Power Calculations for ICT Data Logging systems

A Basic Meteorological system

The SL5 logger is installed with a 6V 7.2 Ah (amp hour) battery. The 7.2 Ah rating means that the battery can supply a load of 0.1 amps for 72 hours however this will depend on the rate of discharge. A high rate of discharge will reduce the amp hour capacity.

The logger battery capacity has been selected to provide power to a typical suite of meteorological sensors. Table 1 lists a selection of sensors and the amount of current they draw from the battery.

Device	Load
SL5 Logger	0.6 mA
Air temperature	1.5 mA
Anemometer	1 mA
Humidity	2 mA
Solar radiation	1 mA
Rain gauge	1 mA

Table 1.

As an example for a simple meteorological set-up we can see that the Total Load drawn by the logger and sensors (Items 1 to 6) is:

Total Load = Sum of all sensor and logger loads.

Total Load = 0.6 + 1.5 + 1 +2 +1 +1 = 7.1 mA (milliamps) = 0.0071 A (amps)

Therefore, the system will use 0.0071 Ah (Amphours) in 1 hour. From this the Total Daily Amphours can be calculated using:

Total Load x 24 = Total Daily Amphours

0.0071 x 24 = 0.1704 Ah per day.

Therefore the duration that the internal battery of the SL5 logger will last when powering a simple meteorological suite of sensors can be calculated with the following formula:

Battery Duration = Battery capacity / Total daily Ah

7.2 / 0.1704 = 42 days

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.

It should also be pointed out that the battery manufacturers' amphour values are obtained under ideal conditions of temperature and constant specific discharge current. The most that could realistically be expected under field conditions would be 80% of the rated amphour capacity and that value would reduce with age and adverse temperatures. In fact, it is usually considered prudent to use only 50% of the rated amphour capacity when calculating the battery capacity required for a system.

Also, a battery will start to self-discharge after a certain period of time depending on its capacity.

Systems with High Loads

The above example is relatively simple due to the constant current drain of the sensors in a meteorological logging situation. However, some ICT sensors will have times of high peak currents (eg, heaters being switched on) as well as the typical standby current they use in between logging events.

An example of calculating battery duration for a more complex sensor can be obtained by investigating the case of a Heat Ratio Method (HRM) Sapflow sensor.

One HRM draws 0.667 Amps (667mA) when the heater is energised. This means, if it were energised for 1 hour it would use 0.667 amp hours (0.667Ah). The heater is only energised for 2.5 seconds for its default Heat Pulse Energy setting of 20 joules. If the Heat pulse energy is set to 40 Joules, then the heater will be energised for 5 seconds. In this case then, for each measurement event, the Amphours per pulse is

0.667/60/(60/5) = 0.00093 Ah. per pulse

If the heat pulse interval of the HRM is set for 1 hour, there will be 24 pulses per day. Therefore, with regard to the heat pulses, the daily amp hour use of one HRM is

0.00093 x 24 = 0.022 Ah per day.

However, the interface also uses 0.5mA continuously in its idle state. This is equal to

0.0005A x 24 = 0.012Ah per day.

So by adding the amphour values of the heat pulse and the interface idle state, the total daily amphour consumption of 1 HRM logging at hourly intervals is

0.022 + 0.012 = 0.034 Ah per day.

Therefore, if an 80Ah 12V auxiliary battery is connected as the auxiliary databus battery, it will power a single HRM sensor for

80/0.023 = 2336 days.

As stated earlier, a battery will start to self-discharge after a certain period of time depending on its capacity but the above can be used to help determine a suitable battery size for a given number of sensors.

For example, consider a system using an 80Ah battery, 20 HRM sensors, 20 Echo sensors and 20 Gypsum blocks.

As already calculated, the daily amphour consumption of the HRM sensor logging at hourly intervals is 0.023Ah per day. Therefore, 20 HRM sensors will use

20 x 0.023 = 0.685Ah per day

With regard to 20 Echo sensors, there will be 5 x SME4 interfaces. In that case, each SME4 uses 3mA. Total daily Amphour is 0.003 X 5 x 24 = 0.36 Ah per day.

Similarly for the 20 Gypsum blocks, there will be 5 x SM4 interfaces. In that case, each SM4 uses 3mA. Total daily Amphour is 0.003 X 5 x 24 = 0.36 Ah per day.

Therefore, total Amphour per day consumption of 20 HRM sensors (1 hr logging intervals, 40 joule Heat Pulse Energy), 20 Echo sensors, & 20 Gypsum blocks is;

0.685 + 0.36 + 0.36 = 1.4Ah. per day

At 50% of an 80Ah battery, this gives about 0.5 x 80/1.4 = 28 days of battery life.

For customers who wish to purchase their own auxiliary 12V battery, ICT recommends that they purchase a deep cycle battery. These batteries have thicker plates than typical automobile batteries that enables them to be cycled more deeply. While they are more expensive to buy, they will last much longer if properly maintained.