Batteries, solar, caravans and RVs

Posted on 22 Nov 2018 by

If you read the reviews in caravan magazines, usually written by the caravan manufacturer, you'll see comments like "...comes with an awesome 120w of solar power" or "fitted with a huge 100ah of battery power". This'd be fine, except that these sort of comments are basically hyperbole, or a lie. Marketing people who don't know the law.

So let's take a look why that is (and we'll talk about caravans, but the same logic applies to RV's, camper trailers, 5th wheelers et al).

Firstly, if you have solar on your house, you'll already know that you don't have enough to avoid paying any bills. The truth is that you need well in excess of 5kw of solar on your roof in summer to cope with typical demands, and at least double that in winter (ignoring batteries for the hours of darkenss - we'll come to that). So how are you going to cope in the caravan with a single panel that is less than 1/40th of what you need on the roof of your house? You're not. Even the most frugal travellers need more than that. As a rule of thumb, a 150w panel will keep up with the most economical of 12v fridges, in summer, just. If you are going to sit in your van and run a fan occasionally, some lights, cooker hood, fridge, charge phones and a computer, well, you are going to need a lot more power. I'd aim for 600w of power - 3 very big panels or 4 biggish solar panels, and then you shouldn't have to run the car or generator very often (but you will need to run the generator when you are using the aircon, unless you are either on a powered site or have invested in a really flash battery system).

Secondly, batteries...a 100ah battery stores about 1.2kwh of power (12v times 100ah). Of that, about 60% is typically usable (more with lithium batteries) and, of course, the battery won't be new, so it won't still live up to spec. Anyway, 700whr of usable power might run your laptop for 9 hours if you don't use lights, fridge, fans, cooker hood, inverter, etc etc. So, clearly, 100ah of battery is nowhere near enough. 120ah might be ok for occasional camping, short trips, a day or two away from 240v power but if you are free camping, you need at least 200ah of lithium battery or 250ah of lead acid battery (75kg of lead coming on holiday with you).

Now I like caravans and RVs, and I like the people that make them for the most part, but they are in business to make a profit so very few of them fit the best components in the systems. You can generally expect batteries from the cheaper end of the market, budget 240v chargers (dodgy make, not powerful enough), cheap solar panels, cheap solar regulators, almost always no DC to DC charger between the car and the caravan battery, etc etc. 

Why do you need a DC to DC charger? Well, car alternators used to be ok for charging batteries up to about 85%, but the alternator in your car is probably 12 metres of positive cable from your caravan battery, and the same amount of negative cable coming back, and the cable won't be the heaviest that could be chosen, so the alternator isn't going to deliver as much as it can to the car battery alongside it...and then just about any tow vehicle built in the last 8 years will have an alternator that adjusts voltage output based on heat or engine load, so the charge rate will often be inadequate long before we worry about cable length and a DC to DC charger is, in my view, virtually an essential to wire in, near the batteries, to take whatever it can get from the front of the car and turn it into a proper charging output that the battery will really love, and so will you when it comes to how your batteries perform and how long they last.

Batteries? I'll just say that there are 3 battery types that I would use in my own vehicle. Wet lead acid maintenance free if the battery is outside, breathing to the open air. AGM (absorbed glass mat) if the battery is inside and sharing the air that I breath and lithium (LiFePo4) if the budget stretches to it or if light weight is essential. I will not say anything specifically bad about other battery types but if you about to be lulled by someone's marketing blurb or woefully bold promises about new lead battery technology then ask someone who knows about batteries first. We shift about 2 tons of batteries a week - we're coming on for about a million kg of batteries installed this decade so far and we're right up to speed with the latest developments. If we were comfortable, we'd sell these fancy new products, and we don't, and we won't, because you only lose your reputation once.

Now, when you are on a powered site, or have the generator running, you'll need a good battery charger to convert 240v to happy battery power. A good one, not a cheap one, and one that delivers AT LEAST 10% of the battery capacity per hour, so if you have a 250ah agm battery, you need a charger that delivers at least 25amps, and preferbaly one that delivers 40amps or more. The batteries like to feel a proper charge, they get charged quicker and, if your charger is too small, it may never work out that the batteries are pretty full and drop to a float voltage output. We regularly see batteries that have been murdered by a little charger stinging them at 14.8volts for months on end. They hate that!

You'll probably want an inverter to convert your battery power to 240v to power some of your appliances when not on a powered site. A good setup can run your aircon for long enough to cool a caravan down a bit, boil a kettle, heat the water, brew coffee, microwave a frozen meal, that sort of thing. a 200ah lithium battery might run your caravan aircon for a couple of hours if you really twist its arm, but be careful, frugal, and have in mind that no battery likes to be hammered within an inch of its life day in, day out. On the choice of inverter, though, there are two things. It MUST be pure sine, not modified sine output (modified sine is cheap and does no good at all to things with complex electrical components (laptops, power supplies, TVs, chargers, mixers, microwaves etc). A modified sine inverter is at the end of its capability with a water heater or kettle, or an electric power tool in an emergency, so pay double and buy pure sine, and make sure the inverter is at least 50% more powerful than the most hungry appliance that you are planning to use. Why? Most appliance manufacturers are a bit economical with their power consumption figures, many appliance spike much more power when they are fired up, most inverters do not like to be run flat out and, to be frank, saving a couple of hundred dollars is going to feel mighty silly when the coffee machine doesn't work and you have an urgent need for caffeine.

Back to batteries - when you run an inverter, the battery voltage drops. When you draw a lot of power, it drops a long way and may drop so far that the inverter can't get enough power and switches off. Then the battery voltage will rise, the inverter will switch back on again, then off, then on, until the inverter gives up the ghost because this is bad for the components inside it. So big inverters need big batteries. If you are planning to run a 2000w inverter, then it can easily draw a current of 180 amps or more from the batteries, and that means at least 200ah of lithium battery or 400ah of lead acid battery to have a chance of coping with that (and do make sure that isn't beyond the peak current rating of your lithium battery because, with some, it will be).

I've said all this without pictures so, just to describe the sort of sytem you might like:


  • 240v input to the side of the van
  • This then goes to a breaker
  • Then to a 240v socket which is only live when plugged in to external power
  • This 240v socket has a battery charger plugged in, permanently (and this has to be plugged in this side of the inverter, not the other side)
  • From the socket, the 240v continues to the input side of an inverter with a 240v changeover switch and the inverter is close to the batteries and connected with lovely fat cables
  • Out of the inverter comes 240v for all your caravan circuits, powered by external power when available, and by the inverter when not. The inverter handles the changeover, very quickly, if 240v is unplugged. You shouldn't notice the change
  • From the car, an anderson plug at the front of the caravan and this wired to a dc to dc charger that feeds the battery (or batteries)
  • On the roof of the van, at least 600w of solar panels, also feeding the dc to dc charger (make sure you buy one that can handle this)
  • The batteries and all cabling protected by appropriate fuses
  • A battery monitor to keep an eye on input, output, voltage, time to go. Either one built into the battery (some lithiums have this) or one wired into circuit

That lot is going to cost a few thousand $, and please don't buy anything cheap. If you think that you have found a bargain then you probably haven't. You might have found something that has wild claims made about it, has been a grey import, old stock, being sold by an unauthorised dealer or being sold as part of a creative online fraud by a website owner who has no intention of being there when you need them. Good systems are never cheap, but they should be great value for the comfort and reliability that they provide for your caravan and RV experiences. 

That'll do for now, although it's just scratching the surface of a very complex topic. If you'd like to delve in deeper, or talk about you system needs, drop me a line at or call 02 9970 6996 for a chat.