage of your battery's capacity that can be used before you need to recharge it, so if your battery has 40% DoD, you can only use 40% of its capacity before it will need to be recharged.

Allow your battery to charge fully – Don’t just charge your battery for a little bit here and there, allow your battery to have the time to charge fully, and keep in mind this could be 10+ hours at least, and up to a day depending on the charger you have.

Control overcharge – most battery chargers are designed to be current and temperature sensing, so they can prevent excess current from entering the battery and detect if the battery is getting too hot and likely to overheat or cause thermal runaway, so make sure your charger has these features.

Mind the temperature - Generally speaking, leisure batteries don’t do well in the cold, you’ll often see a performance drop in the winter. The amp-hours (Ah) figure listed on your battery will have been taken at 25°C and can drop up to 20% when you get down to freezing temperatures. On the opposite end of the spectrum, as lead-acid batteries absorb heat, so hot weather causes an increase to the level of chemical reaction within the battery. The effects of heat can be astounding, for every 10°C above 25°C, the lifespan of the battery can be reduced by as much as 50%. To help minimise heat damage, keep the battery compartment as cool as possible and minimise your usage of the leisure battery or lower the load you are drawing.

Give your battery some TLC in the off season – If your vehicle is going to be in storage for a long time, make a point of checking your leisure battery every two months to recharge and clean it if needed to keep it in good shape.

Treat your terminals – Apply a thin film of grease or petroleum jelly to the battery terminals, this well help to prevent corrosion and provide some lubrication for reattaching your clamps.

Get quality connectors – Make sure to use good quality battery clamps to ensure a strong connection and full contact with the terminals to help minimise corrosion on the clamps themselves, and if you are using spring-loaded clamps with steel contact points, check them routinely for any potential rust.

Terms to be wary of

Maintenance Free: All this means is that the battery is sealed, and you don’t have to top up the electrolyte within the battery, that said, you should ALWAYS get your battery looked at when servicing your caravan just to be on the safe side. The potential damage a faulty battery can cause can be severe.

Fast Charging: Whilst batteries that are advertised as having the ability to do this may be able to do it, for the sake of the long term health and lifespan of your battery, avoid doing it. Leisure batteries are designed to be charged slowly at low voltage according to the manufacturers specifications to keep the battery as healthy as possible.

Rules of Thumb

• The heavier, the better – As mentioned, there is a huge variety of leisure batteries out there, and some of them are just not good quality and won’t last you very long, so a good way to check if a battery will last can be the weight of it – the heavier it is, then generally it contains more lead and will be able to hold a larger capacity of charge.
• Check the shelf life – Some retailers can be a bit lax with recharging their stock of batteries, so make sure you buy a battery that has been produced in the last 12 months to make sure it is as healthy as possible when you purchase it.
• Invest in your leisure battery– Where possible, we would always recommend getting the best battery you can; if you buy a high quality battery and look after it, you’ll definitely see the value with time. Even if you can’t afford a higher end battery, just getting a slightly higher capacity battery than you think you need can improve your battery’s longevity as you are allowing some headroom to account for any unforeseen circumstances.
• Always discharge your battery in line with manufacturer specifications - leisure batteries are not designed to be discharged down to zero every time they’re used, in fact this can cause irreparable damage to your battery, thereby shortening its lifespan or causing it to fail entirely. Manufacturers will advise how much your battery can be safely discharged either on the battery itself or in a user’s manual with a figure called Depth of Discharge (DoD), so if the DoD figure is 40%, that means you can discharge your battery by 40% of its capacity before you need to recharge it.