Motorhome Batteries Explained

The subject of batteries in the motorhome is way more complicated than I first expected. Choosing a battery to suit your needs and budget can be tricky. Hopefully, this blog will be able to help a little when you get into the subject of how to power the electrics in your motorhome.


Throughout this blog, I use the word usually a lot. The reason for this is when you are dealing with an older motorhome there (usually) has been a lot of modifications done over the years. Some of them will make perfect sense, others won’t. Sometimes the modifications can be detrimental to the motorhome or the full implications of the changes made have not been understood. So when I am describing a particular example this is my experience of our motorhome. Your motorhome will have seen different modifications over the years from ours. Just something to watch out for.


The first thing to have a look at is how the power system in the motorhome is actually set up.


Motorhome Battery Setup

A motorhome usually has two 12 volt systems.

One system will be for the starter and is basically a car battery that is used to power the base vehicle in the same way a car battery is used to power a car. It will start the vehicle and then the alternator kicks in and powers everything like the headlights, indicators, cab fan and recharges the battery. Being in a motorhome the alternator will also power things like the fridge and will recharge the leisure battery.


The second system consists of a leisure battery and should be separate from the starter battery, it is only used for the habitation side of the motorhome itself. It will power the lighting, fresh water pumps, taps, fans. This system will be charged by the alternator but can also be charged when connected to electrical hook up and if you have a solar panel this will be used to keep the leisure battery topped up.


The leisure battery is what powers all your electrical demands when you are not connected to an electrical hook up. When you are on electrical hook up the mains powered sockets will work but all the things powered by the leisure battery are still powered by the battery when on electrical hook up, the only difference is that the leisure battery is on constant charge and you don’t have to worry about it going flat.


The two 12 volt systems are kept separate so you don’t suck the power from the starter battery when you are parked up so the engine battery is usually protected from discharge while you are using power in the motorhome.


This usually means that there are two fuse boxes in the motorhome (at least there is in ours, newer models may be different). One is for the running of the vehicle and has fuses for things like headlights, indicators, windscreen wipers, really anything to do with the running of the vehicle. The other fuse box is for the habitation side of the motorhome – for the internal lights, pump, heater fan and anything powered in the habitation area.


Once you manage to separate which parts are for what it makes tracing any faults you may have that bit easier. I found that spending a few days getting to know our motorhome really helped to give me an idea where faults may be and gave me the confidence to tackle most jobs.


Types of Battery


Car Battery
White car battery

There are several different types of battery that you can choose from. Each type has pros and cons and each type can vary greatly in price. It took me a long time to come to the conclusions I did. There are so many variables and things to consider.

Flooded Lead Acid Batteries (sealed)

These are the cheapest and most common type of 12-volt vehicle battery. The chances are if you have a car it will have one of these batteries. They are used mainly for starter batteries but you can get leisure batteries in this type but they are constructed differently from the common starter type and usually cost a bit more.

There are a couple of different types sealed and unsealed. For the purpose of this blog, I will be talking about the sealed type of lead acid battery which is classed as maintenance free.

With the unsealed lead acid, you can top up the electrolyte levels and require more maintenance which brings in a bit more complexity which is really not needed – this subject is complex enough.

Lead acid batteries are old but reliable technology. Older motorhomes will have been designed to have this type of battery fitted and the charging systems will be optimised for this type of battery. Ours certainly is and a battery upgrade has to be considered carefully.


Gel Batteries

As the name suggests Gel batteries have a Gel in them. This makes them different from the lead-acid batteries in that they are not wet so if they are inverted the contents won’t spill out. This makes gel batteries great for things like quad bikes, jet skis and any other use where there is a chance the battery might get inverted.

They are well suited to be leisure batteries and are deep cycle but they tend to be expensive and, from what I have read, not much better than modern glass mat batteries.

They also have to be charged relatively slowly, putting in too much power will damage them cutting the amount of charge they can hold. So, you may have to change your charger to accommodate a gel battery.


Absorbent Glass Mat Batteries(AGM)

Glass mat batteries have a mat in them that contains the electrolyte. Just like the gels, they are not classed as wet batteries. They are well suited to as both starter batteries and deep cycle leisure batteries.

They are a good bit more expensive than the lead acid batteries but are more suited to the function of a leisure battery. They are great at releasing small amounts of power for large amounts of time. Some can be discharged more than the 50% norm that is recommended for lead-acid giving more useable power. Most though give a life of 300-600 cycles quoting a 50% depth of discharge.

When you calculate this out you should see up to 2 years of use before there is a degradation in the battery – assuming you use the battery every day and use 50% of the capacity each day. If you use your motorhome for a month or two every year you should get many years from these batteries if they are looked after.


Lithium Leisure Batteries

These are the top of the range batteries at the moment and are very expensive. They can be charged quickly and can take a lot of charging cycles making them very robust.

