Motorhome Leisure Battery Drain? Causes and Cures

What would cause your leisure battery to drain?

The main causes of battery drain is a faulty leisure battery. The battery may be old, cold or just need to be replaced. A faulty piece of equipment such as the charger or anything connected to the leisure battery could cause a drain.

This blog takes a look at some of the problems we had. The diagnosis of what is causing a battery drain can be difficult to work out. By taking some simple methodical steps you can usually find the battery draining culprit.

Most of the information is based on wet lead-acid batteries as those are the type I use in my motorhome and they tend to be a bit cheaper than other types.

For an explanation of the different types of leisure battery – see here.


Causes of Leisure Battery Drain?

There are many reasons battery drain occurs in a motorhome. The main ones being;


Faulty Leisure Battery

This tends to be the main problem. If your battery is very old – more than 5 years would be considered old, although I personally would use the battery as long as possible. They are not cheap to replace and I like to squeeze as much as possible out of them.

A battery can fail for a number of reasons



Age of the Battery

There are only so many years that a battery will work for. Generally, around 5 to 7 years if you really look after your battery and have a good charger, it may even last longer. If you have a cheap charger and let it run flat a lot you probably won\’t even get to 5 years. These ages are for a typical lead-acid battery, AGM (Absorbent Glass Mat) batteries are about the same.



Extreme cold is very bad for a battery. A fully charged battery can survive much lower temperatures than a poorly charged battery approximately -50℃ (-58℉) for a fully charged and -1℃ (30℉) for a flat battery. So make sure your batteries are fully charged in the winter. A battery will not recover from being frozen.


Inadequately Charged Battery

The charger is very important. If your battery is not charged properly each time then it will radically shorten the life of your battery. This tends to be what people underrate. The charger is every bit as important as the battery. If you buy a really expensive battery and use a really cheap charger the chances are the life of the battery will be shortened.

This is especially true if you tend to run your batteries until they are flat. Once they reach the 12volt mark they are flat and you should not use it anymore as damage will occur. There are some devices that you can buy which will protect the battery by cutting it off once the voltage reaches a certain level. They can be a pain as your power just cuts off abruptly but they do protect your expensive batteries so worth it.

To charge from 12 volts or below you really need a smart charger that will adjust the current to bring it back to a charged state. They will charge it in such a way as to reduce any internal damage that may occur.

Some of the cheaper chargers have such small current ratings that they may never be able to charge your leisure battery. Most of the recommendations I have read is that you should have a charger that is able to charge at a rate of at least 10% of your total amp hours. So if you have a battery set up of just one battery at 110Ah you will need a charger that can charge at a rate of 11A. If you have a double battery set up your charger will have to be able to charge at 22A. At this power, the charger will be able to charge the battery quickly and recondition the battery as well – assuming you are using a smart charger.

Spend time choosing a charger and get the best one you can afford.


Battery Left in a Discharged State

It is very bad for a battery to be left in a discharged state, that is, left to go flat and not charged. The battery cells quickly degrade in this state and is sometimes not recoverable. The battery cells suffer from something called sulphation when they are let go flat and this will greatly reduce the life of the battery.


A Light Left on or Other Hidden Loads

Not just a light but there are a whole number of electrical devices and items that consume power in a motorhome. Even things like signal boosters and amps can be a problem as these things are generally out of sight and forgotten about. Get to know your motorhome and check out all the items connected to the power and work out if you need them.

We had a load of old boxes connected to the power that were not required any more, such as aerial boosters for ariels that are not in use, tv signal booster that didn\’t work but was still consuming power. It was amazing what people had kept connected to the battery and never used, these were relics from the 1990s. All these little devices suck the power out of your battery.


Inverter Left On

This is a favourite of ours. The inverter switch is near the floor and very easy to brush by and switch on the inverter. There is only a tiny light to let you know and even though we have never used it the battery has gone flat within a few days of being left on. A bit of a nuisance. Make sure your inverter is off if not in use as it may still be consuming power.


Poor Connections

As motorhomes age so do the electrical connections. I have spent hours replacing connections and even some cable. If the connections get we they will corrode and will cause various types of faults. Whenever you find one fix it straight away or you will spend hours at some point in the future trying to find a fault that takes a few minutes to fix.

Always check the earthing cables that go onto the body of the motorhome, these are also a favourite place for corrosion and poor connections.


Years of Electrical Alterations


This can be a real pain. Sometimes to find a fault you have to trace a cable from the battery to wherever it goes just to find out what it does. When additions are made to a motorhome they can be wired directly from the battery rather than through the fuse box. This means that these new additions won\’t be isolated by the master switch and can make the process of fault finding take a lot more time.

There is no easy way to do this but a cheap multimeter will come in very handy. You will also need a lot of time.


Very Cold Weather

The amount of power a battery has is measured at 25℃ (77℉). For each 1℃ (1.8℉) the temperature drops the efficiency of the battery drops by 1%.

So if you have a 100ah battery and the temperature is 0℃ the battery will operate like a 75ah battery, you basically lose 25% capacity.

