Prepare to go off-grid: A guide to power management
In years gone by, staying in caravan parks was the only real practical option for many caravanners. However, over the past decade or so many variables have come together to make off-grid camping an aspiration for most outdoor adventurers with a van in tow. Social media reveals many beautiful and remote locations we aspire to experience. Nearly all tow vehicles these days are 4WDs with plenty of pulling power and ground clearance. As a result, vans have grown in size, allowing you to carry more of everything, including water. I would also argue that many vans have fancier offroad suspension systems than the matching tow vehicle. So, we have the ability to get to the coolest of places, but the limiting factor has been having enough power to keep the fridge running and the lights on long enough to enjoy these destinations.
The evolution of the lithium battery from high-end curiosity to mainstream caravanner ‘must-have’ has been the key to bringing more power to the people. Lithium batteries are streets ahead of their predecessors as they are much lighter which means you can carry more battery capacity, often in the form of a bank of batteries joined together. Plus, much more of the battery capacity can be used. Traditional AGM batteries can only use about 50 per cent of the stored energy — and drawing more energy can shorten their life. Lithium batteries typically can use 80 per cent or more of their capacity before they risk harming battery longevity.
As you can put more energy into battery banks and take more energy out, manufacturers look for the best way to feed energy into the system. There are three ways to put energy into a battery. The first is solar panels, which use photovoltaic (PV) cells to convert the sun’s energy into electricity. The solar panels generally create a higher voltage than the caravan battery can handle. Therefore, a solar regulator or controller is used to deliver the appropriate voltage to the battery. The second method is using your car’s starter battery (which is charged by the alternator) to charge your caravan battery. There is a gizmo in between the starter battery; this can be either a voltage-sensitive regulator (VSR) or battery-to-battery charger (BC-BC also known as DC-DC charger) to ensure that power is only drawn from the starter battery when it has sufficient charge, so it does not become flat. The third and simplest method is plugging into mains power where an inbuilt charger (transformer) converts from 240V to the appropriate lower voltage (12V, 24V or 48V).
As battery banks can store more energy and can be readily replenished, naturally manufacturers take advantage of this and add more appliances to run off the system. An important tool in utilising this extra energy is adding a power inverter. The inverter enables lower voltage (nominally 12V, 24V or 48V) in the battery bank to be converted to 240V. A mid-level system can power the likes of coffee machines and air fryers. High-end systems can run the most power-hungry appliances such as a microwaves and air conditioner.
All this energy going in and out of the battery needs to be managed. Putting in energy too fast or just too much can damage the battery over time. If we take too much energy out, this can also damage the battery. Plus having multiple energy inputs creates the need to control which one takes precedence at any given time.
The systems that manage this vary in their functionality. The simpler systems could be referred to as battery management systems (BMS) and have been around for many years. The newer, more sophisticated offerings are often referred to as power management systems (PMS).
An iTechworld lithium battery with an integrated battery management system (BMS)
Simple battery management systems
Advances in printed circuit boards (PCBs) and microchips mean it is not unusual for a lithium battery to have a simple BMS built into it. The most basic of these will monitor voltage and temperature and disconnect the battery to protect it from damage if it is operating outside its safe/optimal parameters. Some of these BMS built into the battery also have Bluetooth connectivity, which provides an interface with your phone so that you can monitor key variables such as voltage and state of charge expressed as a percentage. If it is time to replace your AGM battery and you are after a very simple system this is a great entry-level option to consider.
Another way of achieving a similar result is to externally mount a shunt to a battery. The shunt is connected to the negative terminal of the battery and measures the real-time voltage and the current draw in and out of the battery. A unit like this starts at around $200–$300, excluding installation.
The battery management systems described above will usually have other electrical devices added to them such as solar chargers, DC to DC chargers, 240V chargers and inverters — all of which are good additions. However, there is a growing trend to bring all these devices together into integrated systems that manage all the different functions centrally.
Projecta BM500-BT Comprehensive Kit
Complex power management systems
The first level in these more complex systems is integrated chargers that manage solar charging and DC to DC charging. These systems have smart features such as prioritising charging through solar panels rather than through the tow vehicle, when possible, to minimise fuel consumption. These sorts of systems start at about $400, excluding installation. The systems covered so far would generally be quite viable to retrofit in an existing van as it is usually going to be practical to route existing wiring to them.
The BMPRO ProBoost40 40A DC-DC charger
The next level up also integrates 240V charging with the solar system and DC to DC charging. These systems jump up into roughly the $2k price bracket as they require more sophistication to manage multiple power sources. Retrofitting may be possible, but it would require more work.