They have a larger capacity for the same rating than the glass mat, gel and lead-acid batteries.

What this means is that in a perfect world you have a 100ah glass mat battery that can be discharged 50% before a charge is required. This will give you 50ah of usable power.

A lithium battery rated ar 100ah can be discharged 80% before a charge. This will give you 80ah of usable power. That is 30ah more power before a charge is required, a substantial difference.


Other advantages of lithium batteries are;

They can be charged very quickly as they are able to take a lot of power without damage to the battery.

They are very light – usually less than half the weight of the other battery types.

The output is steady voltage until they are discharged – there is no voltage drop, or at least very little. This means that when they reach 80% discharge they just go off rather than the lights dimming or pumps running slowly as you would see with an ordinary battery.

There are many more positives to having these batteries but they come at a cost – around five time more than a good quality AGM battery. They are coming down in cost so they may become the best option.

In fact, if you are a heavy user of your batteries and live off grid you can use a lithium battery discharged to 50% capacity every day for more than 13 years according to the figures. That is way more than the two to three years of the AGM batteries.

At the moment we use the motorhome for about two months per year (we will be increasing this in the near future). Using the figure of 5000 cycles and assuming we use the battery full cycle every day we are away then a lithium battery should last us just over 83 years. I don’t know if I am young enough to get full use out of a lithium battery and I am sure an 83-year-old battery will have issues.
Its amazing just how many cycles you can get from a lithium battery.

Keep an eye on them. They are getting cheaper all the time.


Enhanced Flooded Battery – EFB Battery

The Enhanced Flooded Battery is relatively new. Many of the big car manufacturers are moving over to these batteries instead of using AGMs. They are able to charge much faster than an ordinary flooded battery and have a good number of cycles. They work well both as starter and leisure batteries.
They are still new but look like a reasonably priced option.


What is Depth of Discharge (DoD)?

The depth of discharge is the amount the battery can be discharged safely. Most manufacturers will specify a 50% depth of discharge and the number of cycles the battery can take at that depth of discharge. So the manufacturer may specify 300 charge cycles at 50% depth of discharge. Which means you can discharge the battery to 50% 300 times before the battery degrades.

If you go more than 50% – say down to 80% you will seriously shorten the life of your batteries.


What is a Battery Cycle?

A battery cycle is a fully charged battery being discharged to flat and recharged – this counts as a full cycle.

However, if the manufacturer specifies so many cycles at 50% depth of discharge then discharging to 50% would count as one cycle.

If your battery is only partially discharged before recharging then this counts as a partial cycle. So if you discharge the battery by 25% and then recharge then you have used half of a cycle if the DoD is specified at 50%.

Batteries vary greatly in the number of charge cycles they have, I have seen from 200 to 5000 charge cycles. Generally, the cost of the battery increases as the number of cycles goes up.


How Do You Know When a Leisure Battery is Flat?

All this talk about discharging the battery to 50% maximum seems obvious but this is not as obvious as it seems.

At first, I thought it was when the battery can no longer power anything and would be reading about 10 or 11 volts, but that would be a dead battery and is very bad for any battery. It can be very difficult for a battery to recover from this level of discharge. Some have thought that the battery would read 6 volts at 50% discharge but this is even worse.

When I purchased our motorhome I found a battery in the system set up by the previous owner which was reading 6 volts when disconnected. Needless to say, this battery was a goner.


In fact;

A charged battery is 12.7v or more

A flat battery is 12v or less

50% Charge is in the 12.3 – 12.4v area  


As you can see there is not a lot of voltage drop before a battery is classed as flat. If you are at 12 volts you will be damaging the battery. 

So if a manufacturer says that their battery can be discharged to 50% before a charge then you will be seeing a voltage reading on the battery of 12.3 -12.4 volts. Anything less than this and you are beginning to degrade the battery. This goes some way to explain how obsessed motorhome owners are with regards to battery levels. You have to keep a keen eye on the power levels or things could get costly.


These readings are across the battery with no load on it. A battery’s voltage can be dragged down when there is a heavy load on the battery. You want to measure the battery resting.

If you want an accurate measurement then disconnect the battery and measure the voltage. The battery should get time to rest for a couple of hours but in practice, especially if you are camping this is not practicable.

I tend to switch everything off and see what the voltage is at – it gives me a good idea of the battery health. Just remember any load on the battery will drag down the voltage and give a false reading.


Leisure Battery or Car Battery? What’s the Difference?

When having a first look at leisure batteries and comparing to ordinary car batteries the first thing you will notice is the price. Leisure batteries tend to be more expensive than a car battery but appear to be very similar. So is a leisure battery actually required?

A leisure battery is actually quite different from a car battery.

Leisure Battery – is designed to give out relatively small continuous amounts of power over a long period.

Car Battery – is designed to give a large amount of power over a very short period of time.


