RV battery and solar guide

If you’re new to 12-volt RV power there are a few options to learn about to get the best system for you.

From just getting a few milliamps of charge into a mobile phone to keeping cool with a reverse-cycle air conditioner at camp, you need the right power set-up to do the work.

The typical 12-volt system uses deep-cycle house batteries fitted to the RV. Unlike a car’s starting battery, the deep-cycle battery is designed to take slow, regular current draws until just about 50 per cent charge remains.

Battery types

The wet-cell, lead-acid deep-cycle battery is the cheapest but not ideal: it vents gas to the atmosphere, so it must be kept upright and in a well-ventilated area and needs topping up with distilled water. It can’t charge up as quickly as other deep-cycle batteries either.

Better than the wet-cell is the Absorbed Glass Mat (AGM) battery. The AGM is also a lead acid battery, but it has fibreglass matting to absorb its electrolyte and does not vent gas to the atmosphere unless overcharging. AGMs have much lower resistance than wet-cell batteries so they charge more quickly and perform better when under heavy load.

Next up the chain is the Gel lead-acid battery, which suspends electrolyte in a gel paste. The gel battery doesn’t vent to the atmosphere and if you do flatten it, it’ll handle it better than any other lead-acid battery. This type can only take a relatively low rate of charge.

For most deep-cycle batteries, voltage slowly falls during discharge - but voltage also decreases as load increases. They are not good at discharging quickly.

There's also another option now available: lead crystal batteries.

Pricey lithium 

However, the best (and most expensive) battery is the lithium battery. Unlike other batteries, lithium batteries will give the peak power required and maintain constant voltage regardless of the load. In practice that voltage is likely to be up to about 13 volts.

With enough inverter capacity, just about all RV electrical appliances can be run simultaneously (if required), including high-current draw equipment such as 240 volt air conditioners.

You can also pump in more power more quickly with lithium and they can last as much as four times longer than an AGM and lithium will easily cope with a drop to 20 per cent state of charge. Lithium doesn’t cope too well with being run to zero SOC - cells can reverse their polarity and might not be able to be fixed.

There is a catch: lithium is expensive, up to twice more than lead-acid batteries. Specific charging equipment controlled by a Battery Management System is needed and existing charging sources like AC battery chargers, solar regulators, alternators and DC-DC chargers may not be compatible with a lithium battery set-up.

AGM batteries provide the best compromise of performance, price and longevity when running RV gear such as 12 volt LED lights, water pump, TV and radio. But if you want to run big 240-volt gear like an air conditioner, a washing machine or microwave when free camping, you will want to think about lithium batteries.

Petrol power

Another option - which will be cheaper - is to use AGM batteries and a generator as required.

If you need more charge than one battery can hold, you can run up to four AGM batteries and hook them up in parallel. This’ll give you their combined amperage output, while still remaining at 12 volts. Make sure the wiring is not any longer than necessary and has a thick enough core to do the job.

However lithium battery packs can’t be run in parallel. That will lead to early failure of the battery packs as the lowest resistance battery pack will do all the work. You’re better off getting a high amp-output single battery pack to run your high-current draw gear.

240V inverter options

To run 240-volt AC gear you need an inverter, which changes 12 volt DC input to 240 volt output.

There are a few different inverter types —  square wave, modified sine wave and pure sine wave. Pure sine wave inverters are the go because they smooth out any voltage spikes and so can run sensitive electrical gear like laptops without damaging them.

House batteries are also going to need to be charged up. At the most basic level, a 240v to 12v charger is the way to go. For a 100 amp hour battery, you’ll need a 25 amp charger; for more than 200 amp hour, go for a 40 amp charger. Each type of battery (eg AGM or lithium) needs a specific charger type, which must be wired up as close as possible to the house batteries to avoid voltage drop.

Such a charger requires that you have 240v mains power or a generator handy.

Another option is using your tow vehicle’s alternator, wired through to the RV house battery.  However, it’s not aways so simple: most vehicles have a ‘smart’ alternator, which allows the vehicle to control the output voltage from the alternator based on vehicle operating conditions to reduce electrical load. This type of alternator doesn’t do a great job of charging a house battery.

To get around this, you’ll need a DC to DC charger. A good DC to DC charger will fool the smart alternator into thinking that the battery is a fixed current draw accessory, and so be able to charge it up.

As for wiring, you need to run a minimum of 6mm wires and use a 50 amp Anderson plug to connect the wires at the towbar. If you don’t, you’ll have too much voltage drop.

You’ll need an inline fuse or a reset breaker switch and it's a good idea to fit a relay so that the plug is only powered when the vehicle is switched on. That’s so if you leave the caravan accessories like a fridge on and forget to unplug the vehicle overnight, for example, you won’t run the vehicle battery flat.

The solar option

Another option is to use solar panels.  There are three types of solar panels cells: Polycrystalline,  Monocrystalline and Amorphous.

Polycrystalline are less efficient than monocrystalline cells and need more surface area for the same output. Both types need to be as perpendicular to the sun as possible for best performance.

Amorphous panels have the most efficient solar cells, although they do require twice as much surface area for the same power output as a monocrystalline blanket or panel. They're more flexible though, and can handle higher temperatures better without output being affected.

Amorphous cells are constructed from a fine layer of silicon which makes the solar panels more flexible and lighter. Amorphous cells perform better in low light conditions compared to crystalline cells too.

As for how much solar you need to keep batteries charged when free camping, there are plenty of variables. How much power you draw and how much sun the panels get to produce charge from is just the start.

However, generally speaking solar panels with about 250watts capacity will put 50 amps back into the house battery each day.

Solar panels need a regulator so that the voltage going into the battery is consistent. A Maximum Power Point Tracking (MPPT) solar regulator is the best type because it will simulate the load required by the solar panel to achieve the maximum power from the cell to ensure  that you get the most power possible from your solar panels at any point in time. It is particularly effective during low-light conditions.

What's best for me?

The best way to work out which 12 volt battery system and which charging method is best for you is to determine what gear you typically use and how much power it draws.

For example, say you have eight LED down lights in your van, each one rated at 3.5 watts. First, multiply the number of lights you have by the rated watts per light (28). Then divide by 12volts to get the amps the lights draw (2.3). Then multiply this by the number of hours you will run them (let’s say five hours). In this example, you’re pulling a total of 11.5amps out of the battery a day.

Most people run a fridge at camp. Say you’ve got a 60-watt compressor fridge that uses 5 amps per hour, but only when the compressor is running. Say the compressor runs about 15 per cent of the time, and you have  0.75amps. Multiply that by 24 hours, and you have 18amps.

With just these two basic camping requirements you’re using 29.5 amps a day. With a fully-charged 100 amp hour AGM battery, you’ll get not quite two days’ use out of it before it has dropped to its safe minimum amps (50amps) and needs to be recharged.

Reverse-cycle 240v air-conditioning can cause big problems if you try to run them on batteries.

Even a smaller, power-saving unit like the Aventa Eco draws 62 amps DC each hour.  You’ll only be able to run the aircon for a bit less than an hour and a half with two fully charged 100a/h AGM batteries running parallel (with 200amps power in total).

Although a bank of three or four AGM batteries could run the aircon for longer, either lithium batteries or a 2.5kVa gennie are the way to go for powering 240 volt appliances at camp.

Monitor your usage

The best way to make sure you don’t run out of power is to have a battery monitoring system that gives you estimated state of charge, battery voltage, live current draw, amp hours charged and amp hours used.

One step better is to invest in a battery management system, which will handle solar regulation (with MPPT)  and 12 volt and 240v mains charging all in the one unit.