Moving up to the top of the food chain are fully integrated systems that control potentially all three kinds of chargers (solar, DC to DC and mains power) plus the inverter and provide connectivity for all of the output devices. These high-end products can have 5kWh, 10kWh or even greater battery banks. Including batteries, these systems are in the tens of thousands of dollars. It pays to do your homework on how you want to do your charging as we have come across systems that do not include DC to DC charging in the package. This level of sophistication does not lend itself to retrofitting systems into existing caravans as very specific wiring and appliances are required.
REDARCpower management system
How much power do I need?
With all these options available, how do you decide how much power is required for your specific needs? To answer this, you have to address the three questions below, and ask them in a loop because they are all interrelated.
You may have a big wish-list when it comes to “How much power do I want to use?”, but you may find you need to limit your ambition due to the budget/physics constraints on “How much power can I store?” and “How much power can I put back in?”
The tables created below are extremely simplified, as are the explanations. There are many variables we have not addressed and there are assumptions that may not apply to your installation, requirements and/or environment. However, the below will arm you with some basic information to do some rough calculations to contemplate the scale of the system you should be considering. It will be necessary to consult a professional to help you configure and install your system.
How much power do I want to use?
Below is a table I adapted from my colleague Malcolm Street. It shows the typical power consumption of a variety of appliances you are likely to find in a caravan with an estimate of the average hours per day the appliance is likely to be used. Multiplying these two numbers together gives us the far-right column of watt hours per day. It can be used as a guide with appliances being added or removed and run times adjusted to meet your requirements.
You can see that the table is broken down into regular light load 12V appliances in the top half and heavy load 240V appliances in the bottom half. The numbers clearly show that 240V appliances are way more hungry requiring much more battery and charging capacity.
You would also need to consider what rating inverter you require based on which 240V appliances you wish to run and if you want to run them at the same time. In simple terms, you would add up the wattages (first data column) for the appliances you want to run concurrently, and this would determine your inverter requirement.
How much power can I store?
Manufacturers have tended to rate their batteries in amp hours (Ah). While most RVs ran 12V systems it was fairly straightforward to have an understanding of the power available because of the relationship amps x volts = watts. Therefore a 100-amp hour battery can deliver 100Ah x 12V = 1200 watt hours (Wh). Now that 48V systems are emerging, albeit at the top end of the market, speaking in amp hours alone is no longer comparing apples for apples. A 48V 100Ah battery bank will give you 100Ah x 48V = 4800Wh. In other words, four times as much power. Therefore, if we want to compare apples with apples, watt hours is the way to go when considering your battery capacity. And as the numbers get bigger, kilowatts (1000W) is the apple of choice to compare. It’s evidenced in this issue with a number of the vans reviewed carrying 5kWh or 10kWh rated battery banks. The table below is showing the capacity of three different ‘battery sizes’ expressed as amps, watts and kilowatts.
How much power can I put back in?
We’ve spoken about the three forms of charging a battery: solar, DC to DC and mains power. What we all need to know is how long does it take to put power back into the battery? Here are some very rough rules of thumb:
Solar systems
Solar panels tend to be rated in watts, so this makes things a little easier for the common language we are adopting in this article. However, as we know, sunshine is highly variable and solar systems also vary in their efficiency so these numbers will change based on your environment and circumstances.
DC to DC charging
DC to DC chargers tend to be rated in amps, and with most tow vehicles running nominal 12V systems we can calculate the watts available to put back in the battery. As manufacturers tend to quote optimal performance ratings, these are 'the good as they get' numbers.
AC charging
We will keep it simpler for mains power charging as you are likely to be under less time pressure relaxing at a caravan park. A mid-range charging system can deliver around 400W hours of charging. If you charge for 12 hours, you will be able to put 4800W into the battery.
The wrap
Modern power systems enable a plethora of appliances to be run in your van off-grid. Advances in lithium and lithium-ion batteries and the electronics that manage integrated systems enable you to capture, store and deliver power in ways once only dreamed of. If you have a big enough budget, you can run appliances just about forever. Even with a modest budget, you can do a lot more than you could a decade ago. This article has been simplified to introduce concepts and is by no means complete. It can not be relied upon to configure a power system for your needs.
(Image Cam Inniss)
Thanks to the following companies who provided invaluable background information and images for this article:
Check out the below articles to learn more about how to upgrade your van's electrical system:
THE NEXT STEP
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