System Components

An overall mobile power system will include a "house" battery that is separate from the vehicle's starting or "chassis" battery and powers the electrical components in the living space. Methods of charging these batteries include: roof mounted or portable solar panels, the alternator of the vehicle, plug-in shore power at a camp site and fuel powered generators. To make safe use of the battery power, fuse blocks, wire and switches are selected to power the DC components such as lights, water pumps, fans etc. An inverter is used to convert the battery's DC power to 120 volts AC. This allows the use of regular home appliances such as laptop chargers, blenders, induction cooktops or large air conditioning ("A/C") units. 

The complexity of the design is in part because there are so many choices of how each person wants to live in their mobile space, what appliances they want to use and how many hours per day they want to use them. Some of the fun of the design is figuring out what might be a nice-to-have vs. a need and how these personal decisions can raise and lower the electrical needs and thus the size of the system.

Below, is a brief description of the various components mentioned above.  

Battery Bank

The battery bank is the site of energy storage and is the electrical hub of the system.  This section discusses battery chemistry, selecting the best voltage and the cost benefits of building a battery pack from raw cells. 

Battery Chemistry

For many decades, various forms of lead acid batteries have been the norm for vehicle starter batteries and "house batteries" (separate batteries that power the living space) for mobile systems. In the past 5 or so years, Lithium-iron-phosphate batteries (abbreviated "LiFePO4" or "LFP") have become the best choice for new mobile systems. This is because they are lighter for the same energy, they don't give off poisonous gas so can be used inside. They last much longer and have dropped in price, meaning they are now a cheaper option than lead acid over the long term. 

LiFePO4 is one of a variety of "Lithium Ion" battery chemistries but unlike cell phones and early electric vehicles (using combinations of lithium, manganese, cobalt or aluminum) they have a very low fire risk. The one safety concern for LiFePO4 is that they have very low internal resistance meaning they can release very high amperage causing sparks when connecting to other components. Safety glasses and care when assembling components can manage this risk. 

Compared to lead acid, LiFePO4 batteries have a different charging curve. This means that equipment used to charge these batteries (from solar panels, alternators and AC shore power and generators) should have charge settings specific to LiFePO4 requirements, or at minimum, a custom settings option. 

Choosing a Battery Bank Voltage

Most vehicles are designed around a 12 Volt system and 12 volt lights, fans, water pumps and other accessories are commonly available. For small to medium sized systems, having a 12 volt battery bank can work well. To increase the available energy (Watt-hours) more than 1 battery can be connected in parallel. This keeps the voltage at 12V and increases the available amp-hours. Small to medium solar systems, 100 - 600 Watts, will use up to 50 Amps to charge a 12V battery and that is a reasonable size solar charge controller.  On the inverter side, a 1,000 Watt inverter would  use 80-90 amps from a 12V battery which is also very reasonable.

However, for systems with larger solar arrays and/or inverters, shifting to a 24 volt or 48V system has advantages. The main one is that the same power (Watts) can be generated using 1/2 or 1/4 of the Amps because the higher voltage means lower Amps for the same Watts. When Amps are reduced, wires can be smaller and cheaper and solar charge controllers will be cheaper as they are generally priced on maximum output current (Amps). 

A disadvantage of higher voltage systems is that 24V or 48V components like battery chargers may be less available out on the road or in other countries, which would make it harder to find replacements.  In addition, a DC-DC converter will be needed to run 12V lights from a 24V or 48V battery bank which adds one more component to the system. 

Rules of thumb - there is some overlap, but battery voltage can relate to the size of the inverter or solar panels.

Sizing Based on Solar panels

• • 12 Volts: 100 - 1200 Watts of solar panels

  • 24 Volts: 600 - 2,400 Watts of solar panels

  • 48 Volts: .≥ 2000 Watts of solar panels

Sizing based on the size of the Inverter:

• • 12 Volts: 100 - 3,000 Watt Inverter

  • 24 Volts: 1,500 - 4,000 Watt inverter

  • 48 Volts: ≥ 3,000 Watt inverter

Remember that system design should also plan for possible future expansion. If you are planning to start with 800 Watts of solar panels, but might add more in the future, starting with a 24V battery now could make the later additions simpler. 

Solar Power System

Solar power can be a wonderful addition to a mobile system as once it is installed, you get free "green" power from the sun to run equipment now or store it for later use. Solar systems consist of the panels, a combiner box and a solar charge controller as well as the wires and breakers/fuses needed to make the connections. 

