Caravan batteries, AGM versus lithium

Posted on 12 Sep 2020 by

Caravan batteries, agm versus lithium, and how so many caravans have a sub-optimal battery installation. You might just find this compelling!

First of all, a personal bugbear. Why do some caravans have two or more batteries installed in parallel?

The reason is usually fairly clear – to provide more power for free camping – but putting batteries in parallel is generally a really bad idea because:

  • They will age differently, and one will fail before the other, but that will probably mean that both need to be replaced at the same time, so paralleling tends to make the failure date sooner than would be the case with a single (larger) battery
  • They will get out of balance and are usually wired incorrectly. As a bare minimum, large cables are needed and all positives should go to one battery and all negatives to the other battery
  • Two batteries have more casing than one larger battery, so there’s a weight penalty
  • Very few lithium batteries are designed to work in parallel, and things can go badly wrong if they are installed this way when they shouldn't be, and very few installers are aware of this, so buyer beware!

In fairness, sometimes space dictates using two small batteries instead of one large one but, if space allows, a single large battery is always a better solution.

Now, lithium versus agm batteries for caravans. The great debate continues.

First of all, inside a caravan, you MUST use a battery that doesn’t vent toxic fumes, so if your battery is located under a bed, in a cupboard or somewhere else where it shares the air that you breathe, then you must use AGM or lithium technology (or gel, but that has largely been superseded by agm).

As we’ve said, before, a quality lithium battery will typically handle about 3 times as many charge/discharge cycles as a quality AGM battery. If you compare 100ah of lithium battery to 100ah of AGM battery, you can usually use more of the available lithium power so, as a generalisation, you might find that 100ah of lithium battery does the same sort of job as 150ah of AGM battery (but forget the comments about 2 to 3 times the capacity because they are, simply, wrong).

So, if the lithium battery costs about 3 times as much as the AGM battery, it’ll typically last a lot longer and provide more available power, at a serious price disadvantage. So the jury is out, at this point…

… which is where we come to the knockout punch:

I’m looking at a caravan advert, now, and the van has a tare around 1950kg and a loading weight of just over 450kg, allowing for an ATM (aggregate trailer mass) of 2400kg, so that’s the maximum that the caravan can legally weigh, fully loaded, ready to tow (and I’m going to ignore towball weight for this article, but that’s important, too, so how you load the caravan and where the heavy things go is vital stuff).

Now, where were we? 450kg available above the tare weight – that sounds like plenty of payload available for clothes, cameras, food, and beer. Or does it?


The tare weight is the caravan weight as supplied by the dealer. It ignores water in the tanks, gas in the cylinders, anything that has been added afterwards. Some dealers are even cheeky enough to remove things like batteries before they weigh the van! 

So this particular caravan has two freshwater tanks, 120 litres each, and if you tow the van with both of them full you are using up 240kg (1 litre of water = 1kg) and now your 450kg of available payload is down to 210kg (assuming the grey water tank is empty before you tow, otherwise things are even worse).

This van has two 9kg gas bottles. The 9kg is measured empty and their weight, full of compressed gas, is up to 19kg each, so another 20kg has gone and you are down to 190kg.

You might have added a camping table and chairs (22kg), your golf clubs (14kg), a barbecue (10kg), 2 mountain bikes and their rack (34kg), a carton of beer (14kg) and 6 bottles of wine (8kg).

Sensational. Now you have the necessities sorted, you have a glorious 88kg left for the clothes, food, books, computers, camera gear, water and orange juice, broom, dustpan and brush, hoses, extension leads, sides and floor for the awning, coffee machine, toaster, toiletries, the list goes on…

… and this caravan, which starts with 450kg available, is pretty generous. Many have much less in the way of available payload, and you can’t always empty the grey water tank before you tow, can you?

Thank heavens you didn’t add solar panels to the roof!

You did? Now you are really pushing the limits, and the chaps at the weighbridges really don’t like that.

But the other thing that this caravan has is a "generous" 2 agm batteries, 100ah each, because the manufacturer didn’t bother to think hard about this part of the equation. Those batteries weigh 31kg each. A 200ah agm battery might have saved a couple of kg of case material, but how about a 150ah lithium battery instead? 18kg? That’s a saving of 44kg...

... or, more to the point, 50% extra on that paltry 88kg that you had to play with.

So, if the jury was out before we considered weight, the jury is absolutely convinced beyond reasonable doubt when it comes down to keeping within the legal limits without sacrificing the things that make a caravanning holiday a pleasure.

Of course, there are other matters to consider – battery chargers, solar regulator – they’ve got to have a lithium setting, but that’s all easy to sort out, and you have the side benefit that your lithium battery can be disconnected and left to stand, without harming itself, when you aren’t using the van for a few weeks or months. No need to keep it trickle charged? No battery shop telling you that you’ve let your battery sit in a discharged state and it’s your fault, not a warranty issue!

Case closed?

Lithium battery technology isn't for everyone, but if you are amongst the 100% of caravanners who need to worry about the weight of their rig, then a swap to lithium might be worth considering.

One last thing, though. About twice a week we get people wandering into one of our shops with bargain lithium batteries that they've purchased from a dodgy supplier, at half the normal price, via the internet, and they'd like us to tell them why they don't perform. You're smart enough, I'm sure, to know that if something looks too good to be true then it probably is. A cheap lithium battery is a fire risk and, most likely, a dodgy performer, so buy a good one first time because buying cheap, now, is just going to mean that you'll need to buy good one a few months later!