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Power Use Calculations for MP406 Soil Moisture Sensor & Interfaces

1. Power Use Calculations

The following sets out the power supply calculations for a single SMD4-P interface with 4 x MP406 connected and then calculates the power consumption for a typical soil moisture station with 6 x SMD4-P and 24 x MP406 measuring soil moisture once per 20 minutes.

1 x SMD4-P interface uses 0.12 mA continuously for maintenance of the DataBus connection. In this "idle" state the interface is fully responsive and its clock continues to keep time. Power usage will accelerate up to 2.4 mA if a response is required but immediately drops back once the message is sent. The actual power consumption at 12 volts is 0.12 mA continuous and 25 mA for 5 seconds during measurement of the single channel.

Then 1 x SMD4-P uses/day 0.12 mA x 24 hours = 2.88 mA hours

1 x MP406 sensor uses 25 mA when turned on and measuring soil moisture.

Note: All 4 x MP406 connected to the SMD4-P interface are powered for the full time needed to take a reading by all 4 x MP406 in each 20 minute reading cycle. This is 5 seconds / MP406 or 20 seconds each / 20 minutes for each sensor.

Then	= 25 mA x 20 seconds x 3/hour x 24 hours/3,600 (seconds/hr)
	= 10.00 mA hours for each sensor each day.

Then power use to all 4 x MP406 connected to the SMD4-P interface per day is:

4 x MP406	= 10.00 mA hours / day x 4
	= 40.00 mA hours each day

Then the power use of the SMD4-P interface with 4 x MP406 sensors with 20 minute readings per day is:

= 40.00 mA hours	- 4 x MP406
2.88 mA hours	- Continuous
42.88 mA hours	- Total Daily

Then power use of 6 x SMD4-P interfaces each with a 4 x MP406 connected with 20 minute readings is per day
= 6 x 42.88 mA hours
= 257.28 mA hours / day.

2. Power Supply Options

A.	A 40 ampere hour Gel Cell battery connected to the DataBus in order to power 6 x SMD4-P and 24 x MP406 with 3 readings/hour

	=	40	.00 / 0.257
	=	155	days
	=	77	days (only 50% available at 25° C)

B.	A 20 ampere hour Gel Cell battery connected to the DataBus in order to power 6 x SMD4-P and 24 x MP406

	=	20	.00 / 0.257
	=	77	days
	=	38	days (only 50% available at 25° C)

	Note: This is theoretical as in practice the battery will self discharge in approximately 30 days without recharge by either continuous recharge from a solar panel or rapid recharge from mains power. A 20 ampere hour battery is sufficient to power this system for a minimum of 30 days of cloudy weather without further recharge by a solar panel.

C.	Solar Panel

	A 20 watt solar panel will typically generate 6 watts / day on a clear sunny day in summer. This is equivalent of 500 mA hours / day of recharge to a 12 volt battery. The safety margin is calculated to be:
	=	500	mA hours divided by 257 mA hours
		(re	charge) (system power use)
	=	2	.0 days

	Then this requires a clear sunny day every 2 days on average to maintain the 20 ampere hour battery in a constant state of charge.

A 20 watt solar panel used in conjunction with a 20 ampere hour Gel Cell battery will provide sufficient power supply for continuous functioning of a system with 6 x SMDP-A interfaces and 24 x MP406 sensors.