This is very significant in very cold countries. Your battery can be greatly affected by temperature and if the battery is old then the effect is even greater. This is why, especially here in Scotland, a lot of batteries fail in the winter time.

I was shocked when I found this out, a lot of the time it can be -5℃ here – that\’s a 30% drop in battery capacity.


The Radio – Especially if Newly Fitted

A lot of new radios will have a power draw so they can keep settings in the memory or have their lights on so you can find the power switch.

The radio in our motorhome would have been wired through the ignition switch and would only come on when the engine was on or the ignition on. As I like to listen to music while we are camped I wired the stereo to the leisure battery rather than the starter battery. This helps to protect the starter battery but will draw a constant current from the leisure battery. Doing this helps to protect the starter battery from being depleted and leaving you stranded somewhere.

Our new radio has a constant light on which is a real nuisance, however, removal of the facia plate cuts all the power. If your stereo has a removable facia try that and see if it cuts all power usage.


Defective Charge Relay?

It is possible for the split charge relay to become stuck and you would have the starter battery connected to the leisure battery. Not a huge problem and you were not in a campsite for some time. You would only notice when you have drained your leisure battery and find that the motorhome won’t start. This would give the impression of having a faulty starter battery.


Tracking Down the Cause of a Battery Drain

This is by no means a complete guide but may point you in the right direction.

The best way to track down an electrical fault is to work in a logical fashion. You will need a lot of patience and a multimeter. The first step is to;


Start at the Battery.

First of all, make sure all the connections are good. Look for any corrosion or loose fittings and connectors. All the cables should look nice and clean with connectors tightly fitting  – they should not come away with the slightest tug. If you are happy that the battery terminals are good then;

Disconnect the battery and take a reading of the voltage.

If the reading is low charge the battery with an external charger if you have one, otherwise you will have to charge the battery on hook up.

See if the battery holds a charge, try and give it a day or two to make sure it\’s going to hold any charge. If it seems to lose charge then you may have a faulty battery.

If you determine that the battery is good and the battery is holding a charge then refit the battery and move onto the next step.


Switch off the Power for the Habitation Side of the Motorhome.

There will be a main switch for your van that will switch off everything that is connected to the motorhome leisure battery, at least that is the theory.

If your motorhome is any age at all you may find that there are a few additions or modifications that actually bypass this switch.

So you want to determine if there is anything in the motorhome that will be using power when the main switch is off.

You can use a meter to see if there is any power usage or if you have a good power meter in your motorhome this will also be able to let you know. If you don\’t have a meter then it\’s a waiting game to see if your battery is being used by something. Give it a few days and you should be able to see. By switching the main switch off you cut out all the appliances that are powered through the fuse box. If the battery is not showing a drain you know its problem with something that\’s powered through the fuse box. If it still shows a drain then its something that has been fitted later and does not go through the fuse box. Simple.

If the battery holds its charge then the drain is something that is powered through the fuse box.

Next stage is to check the fuse box. Take a photo of your fuse box and pull out all the fuses. Put them in one by one until you find which circuit in the motorhome is causing the power drain. Either measure the current of each circuit with a multimeter as you put the fuses back in or keep an eye on the meter at the motorhome control panel if you have one.

Usually, there are about 10 fuses.

In our older motorhome, we have really ancient fuses, the type before the blade fuses. These sometimes need a little rub with sandpaper on the fuse and the fuse holder to freshen up the connection.

Once you determine what is on the problem circuit then it should be relatively easy to fix from there. Just see what that particular circuit is running and you should be able to narrow down to the offending item.


Personal Battery Problems so Far

In our motorhome the leisure battery had a second battery in parallel so should be adding power to the leisure side of the motorhome. I was finding that the leisure battery was draining way too fast once the electrical hook up was removed.

This seemed very strange as all seemed normal. Both were giving a reading of about 12.6 volts which seemed fine but once disconnected from the charger would quickly lose power and be useless in a couple of days.

All became clear when I disconnected the batteries from the motorhome and measured them independently. One of the batteries was reading 6 volts. This is a dead battery and if yours is reading anywhere near 6 volts then get rid of it. The poor battery was pulling down the good battery and the good battery was making the poor battery look good.

Once the bad battery was removed, I did a recondition charge in the good battery and checked to make sure that it was still able to hold a meaningful charge.

I used a tester that puts a load on to the battery and lets you see if it is still functional. This is a very useful addition to your toolbox if you are having battery issues. Can save you a lot of time and expense if you can determine how good a battery is. A battery can read 12.8volts and still be a dead battery. The only way to determine is to put a load on it and see if it can actually power that load.

I have reduced the leisure battery to one battery and find that it works well. We have not had a problem since.

I actually have two leisure batteries but I don\’t have them connected in parallel, they are actually both independent and only use them one at a time. This is due to the charger, which is the original, not being powerful enough to charge both the batteries together.

I will explain this in another post if anyone is interested – Motorhome Batteries Explained.

The battery side of a leisure vehicle is quite complex there has been so much written about batteries and how to care for them and it can all seem a bit overwhelming. If you have had any experiences that may help others please leave a comment.