To carry out the different jobs both these batteries are constructed very differently.

The leisure battery usually has thicker plates and glass mat separators. This allows them to be discharged slowly and to a deeper level than a car battery and be charged with very little damage. A car battery does not like to be discharged and will not last very long being used in this way.


You can use a car battery as a leisure battery but the life of the battery will be greatly reduced. So it may appear it be a cheaper option but in the long term, you will pay more if you are discharging the battery a lot.


Although that is the thinking I have met people who have used ordinary car batteries for many years without a problem. This may be due to their actual power consumption being very low and taking good care of the batteries.

I would say if you use a lot of power, you are going to need decent batteries.


Which Battery Type is best for a Motorhome?

They type of battery best for a motorhome is… Well, it depends.

It completely depends on how much you are going to use your motorhome and for how long.

The problem with all batteries is they have a limited life span whether you use them or not. The chemicals inside the battery degrade over time.

So, you may have plenty of spare cash and want only the best lithium batteries at £800 to £1000 each. These batteries are the best you can get – they can be discharged way more than lead acid or AGM batteries and they can do that for many more cycles.

But if you always go to campsites and use electric hook up tend you only use your motorhome a few times a year then you are wasting your money.

There is no way you will get the use out of the batteries to justify the cost. After about ten years (this is a generous ballpark figure) they will be degrading and will need to be replaced eventually no matter what type of battery you get.

There is a lot of compelling information for using lithium batteries in your motorhome. These are top end batteries but in my opinion and with our usage I cannot justify the cost.

If you live in your motorhome and do a lot of free camping, have a lot of things to power, these would be the ones to get. For us at the moment – we don’t need them.


The conclusion I eventually came to is that the Glass Mat batteries are probably the best for us. They can take a lot of punishment. They are safer in a crash or if tipped. They can take a lot of abuse in that they can be discharged and charged more than the other batteries (not the Lithium).


Why not Lithium Batteries? (For me)

The lithium batteries are so much more expensive at the moment. There is no doubt that they are great batteries but at the moment they are just too expensive – in my opinion of course. If I had the money I may indeed buy a couple of lithium batteries for the motorhome.

There are also some problems with the charging of lithium batteries when the temperature is around freezing. The battery can be damaged if charged in freezing temperatures. Once damaged a lithium battery loses a lot of its capacity and is not repairable. That would be an expensive mistake.

Living in a place where it often gets below freezing in the winter I would rather not take the chance of damaging these expensive batteries.


Have a look at this beauty! Free delivery – Bargain!


What Battery Capacity Do I Need

The capacity of the battery you will need is completely up to you. It will depend on your personal power usage. If you want to run a television you will need more power than if you want to read a book.

Work out what you want to power, how much power the items use and start calculating. If you know the wattage of the appliance you can work out how many amps you will require for each. Add it all together and you will have a rough total of your power requirements. I say rough as the calculations are never completely accurate, an appliance may use a bit more or less than you have anticipated and you may use things more or less than you originally thought. The calculation will give you a ballpark figure.


If you have something that consumes 60watts then just divide the wattage by 12(volts) and you will get 5 amps.

Now, if you have a 100Ah battery you should be able to run that item for 20hours (100Ah using 5 amps per hour).

That is the theory, simple. The reality is very different for a few reasons.


  1. You don’t want to run the battery completely flat.
  2. You will not get the full capacity of the battery.


The amount of capacity you can get from the battery without damage and shortening the life of the battery is about a third. So a 100Ah battery will give you about 33Ah of good power before the battery is being strained – this is the sweet spot of the battery.

The reasoning for this is that you will not get the battery to charge up to 100% capacity and this will decrease with age. So you will get the charge to about 85% or thereabouts and you can only discharge to about 50% capacity giving you about a third usable power.

That does not sound like much but in practice, I have found it to be ample for our needs. It can be a bit tighter in the winter where a lot more power is required. We do have an old solar panel which will top up the battery during the day which does help a lot.


How Does Temperature Affect Battery Capacity? 

This is something I never gave much thought to but is something to be aware of, especially if you are travelling in colder places or in the winter months.

The capacity of a battery is calculated at 25 degrees c. For each degree below this, you lose roughly 1% per degree. So if the temperature of the battery is zero degrees and the battery is rated at 100Ah then it will behave like a 75Ah at zero degrees. This is why on a cold morning the turnover of the engine can seem a bit sluggish.

So maybe if you are in hot countries that would be better for a battery? Well no.

The battery will start to be affected once the temperature rises above 30 degrees shortening the cycle life by 20%. This means if your battery can survive 300 charging cycles this will be reduced to 240 if the temperature is more than 30 degrees. The hotter it gets the shorter the cycle life.

Turns out that batteries are very temperature sensitive – who knew?


