SignalFire Remote Sensing Systems (SFRSS) consist of radio nodes that integrate with different sensors to send process data to a Gateway, where it is transferred to a PC, PLC, or other control system to monitor assets such as silo temperatures, tank levels and other parameters.  When using long-range Modbus stick nodes or wireless I/O modules (or Multiple I/O modules) as part of the SFRSS, these components are typically externally powered by a solar power system that consists of a 12V battery charged and a solar power panel.

Correct configuration of the solar powered battery system ensures autonomous battery operation without the presence of light for extended periods. SignalFire recommends following these specifications when sizing a solar power system for the SignalFire Modbus stick, wireless I/O module, RSD Stick or Gateway:

• Operate battery at ten days of autonomy
• Only allow ½ of the battery to be discharged over the autonomy period
• Charge the battery from ½ discharged to full charge during 6 hours of full sunlight

Step 1: Sizing the Battery

Based on using a 12 Volt system, battery “size” is expressed in amp-hours and is a measure of how much total power or capacity a battery can generate.  The required battery capacity depends on the amount of (average) current to be drawn from the battery and the autonomy length.  Following our basic rules, we want 10 days of autonomy to use ½ of the battery capacity.

Current draws vary by load.  The current draw for standard configurations of SignalFire equipment is outlined in Table 1.  Let’s say, we want to power a SignalFire Remote Shutdown Stick  (Modbus Stick with Remote Shutdown Module) with a single relay that is almost always energized. According to Table 1, the current draw for the stick and module at 12 Volts is 34 mA.

Using this value, we can calculate the amount of capacity used in a day:

0.034 Amps * 24 Hours/Day = 0.816 Amp-Hours/Day

Based on 10 days of autonomy, capacity increases to:

10 Days * 0.816 Amp-Hours/Day = 8.16 Amp-Hours

As these calculations are based on only half the battery capacity, we need a battery with 2X this capacity or:

2 x 8.16 Amp Hours = 16.3 Amp-Hours.

As batteries are available in capacities at rounded numbers such as 5, 10, 12, 15, 20, a 20 Amp-Hour battery would provide a little extra capacity or a 15 Amp-Hour battery would offer a little less capacity.  If the SFRSS operates for long periods without sun, a 15 Amp-Hour battery is adequate but in areas with extended dark periods, a 20 Amp-Hour battery is recommended.  Keep in mind that as the battery increases in capacity, so does its physical size.

Step 2:  Determining the Solar Panel Size

Based on the battery size, we can determine the solar panel needed to charge this system.  If using a 20 Amp-hour battery, we must half charge the battery at 10 Amp-Hours of capacity in 6 hours.  The calculation for determining the needed charging current is:

10 Amp-Hours / 6 Hours = 1.67 Amps

 Using this value, the size of the solar panel is calculated.  Solar panels are measured in Watts, which is a measure of their power output.  Watts equals current times voltage:

1.67 Amps * 12 Volts = 20 Watts

Outcome:

Based on these calculations, a 20 Watt solar panel is needed to charge a battery from half charge in about 6 hours. After determining the battery size (12 Volt, 20 Amp-Hour) and solar panel size (20 Watt) needed for this system, a charger is needed that can output at least 2 amps (5-10 Amps is common).  SignalFire recommends the SunSaver solar charge controller from Morningstar to connect to the solar panel to charge the battery.

Table 1: Outlined is the current draw for standard configurations of SignalFire equipment. (Contact the factory for other configurations.)

For assistance in determining the solar-powered battery system for SignalFire equipment, contact technical support at: 978-212-2868 x2 or support@signal-fire.com.  Read about how the Remote Sensing System with Solar-Powered Repeater Automates Liquid Storage Tank Level Monitoring here.