Solar Panels

Solar panels come in a variety of brands and dimensions but they all work using the same basic photovoltaic (PV) cell. When light hits the silicon in the cell, electrons are excited and move within the cell creating an electrical potential. When the sunlit panels are connected to a circuit, current flows to power components and charge batteries. Most solar panels cover the PV calls with glass and mount them in an aluminum frame. A positive and negative wire extend from the panel and can be connected to other panels and the combiner box. 

Combiner Box

Solar panels mounted on a roof of a vehicle need to be wired to the solar charge controller inside the vehicle. One way to do this is to use a combiner box that serves 2 main functions. 1) The box simplifies the process of connecting more than 1 panel together. 2) The box can be mounted over a hole drilled in the roof and seals out the weather from entering the hole. 

Solar Charge Controller (SCC)  

The solar charge controller is an electrical device that takes the energy from the solar panels and converts it to the proper voltage to charge the batteries. A good quality SCC will include maximum power point tracking (MPPT) which adjusts the voltage at the solar panel input to find the maximum power available from the panels at any given time. The SCC also adjusts the output volts to match the batteries and respond when the batteries are full by shutting off charging. The battery parameters are typically programmable and so can be adjusted to match specific batteries. 

SCCs are listed with 2 electrical ratings. The maximum voltage allowed in, and the maximum current that will flow out to the batteries. The SCC price generally follows the maximum output current in Amps meaning that a 20A SCC will often be about 1/2 the price of a 40A SCC. 

Choosing size and number of solar panels

The size and number of panels is often constrained by space on the roof of the vehicle. If enough space is available, then we will look at the energy audit results to see what the planned daily usage is in Wh. If, for example 2,500 Wh of energy are needed each day,  800 Watts of solar panels might be a good plan as a good day with 5 hours of sun would offer about 4,000 Wh which could cover today's need and fill the batteries a bit for cloudy day use. 

Ultimately, balancing the solar panels with the SCCs and the size of the battery bank takes a few steps to design the best overall system. 

Alternator Charging System

Whatever engine is used to move the mobile vehicle, the alternator of that engine usually has additional capacity that can be used to charge the house batteries (as well as the vehicle batteries). When LiFePO4 batteries are used for the house batteries, the best practice is to use a Battery to Battery charger (also called a "DC to DC" or "B2B" charger) to perform this function.

Battery to Battery Charger

These units function much like an SCC in that they have an input side and track the voltage of the vehicle's battery/alternator system and only charge the house batteries when the alternator is running and has raised the voltage above a programmed threshold. 

These devices also keep the house batteries from sharing charge with the vehicle battery when the vehicle is not running. 

For larger amperage B2B chargers, additional large gauge wiring may need to be added to the vehicle to safely carry the needed current. 

AC Battery Chargers

A third way to charge the house battery bank is using a battery charger plugged into an AC circuit from a campsite or other power source such as a fuel powered generator. These allow the batteries to be charged back up when the vehicle is stationary and no solar power is available. 

These chargers can be thought of as performing the reverse function if an inverter, taking 120V AC and converting it to the DC power in the battery bank. In fact, many companies offer all-in-one (AIO) systems that combine an inverter and charger in the same unit. They often add a transfer switch and clever circuitry to automate the process of connecting and disconnecting from shore power. 

Other Elements

In addition to the main components above, smaller components such as wire, fuses, breakers and switches tie the whole system together. 

Wire

For most mobile applications, stranded copper wire is selected based on the wire gauge and the quality and feature of the insulating material. Insulation is rated by temperature that it can withstand and it's tolerance for UV light and other substances like oils. The American Wire Gauge (AWG) system defines the cross sectional area of the copper in the wire. A lower AWG number means larger wires that cost more and can handle more amps before heating up too much. Part of the design process is to select the best AWG size for each portion of the system balancing cost, safety and efficiency of the system.

Fuses and Breakers

Fuses are 1-time use devices that intentionally break if too much current is passed through them. Fuses are rated in amps and should be selected such that their amp rating will allow the components they are connected to to work under normal conditions and will break before the amps get high enough to create a fire hazard in any of the downstream wires. 

Breakers serve the same function as a fuse but for them, a high current will flip a switch that can be reset without having to replace the fuse. In many circumstances, these can also function as a switch to temporarily disable a component for service or repair.