Battery Charging Information

Battery charging is crucial to the life of the battery. Our old charger is very basic and pumps out 7 amps. I plan to upgrade the charger to a 15 or 20A charger in the near future so we have the capacity to charge the batteries at the optimum level to protect and preserve the life of the battery.

I have put together another blog specifically about the charger we have chosen for our motorhome.

When choosing a charger


Leisure Battery charging


How do the batteries charge in a motorhome?

There are several common ways to keep the batteries of your motorhome charged up and in tip-top condition ready for action.

Charging from the Alternator

The batteries are charged by the alternator when the engine is running. Both the starter and the leisure battery will get a charge. This means that after a good run the batteries should be ready for action. If you are parked you can let the engine tick over for a while to top up the batteries, just make sure there are enough revs to engage the alternator. You should be able to see from your built-in meter when there is power going to the leisure battery.


Charging on Electric Hook Up

Motorhome power supply
This is where the power comes into the motorhome. Old Charger on the right.

When you connect to electric hook up the leisure battery will get a charge. There is a battery charger in every motorhome that will come on when you hook up to the mains. If you have bought a different type of battery it is essential that you make sure the built-in charger can charge the battery you have chosen. When we get a new AGM battery we will have to upgrade the charger to charge an AGM instead of the lead acid and to provide more power than the standard charger. You can read about choosing a charger here.

We have added a small split relay so that when the leisure battery is fully charged the engine battery will get a small top up charge.


Charging Off Grid

Off-grid methods- when you are not plugged in or driving there are a few ways to charge your batteries. The main methods seem to be wind, solar and a generator.


Generator – We don’t have a generator and have no plans to get one. I did look into it a while back and had one picked out but for the type of camping we are doing at the moment, it is not necessary. They tend to be a bit noisy and seem to detract from the point of getting away for some peace and quiet. If you have any neighbours they may not be too happy. But they do provide plenty of power wherever you are so long as you have some fuel.


Wind power. This is probably not the best on its own but with a couple of solar panels should meet your power needs. They are good as they can work at night while you sleep but you do need a good wind for them to kick out the power.


Solar – Solar panels are great for keeping your batteries topped up for when you need them. Solar panels are becoming quite cheap and whole systems can be picked up for s few hundred pounds. I have my eye on this kit of 250W solar panels. Not purchased it so can’t comment on the quality but each time I look the deals get better and better.


How Long do Leisure Batteries Last?

The life of the battery is difficult to determine. They usually last for about 5 years but can last way longer or way less than 5 years. It really is determined by the way you look after your battery.

If you abuse the batteries and constantly discharge them below 50% you can wreck them in a year or less. If you look after them you could get way over 5 years use.

They are more delicate than I first thought and have decided to take good care of them.

The battery is essential to the workings of the motorhome so if the battery goes you have to fork out for a new one. Make sure you get the most from them.


Battery Care

In order to care for your battery, you should charge it regularly. Especially if you do not use your van in the winter. By keeping the battery charged the battery will be more resistant to freezing and will be healthier, ready to use in the spring when you need it.


If you are able to take the batteries out and keep them in a warm place charged. This is even better but not many people have space and if your battery is anything like ours they are not easy to remove.


Extremely cold temperatures are not good for any type of battery so removing them is probably the best option. Just be aware that if you have an alarm system it will not work without power.


Extremely hot places can shorten the like of your battery as well so in the heat try to keep them as cool as possible.


For a battery, the temperature sweet spot is about 25℃. Below this and you lose capacity and above the number of charge cycles begin to decrease. Not a lot we can do about that in Scotland virtually never gets to 25℃, but we can protect the batteries from the extremes.


Make sure you have a decent charge that is able to charge at a decent rate. As a guide, the charger should be able to charge at 10% of battery capacity. So a 100ah battery should have a charger that can give out about 10 Amps. This means the battery can be charged at a decent rate and have a good chance of reaching full capacity.

Charging is a blog on its own.

The main point of battery care is not to discharge them below 50%. Go below that each cycle and the life of your new battery can be reduced by more than 70%. This is without a doubt the most important thing to keep a track of when you are using your battery. There are some power monitors that will cut off the power when the battery reaches a preset level. I am experimenting with this at the moment.

Batteries Dos and Donts


  • Keep your battery charged
  • Keep your battery warm
  • Monitor your battery obsessively – only then are you a proper motorhome owner
  • Make sure you have a decent charger



  • Let your battery be discharged below 50% of capacity – this will greatly shorten the life of the battery
  • Leave your battery for long periods (2 months or more) without charging
  • Let your battery freeze
  • Let your battery get too hot


Hopefully, this has given a glimpse into the world of the battery. There are so many variables but with a little thought and some research finding your perfect battery should be relatively simple. There is so much more to learn about batteries but hopefully, this is enough to go out and confidently buy a battery and know what you are getting for your money.


Good luck.