Control the power

Setting up a vehicle-based 12V system to charge the battery or batteries that will power all your off-grid appliances involves quite a few choices. Once you can establish your typical camping power draws in Watts or Amps over each 24-hour period (from the fridge, lights, pumps, and other devices), you can size the battery needed in terms of Amp-hours. From there, one can establish the output needed from the solar panel array required to keep the battery (or series of batteries) at a decent state of charge.

Because solar panels put out a higher voltage than is optimal for charging a battery through its various charging phases, you will also need to invest in a solar charge controller or regulator that moderates the charge going into the battery. After being fully discharged (down to around 10.8V), a battery typically needs a bulk charge at a higher voltage, before going into a longer absorption phase at a lower voltage (this could be around 14.4V), before entering a fully-charged float phase.

In most dual-battery systems fitted to overlanding vehicles, a modern intelligent DC to DC charger is specified. When driving, this electronic box uses power from the alternator (usually up to 14.4V) to ensure that when the cranking battery is juiced up, the second battery is also given a charge. Few systems are able to fully charge a second battery, even with a full day’s driving, so many of these DC to DC chargers allow for solar input for when you park up or stay in the bush for a few days, and thus incorporate a solar charge controller in their circuitry.

Why a regulator?

When a different off-grid system is needed, for example, when multiple batteries are used, or an inverter is needed to charge 220V devices, then a separate controller/regulator is needed to ensure input from the solar panel or panels, usually at 18-22 Volts, is matched to battery capacity. These can be simple switches or sophisticated units with LED readouts showing temperatures, charge rates and more.

There are two main types of solar controllers: PWM (Pulse Width Modulation), and the slightly more intelligent MPPT (Maximum Power Point Tracker) option. The PWM is essentially a simple switch that directly connects solar panels to the battery, staying open during bulk charge, then rapidly opening and closing when the battery is at absorption voltage. This modulates the current, but pulls the output voltage from the panels down to the battery voltage – effectively reducing their power and efficiency. A PWM regulator is fine for a simple system designed to power lights or USB chargers, for example.

A more expensive MPPT controller is better for systems that require faster, efficient charging, such as most overlanding systems, as they can get up to 30% more from a given solar array than a PWM regulator. The output from solar panels constantly changes throughout the day due to changing angles of the sun, cloud cover, shade, and when temperatures increase above the optimal 25°C. Voltage drops by some 4% for every 10°C above the optimal 25°C. What the MPPT does is it track and adjust the voltage being received from the panel or panels and thus transmit more power for longer to the battery, because it is drawn out at the panel’s maximum power voltage (or Vmp), usually around 18V. A solar panel needs to be giving the MPPT controller 4-5V more than the battery’s absorption voltage to do its best work.

Sizing the system

To size an off-grid system, the first step is to estimate the average loads drawn in a day, measured in Amp-hours. The calculation is Amps drawn multiplied by time in hours. Next, determine the battery size required, also in Amp-hours, to meet these loads, taking into account that a lead-acid battery cannot handle more than a 70% daily discharge without severe damage and a shorter lifespan, while a lithium battery can go much lower, to around a 20% State of Charge (SoC).

You want the solar panel to fully charge the battery in a day, and you also want to oversize the panel output to compensate for the various losses, which can be 20-30% of the panel’s stated output in Watts. The solar charge controller needs to be 10-20% of the battery’s Amp-hour rating. Doing the calculations for a typical 100Ah battery, the rule of thumb will be to size the solar charge controller at 10-20 Amps. The solar panel or panels should have an output of 150-200W to ensure the battery gets the daily recharge it needs.

It’s worth sitting down with your supplier or chosen manufacturer with a calculator to ensure the solar panel output, battery size and type, and solar charge regulator or controller you buy for your vehicle will match your particular type of camping and off-grid travel needs. The wiring of these systems must also be professionally done, to ensure minimal voltage drops and a trouble-free installation.

If you do not need to be vehicle-linked, it’s worth exploring the options now widely available of separate external battery packs with DC and AC outlets that can power a full range of 12V camping fridges, lights and laptops. This new wave of technology presents a portable and increasingly affordable alternative to off-grid power. They can be charged via wind, solar and 220V sources, and are easily moved to where power is needed.

