Previous articles in this series covered portable power stations and solar panels, but the most flexible and expandable option is to install solar panels on your roof and install batteries and other electrical subsystems permanently in your RV.
Today’s article covers the devices you need to take energy from your solar panels, store it in your house batteries, and make the best use of the solar energy stored in your batteries.
Charge controllers
For all solar panel options other than those that use a portable power station, one required item is a solar charge controller that works with the type of house battery you have, either lead-acid or lithium (LiFePO4).
A charge controller converts the variable current and voltage provided by solar panels and converts it to the voltage and current required for properly charging your house batteries.
There are two types of charge controller: low-cost PWM (pulse width modulator) chargers and MPPT (Maximum Power Point Tracking) versions. PWM controllers are a little less expensive, but generally, the MPPT is best for RV applications because it will squeeze the maximum available energy from whatever solar panels are connected to it, and they can generally charge either lead-acid or lithium batteries.
Charge controllers are rated for maximum input and output
Charge controllers are rated with a maximum input voltage from the solar panels and a maximum output charge current supplied to the battery it’s charging. A unit advertised as 100/30 generally means it can handle input voltage up to but no more than 100VDC from the solar panel(s), and can output 30A, at most, to the battery being charged.
The maximum input voltage must not be exceeded or it may damage the charge controller. The output current is automatically limited to the MPPT controller’s maximum or less. If you have a charge controller rated at 100V input and 30 amp output, the maximum power it can deliver from solar panels to the battery is 30A at the battery voltage. So, for a typical 12VDC battery, that maximum output is the battery voltage times current or 12*30 = 360 watts, but it’s usually less than that depending on the panel rating and operating conditions.
A single controller can be used for the same type of panels with the same voltage and current specs when combined, but if you mix different types, you need to have a different controller for each type of panel. My rig has a 150-volt 70-amp MPPT controller for the roof-mounted panels, and a 100-volt 30-amp MPPT controller for use with my portable panels since the rigid roof-mounted panels are different than the folding portable panels.
Charge controller may limit battery charge current
All solar panels have a specification abbreviated as Voc (unloaded, or Open Circuit voltage), which is the maximum voltage they will generate when not connected to a load. That panel’s Voc must be less than the voltage specified as the maximum solar input for a power station, or the station can be damaged.
Note that panels connected in series output the single panel Voc times the number of panels in series, so you may need to connect panels in parallel to limit the voltage going into the power station.
The solar panel input voltage must always be less than the maximum voltage the power station or battery charger is rated to accept. Connecting a higher input voltage than specified will likely damage the unit.
Panels or batteries in series add their voltages, while those connected in parallel add their currents. For example, if a power station or charger input is rated to accept up to 100V, make sure your total solar panel voltage does not exceed that.
Regardless of how much current the panels can provide, power stations and battery chargers limit the input current to whatever they are designed to handle. Battery chargers will also limit battery charge current to their rated output.
For example, consider a 100 Volt, 30 Amp solar battery charger and three panels in series, each with Voc = 25V and Isc (short circuit current) = 10A. In series, the total voltage becomes 3 × 25V = 75V. If you connect two of these 3-panel series sets in parallel (six panels total), the voltage remains 75V, but the current adds: 10A + 10A = 20A.
That means the maximum possible power the panels could produce would be 75V × 20A = 1500W.
Solar charger output limited by its design
Power stations and solar chargers are rated in DC voltage (Volts) and current (Amps). However, even if the charger can accept 1500W input, its output is still limited by its design. For instance, a 100V/30A solar battery charger charging a 12V battery can only output up to 12V × 30A = 360W. That’s far less than if the same charger is used with a 48V battery at 30A, which would provide 48V × 30A = 1440W.
This is one of the advantages of a higher-voltage battery bank.
The disadvantage? A higher-voltage battery bank (typically 24V, 36V, or 48V) can’t be charged directly by a standard engine alternator, which only puts out 12–14V. In this case, a step-up charger is needed to convert the alternator’s output to charge the 48V battery, and a step-down converter is needed to power typical 12V RV systems from that 48V battery bank.
A substantially more expensive charger with higher battery charge current could be used with a 12V battery, say a 150V input, 70A output charger, and that would provide a maximum of 12V*70A=840W, but this is not so bad. Under typical conditions, a 1500W array of panels flat mounted on a roof may only produce 50-60% of peak rated power, so it can charge a maximum of 0.5*1500W=750W to 0.6*1500W=900W into the batteries compared to the charge controllers’ 840W maximum rated output at 12V.
Consider limitations of batteries being charged
You must also consider the limitations of the batteries you are charging. The maximum charge rate of your house batteries may be the limit to how much power can be used to charge them. A single typical lithium 100Ah battery may only accept 50A of charge current or 12V*50A=600Watts. If two 12V batteries of this type are connected in parallel, then they can accept 2*50=100Amps or 1200W of charging power.
Of course, other energy sources such as gas, diesel, or propane generators can also be used to charge the house batteries, but the total combined charging current from all sources should not be allowed to exceed the maximum rated charge current for your batteries.
Inverters convert DC battery power to 120Vac power
If you want to use household appliances in your RV, you will also need an inverter that converts low-voltage DC battery power into the 120Vac power used by standard appliances like coffee makers, microwave ovens and the like, but that’s an involved decision topic for a different article. There are also products that combine two or more of the various functions of an inverter, battery charger, transfer switch, and other features in one package, but with increased capability, the cost goes up.
MORE FROM KEN ON RV SOLAR:
- Don’t pay for more solar than you need
- Don’t pay for more solar than you need, part two
- Don’t pay for more solar than you need, part three
- How many solar panels you need for your RV
RVT1219
Thank you for going through this information so carefully and in such manageable pieces, Ken! I appreciate this opportunity to learn much more about solar power and its many attendant aspects. Have a great week and safe travels!