System Basics
Electricity basics
Language of Electricity
A few terms are used regularly to describe the storage and flow of electricity through wires and in and out of components. The terms interrelate to each other so some simple math is needed.
Voltage (Volts)
Voltage is a measure of the potential energy between two points in the electrical circuit. It is measured in volts using a voltmeter and abbreviated with an uppercase V (E is also used). Also called electromotive force, voltage is analogous to water pressure in a garden hose. For a battery, the voltage is defined by the battery design. Many small batteries measure about 1.5 volts. Typical car batteries are ~12 volts. The voltage of a battery pack can be increased by connecting more than one battery in series (meaning positive to negative). A simple example of a series connection is a flashlight with 2 AA batteries stacked in the handle. Assuming the batteries are pointing the same way, the voltage measured from the negative end of the first battery to the positive end of the second one is about 3.0 volts. Similarly, two 12 volt batteries can be connected by wiring the positive of one battery to the negative of the other. The voltage between the free terminals will now be 24 volts. Four batteries connected in series would create a 48 volt battery pack. Packs of many battery cells connected in series for hybrid cars can be over 200 volts.
Current (Amps)
Current is the flow of electricity through a conductor such as a copper wire. It is measured in amps using an ammeter and abbreviated with an uppercase A (I is also used in formulas). For the garden hose analogy, current is the flow of water. Current, voltage and resistance (R) of a circuit are related by the formula A = V/R. Current is directly related to voltage meaning that increasing the voltage will increase the current if the resistance is unchanged. Current is inversely related to resistance meaning that current will increase if resistance drops and voltage is unchanged.
Current Type (DC & AC)
Direct Current (DC) is continuous one-directional flow which comes from batteries and solar panels. A basic DC circuit connects the positive side of a battery to a switch, then to a light and then back to the negative side of the battery. When the switch is on, current flows out of the battery, through the light turning it on and back to the battery. The amount of current will be based on the voltage of the battery and the resistance in the wires and light.
Alternating current (AC) is the form of current created by rotating turbines and generators and the flow alternates back and forth 60 times every second in the US. On our human time scale, this 1/60th of a second may see fast, but electricity is so quick, it can travel about 3,100 miles in that time. AC is used to power homes and so many appliances are designed to use 120 VAC.
Because it is convenient to use household appliances in a mobile vehicle, inverters are used to change the DC voltage of the battery bank into the AC current needed to run the appliances. In reverse, a battery charger takes the 120 VAC current from a home or campsite and changes it to the appropriate DC voltage to charge a battery bank.
Resistance (Ohms)
Resistance is included here because it is commonly referenced in connection with voltage and current. It is measured in Ohms using an ohmmeter and is abbreviated with an uppercase R. In designing mobile electrical systems, we will not use or measure resistance very often. Resistance comes into play when selecting the size of wire for a circuit. Thinner wires have more resistance than thicker ones and that resistance creates heat and can create a voltage drop along the wire. Therefore, when picking wires, there is a balance between the lower resistance and the higher cost of larger wires.
Power (Watts)
Power is a measure of the electrical energy transferred by an electrical circuit. It is also a measure of the electrical work performed by the circuit to do things such as pump water or cook dinner. Power is calculated by multiplying voltage and current and is abbreviated with the uppercase W (so W = V x A). As an example, if a 12V light that draws 0.5A of current is using 6W of power.
Power is a particularly useful way to plan and keep track of energy usage because it is independent of the voltage. For example, a DC powered refrigerator might accept either 12V or 24V and require 72 Watts to run. This 72W number stays consistent so if you power the fridge with a 12V battery it will take 6A of current (72W/12V = 6A); if you use a 24V battery, the current will be just 3A (72W/24V = 3A).
Energy (Watt-hour)
Energy is a measure of the total amount of power available over time. It is useful when measuring battery banks and estimating how long different appliances could be run. It is measured in Watt-hours and abbreviated as "Wh". It is also sometimes simpler to talk about Kilowatt-hours which are abbreviated as "kWh" and are simply 1,000 Wh.
Battery examples are useful because batteries are rated in Amp-hours and so multiplying this by the battery voltage gives the Wh of the battery bank. For example, a 12V battery rated at 200 Ah has a total energy of 2,400 Wh or 2.4 kWh. On the solar panel end, if a panel is in full sun for 3 hours and is producing its expected 250 Watts, it will have produced 750 Wh after the 3 hours are over.
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 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. In addition, main battery fuses need a high AIC rating meaning that MRBF or Class T fuses are generally the best options to function as a main battery fuse.
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, or at a minimum, a custom settings option.
Choosing a Battery Bank Voltage
Most vehicles are designed around a 12 Volt system to power lights, fans, water pumps and other accessories. 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 and 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 battery voltage 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 battery voltage based on the size of the inverter:
- 12 Volts: 100 - 3,000 Watt Inverter
- 24 Volts: 1,500 - 6,000 Watt inverter
- 48 Volts: ≥ 3,000 Watt inverter
A good 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.
Flexible panels are also available and can be simpler to install.
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.
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.
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. The Victron Multiplus line of Inverter / Chargers combines these two functions and provides battery charging and an inverter in a single package. They include a transfer switch and smart 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 designed to 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.