The vital component between your solar panels and the second battery is a solar charge controller. Why do you need this, and how should it be sized for your 12V off-grid system? Angus Boswell makes sense of it all.

Setting up a vehicle-based 12V system to charge the battery or batteries that will power all your off-grid appliances involves quite a few choices. Once you can establish your typical camping power draws in Watts or Amps over each 24-hour period (from the fridge, lights, pumps, and other devices), you can size the battery needed in terms of Amp-hours. From there, one can establish the output needed from the solar panel array required to keep the battery (or series of batteries) at a decent state of charge.

Because solar panels put out a higher voltage than is optimal for charging a battery through its various charging phases, you will also need to invest in a solar charge controller or regulator that moderates the charge going into the battery. After being fully discharged (down to around 10.8V), a battery typically needs a bulk charge at a higher voltage, before going into a longer absorption phase at a lower voltage (this could be around 14.4V), before entering a fully-charged float phase.

In most dual-battery systems fitted to overlanding vehicles, a modern intelligent DC to DC charger is specified. When driving, this electronic box uses power from the alternator (usually up to 14.4V) to ensure that when the cranking battery is juiced up, the second battery is also given a charge. Few systems are able to fully charge a second battery, even with a full day’s driving, so many of these DC to DC chargers allow for solar input for when you park up or stay in the bush for a few days, and thus incorporate a solar charge controller in their circuitry.

Why a regulator?

When a different off-grid system is needed, for example, when multiple batteries are used, or an inverter is needed to charge 220V devices, then a separate controller/regulator is needed to ensure input from the solar panel or panels, usually at 18-22 Volts, is matched to battery capacity. These can be simple switches or sophisticated units with LED readouts showing temperatures, charge rates and more.

There are two main types of solar controllers: PWM (Pulse Width Modulation), and the slightly more intelligent MPPT (Maximum Power Point Tracker) option. The PWM is essentially a simple switch that directly connects solar panels to the battery, staying open during bulk charge, then rapidly opening and closing when the battery is at absorption voltage. This modulates the current, but pulls the output voltage from the panels down to the battery voltage – effectively reducing their power and efficiency. A PWM regulator is fine for a simple system designed to power lights or USB chargers, for example.

A more expensive MPPT controller is better for systems that require faster, efficient charging, such as most overlanding systems, as they can get up to 30% more from a given solar array than a PWM regulator. The output from solar panels constantly changes throughout the day due to changing angles of the sun, cloud cover, shade, and when temperatures increase above the optimal 25°C. Voltage drops by some 4% for every 10°C above the optimal 25°C. What the MPPT does is it track and adjust the voltage being received from the panel or panels and thus transmit more power for longer to the battery, because it is drawn out at the panel’s maximum power voltage (or Vmp), usually around 18V. A solar panel needs to be giving the MPPT controller 4-5V more than the battery’s absorption voltage to do its best work.

Sizing the system

To size an off-grid system, the first step is to estimate the average loads drawn in a day, measured in Amp-hours. The calculation is Amps drawn multiplied by time in hours. Next, determine the battery size required, also in Amp-hours, to meet these loads, taking into account that a lead-acid battery cannot handle more than a 70% daily discharge without severe damage and a shorter lifespan, while a lithium battery can go much lower, to around a 20% State of Charge (SoC).

You want the solar panel to fully charge the battery in a day, and you also want to oversize the panel output to compensate for the various losses, which can be 20-30% of the panel’s stated output in Watts. The solar charge controller needs to be 10-20% of the battery’s Amp-hour rating. Doing the calculations for a typical 100Ah battery, the rule of thumb will be to size the solar charge controller at 10-20 Amps. The solar panel or panels should have an output of 150-200W to ensure the battery gets the daily recharge it needs.

It’s worth sitting down with your supplier or chosen manufacturer with a calculator to ensure the solar panel output, battery size and type, and solar charge regulator or controller you buy for your vehicle will match your particular type of camping and off-grid travel needs. The wiring of these systems must also be professionally done, to ensure minimal voltage drops and a trouble-free installation.

If you do not need to be vehicle-linked, it’s worth exploring the options now widely available of separate external battery packs with DC and AC outlets that can power a full range of 12V camping fridges, lights and laptops. This new wave of technology presents a portable and increasingly affordable alternative to off-grid power. They can be charged via wind, solar and 220V sources, and are easily moved to where power is needed.

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