There’s nothing worse than warm Chardonnay and spoiled food on a camping trip, especially when it can be so easily avoided. Understanding battery management and your camper’s 12V system is paramount to your camping enjoyment. Here’s our take on Battery Management 101!
Starting vs Deep Cycle Batteries
Starting batteries and deep cycle batteries are two very different animals. The starting battery delivers high power in short bursts, such as starting a car. This power used can be replenished quite quickly, usually within a few minutes of running the car. In comparison, a deep cycle battery such as that in a camper trailer, delivers low current over an extended period of time such as running a fridge. These batteries take a long time to recharge, sometime 10 hours or more.
To get the best out of your battery reserves, you need to understand the effective operating range and to do that, you need a voltmeter.
Understanding your Voltmeter
A voltmeter measures the batteries potential in volts. A deep cycle AGM battery is considered fully charged at 12.8V when measured at rest and dead flat at 10.5V. When charging, a battery will read higher than 12.8V, often up to 14.4V. Regularly discharging your battery to 10.5V will result in a drastically shortened life. It’s recommended to discharge to 12.2V only. You may notice your fridge starting to cycle more frequently at this voltage hinting towards the depleting charge.
The most accurate time to measure voltage is when the battery is at rest, without any load. This is usually in the morning, assuming a cold climate when there has been little electrical demand overnight.
How many batteries?
Generally speaking, two is better than one if you do more than weekend trips, as it maintains a reserve in times of high usage or low replenishment. There won’t always be sun to drive solar and you may be camping in a national park which prevents generator use, so extra battery reserves will go a long way to power your devices. You may need to consider reducing your power usage and to do that you need to understand what each item draws.
A deep cycle battery is usually measured in Amps, such as 100Ah. It is important to note, you can only safely discharge an AGM battery to around 50% of its capacity, which means a 100Amp battery only has 50 Amps utilisation. If a fridge draws 2 amps per hour (Ah), the battery will only last just over 2 full days (48hrs). Put like that, it’s easy to appreciate why you need more than one battery!
Calculating Power Use
Understanding power usage is helpful for understanding how much you need to replenish each day and how long that may take. There are four methods for calculating power use.
Formula: Watts / Voltage = Amps. If a fridge draws 24W, divide by 12V = 2 Amps per hour. A fridge/freezer will cycle on and off as required so will only draw 2 Amps while running. If an LED light draws 6W, it will draw 0.5A per hour.
Watts Meter: This device measures the power used or gained over a period of time. Rob Sanderson from the Camper Trailers group wrote an interesting column on how to make a Watt’s Up Meter and how it can add value to your camping experience. You can read about it here.
Battery Management system: a battery management system shows the current draw on the batteries at any given time. By switching on your devices one by one, you can see how much power each one draws. This is by far the easiest method.
Trial and Error: monitoring voltage off a voltmeter is another method. Record the voltage at the start and end of the day and make some notes on inputs (solar, 240V, 12V) and outputs (fridge, lighting, etc). If you are really keen, you can check how often your fridge compressor runs and for how long. But that takes real commitment.
This is a more generic approach as you’re measuring overall voltage from the use of all appliances and it doesn’t really help you understand power use on a per item basis.
With your list of devices and the power drawn by each, you can begin to estimate how long your batteries will last without any additional charge. Ideally you will be harvesting some power each day to replenish what you use.
Most campers these days come with a 240V charger to renew the battery to 100% charge. It does this using a stepped charging profile, gradually reducing the incoming charge as it nears full charge. Generally, the charger size is equal to 10% of the total size of the battery, so a dual battery bank of 200Amps would need a minimum charger of around 20Amps/hr.
The Anderson plug is a common way of getting some charge into your camper’s battery reserve. The terminals in an Anderson plug are 16mm, so 6B&S power cable is the best fit (not 6mm). This cable size also minimises power drop over the length of the charging cable, ensuring you get enough voltage at the battery.
Anderson plug charging will never get you a full 100% charge, maybe 75%-80% at best. Keeping in mind you can only discharge your batteries safely to 50%, this means you’re missing up to half of the batteries utilisation. Most people who do extended trips regularly, will fit a DC DC charger to gain that additional 20%-25% charge. At least this way you know you’ll arrive at your camping destination with a full 100% charge.
DC DC Charger
As stated above, a DC DC charger will help get you full utilisation of your battery resources. Like a 240V charger, it uses a stepped charging profile to safely top the battery up without causing heat issues. Modern cars with smart alternators may need the DC DC charger to step up the voltage to fully charge the camper’s batteries.
A camper plugged into a generator should utilise the camper’s on-board 240V charger to increase the rate of charge. Most generators chug out around 8 amps per hour, whereas a 240V charger of 20A capacity will do the job much quicker.
Battery Management Systems
One method of taking the guess work out of battery management is to install a premium battery management system. The Redarc Manager30 fitted to the Pioneer Mitchell is one such system. It consists of a 30A 240V charger, 30A DC DC charger, multi power point tracking (MPPT) solar regulator, LCD screen and monitoring software to help manage the state of charge.
The LCD screen shows a range of information, from a simple fuel gauge and percentage charge, to the current discharge rate, time until empty, the current recharge rate, time until full, and a maintenance log to see the state of charge over hours, days and weeks.
Unlike a voltmeter, which only measures surface charge, the Redarc system uses a current shunt to accurately measure the state of charge in real time. The Redarc system also has a voltage sensor which acts as an isolator, to prevent accidental discharge of the towing vehicle’s starting battery if left connected.
We used the Redarc Manager15 in a previous camper for an extended 13 month off road trip which helped us best manage our power requirements, particularly as we were using some high draw appliances.
Battery Management Demystified
For the uninitiated, battery management can be a little overwhelming with all the terminology and information. However, properly understood, it can only add value to your camping experiences, with battery reserves to keep the whole family content.
Husband and wife team, Mike & Anita Pavey from The Dirt Off Road Campers in Mt Barker, SA have racked up thousands of dusty kilometres writing for popular camping and 4×4 publications including a 13-month sabbatical in 2011/12. The above information comes from their collective experience over the years from managing battery reserves using both simple and best of breed systems like that offered by Redarc. You can find out more about their outback ready, Australian-made campers here, many with premium Redarc Battery Management systems fitted as standard.