Solar Power

Solar PowerEdit

1) Battery Bank Size

First you need to consider the size of your battery bank to keep the system running.  Battery bank size will depend on the power draw of your units and how many days of autonomous operation you want (i.e. days without sunshine).  I guess that depends if you're an optimist about how often the sun will shine.

Table 1 - Here is an estimate of the battery bank required (measured in total Amp Hours) to run your receivers.  This is for 12 V batteries.  Power useage depends on number of antennas (ant) and how many days of autonomous operation.  Power usage of a Lotek SRX-DL is provided for comparison.

This is also a good time to consider battery types.  Three main 12VDC batteries are Lead Acid, AGM, and Gel.  Lead Acid is cheaper but can spill and requires top up of fluids from time to time.  AGM and Gel are more expensive but no risk of spill and less maintenance.

Some advice from Woody at Fundy Solaron building a battery bank: "There is nothing wrong with connecting several batteries in parallel to increase capacity so long as you don't connect damaged or worn out batteries to good ones. The weakest battery will draw power from the stronger ones.  So...as you prepare to head out into the field you will need to assess the quality of each battery and put similar batteries together. This can be done with a load tester and only takes a few seconds for each battery."

Note - ambient temperature may affect battery performance especially in cold winter months.

2) Number of Solar Panels

Second you need to consider the number of panels you need to recharge your system.  This will depend on many factors: panel size, time of year, and the daily power draw of your receiver.  The goal is to have the average Watt Hours (Wh) generated by your panels replace the daily Wh that you draw from the battery bank.  Because Wh generated changes with seasons, you need to increase the number of panels in the shoulder and winter season. 

Table 2 - Here is an estimate of the number of panels needed to keep up with the receivers at different times of the year.  IMPORTANT - these calculations are based on the following assumptions: 1) using 55W panels, 2) assumes average monthly sunshine hours per day in HALIFAX, NS, 3) assumes 85% battery efficiency and 90% Charge Controller efficiency. 

3) Charge controllers - Use the right kind to avoid brownouts and data loss

Warning:  use a charge controller with dedicated terminals for the "load" (i.e. the sensorgnome)
and one that completely cuts power to the load when the battery voltage
drops too low, then restores full power when the battery has recovered. 
Otherwise, if your power supply gradually drops below required levels, the beaglebone computer in the
sensorgnome will go into "brown-out" mode, and won't recover even if battery power subsequently rises to full levels!
The only way to recover the SG from brown-out is to manually disconnect then reconnect the power.

*Do not use a SunForce charge controller:  as of July 2014, none of them has a low voltage disconnect. *

We have used SunSaver-6L charge controllers.  This charge controller can accomodate a single 55W or 65W panel. 
The SunSaver-10L can accomodate up to 2 55W panels.   The "L" in the SunSaver model stands for the "Low Voltage Disconnect" model.

* If you use a SunSaver charge controller, make sure its model name ends in "L" *

It is important to hook up your receiver to the load terminal with of a Low Voltage Disconnect.  If you go through long periods with no sunshine, this will disconnect power to the receiver (when voltage drops below 11.5) and reconnect power on the next sunny day (when batteries recharge above 12.6V).  This will prevent slow and prolonged drain of your batteries which is important for two reasons: 1) will prevent power "brown out" to the receiver which requires manual reset (described on previous page) and 2) will prevent permanent damage to batteries.

Do's and Don'ts of powering your SensorGnome with solar system:

Do connect your receiver to the terminal load on the charge controller, rather than directly to the battery bank.

Don't mix battery types, ages, or histories in your battery bank.  The "weaker" battery will parasitize your better batteries.  It is okay to mix battery sizes (if they are the same type/age/history etc.)

Do keep solar panels clear of shadows.  Apparently shadows over part of the panel (e.g. shadows from rails, branches, guywires) can reduce the performance of the entire panel.

Don't drop big batteries on your feet!