Motorhome and Caravan Battery Life
Motorhome and Caravan Battery Life – how to select and manage motorhome and caravan batteries
How to get the most out of your expensive deep cycle batteries.
No question, good quality deep cycle batteries are very expensive – and I’m guessing that they are not going to get any cheaper any time soon. So how can we make sure we get the very best life out of these necessary lumps of lead and acid?
There are two factors that affect battery life above all others –
- The type of battery – its physical and chemical composition.
- How you treat the battery – how deep and how often you discharge it and how well you charge it.
Addressing the first point is quite simple – you pretty much get what you pay for in deep cycle batteries. The three most common types used in caravans and motorhomes are :
- Flooded lead-acid – these are often just called deep cycle batteries. You need to keep these topped up with demineralised water and they must be located in a ventilated area (not inside the vehicle). These are the cheapest of the three types.
- Gel Batteries – these batteries use a high viscosity electrolyte and are normally sealed, but can release hydrogen under some circumstances. These are normally more expensive than flooded lead-acid.
- AGM or absorbent glass mat – these batteries have a glass mat between the lead plates that keeps the electrolyte in contact with the plates and helps prevent warping of the plates. AGM batteries are (at least on the surface) the most expensive of the three options.
So why choose Gel or AGM?
Before I answer that, I need to explain “Depth of Discharge”. A battery is simply a device for storing energy – electrical energy. There is no magic; they are storage container (just as a bucket is a storage container for water). Depth of Discharge is how much of the stored electricity we take out.
If we have a 100aH battery (that is 100 amp hours) and we use one third of its stored energy, we can say we have discharged it to 66% – or a depth of discharge of 66%.
Lead acid batteries hate being discharged! (They hate being left in a discharged state even more!) If you want your batteries to last forever (metaphorically speaking) – don’t ever discharge them. Of course a battery that never gets discharged is about as much use as a car that never gets driven – and most things last a lot longer if you don’t use them.
I always think of discharging my batteries as being like inflicting pain on them – the deeper the discharge, the greater the pain and the shorter the life of the battery.
GEL batteries are less vulnerable to the “pain of discharge” than Flooded Lead-Acid. They have a “higher pain threshold” if you like, and can give up more of their stored energy before feeling that pain.
So now that we know about battery pain – we can understand that selecting the right battery is the first step in getting long and trouble free service from our motorhome and caravan batteries.
Selecting the correct number and capacity of batteries is the next step.
Let’s look at a simple example:
If we have a single 100aH battery and we take 50aH from it every night we are discharging it to a DOD (depth of Discharge) of 50% every day.
Now consider if we had two of these batteries. Two times 100 Ah equals 200aH (this is exactly the same as having a single 200aH battery). If we take 50 aH out of this set each night we are only taking them to a DOD of 75% and in so doing inflict far less pain on the pair of batteries. They will probably last perhaps 3 times longer than the single battery setup.
The Real World
Now this is great in theory – all this DOD and AH stuff – but what about the real world? How can we know how deep we are discharging our batteries? Can we use the battery voltage to tell us this stuff?
NO! NO! NO!
If I repeat one phrase more than any other when talking to fellow travellers about their electrics, it would have to be:
Battery voltage is NOT a good indicator of the state of charge of your batteries!
To put it plainly using a voltmeter to determine battery state is like trying to use a temperature gauge to determine how much fuel you have in your car.
You can read a full explanation of why this is in this article.
Unless the batteries have been completely disconnected for at least 12 – 24 hours the battery voltage will tell you nothing useful (the only exception to this it is if the voltage is very low or very high).
So if the trusty voltmeter is useless, how can we really know what is left in the batteries and just how much pain we are inflicting each day?
The answer comes in the form of a real battery monitor. These devices use a shunt (a current sensor) to measure how much charge goes into the battery and how much power you take out. So long as it knows the total battery capacity, it is fairly simple to determine how much power remains at any point in time.
Unfortunately the term “Battery Monitor” is also used to describe the $20 red-light/green-light devices that simply use voltage to try to make some guess about battery state. These devices are worth slightly less than nothing and should be avoided.
Real battery monitors use shunts (current sensors with big brass blocks) and (unfortunately) cost a lot more ($200 – $450 at the time of writing).
The battery monitor that we use is called a “Xantrex battery monitor” and operates just like the fuel gauge in your car (except that it is digital). When it says 100% I am very happy that my batteries are full. When it says 40% – I can almost hear them screaming in pain.
I cannot recommend these devices enough; they are the best thing to happen to caravan and motorhome electrics since the invention of the solar panel.
Xantrex now make two varieties of these devices – a lite version and a pro version. The lite version is about $100 less expensive and should do everything that most people will need it to. It is absolutely critical that it is installed and setup correctly. If it is incorrectly installed or configured it will feed you false information.
The strength of the Australian dollar has made purchasing these devices from the USA quite attractive and a quick search of eBay reveals a number of brand new battery monitors available from the US for around A$200 plus postage.
When I purchased my new batteries for our motorhome, I looked very closely at the specification provided by each battery manufacturer. I have calculated that given my current (full time) use of our motorhome, I should get 7 to 10 years of service from my batteries. With the proper care and treatment I see no reason why everyone should not expect the same from their battery system.
How old are your batteries? What type do you use and how do you manage them – let us all know. Leave a comment in the “Leave a Reply” box below.
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Thanks and happy Motorhoming and Caravanning - Gavin & Tracey.