|ESTIMATING BATTERY LIFE IN DATA LOGGERS
The battery life that can be expected from a data logger depends on several factors, the capacity of the battery, the ambient
temperature, the sensor warm-up time, the sampling interval, and the number of sensors connected along with the current they
Estimating the amount of current a sensor draws can be difficult so you should generally assume the worst case. A 2-wire
sensor, or loop powered sensor, like the WL400 water level sensor draws
whatever the output current happens to be. This type of sensor produces a 4-20mA output so the worst case output is
20mA. If you know that the sensor will be operating within a certain range, you can use that maximum output current in the
battery life estimates. A 3-wire sensor, like the WQ301 conductivity sensor,
draws current equal to the 4-20 output, plus additional current to supply the internal circuitry. You will have to consult
the manual for these sensors to determine the current draw. A typical 3-wire sensor might draw 10mA plus the worst case output
current of 20mA, or 30mA all together. The supply current of a sensor with a voltage output is just the current drawn by the
internal circuitry, since it does not have a 4-20mA output.
During normal operation, the logger is powering the sensors during the sensor warm-up time only, typically 3 seconds but it depends
on the sensor type. If multiple sensors are connected to the same logger, the maximum warm-up time must be programmed into the
logger. During this warm-up time, the internal circuitry of the data logger draws about 7mA in addition to whatever the sensor
currents are. The rest of the time the logger has the sensors turned off and is powered down, drawing a small stand-by
current. Global Water data loggers draw about 0.07mA during powerdown.
The ambient temperature can greatly affect the battery life but it can be very difficult to predict how much. If the temperatures
are expected to regularly fall below 32°F, lithium batteries are strongly recommended. For the purposes of this discussion, we
will assume normal ambient temperatures.
The capacity of a battery is measured in mA-Hrs (milliamp hours), its ability to produce a certain amount of current for a certain period
of time. 9 volt alkaline batteries have about 625mA-Hrs and are used in several of Global Water's products. Global Water also
offers 12 volt rechargeable batteries in some of our systems, with 2200mA-Hr and 5000mA-Hr capacities. Most of our products use two
9 volt batteries but since they are connected in series to produce 18 volts, the capacity is still 625mA-Hrs, not twice that amount.
The amount of current that the logger draws from the batteries is the average over time of how much the current is being supplied during
the sensor warm-up time, and how long the logger remains powered down drawing the standby current only. The battery life is the capacity
in mA-Hrs divided by the average current drain in mA's. It is also safer to assume that only 80% of this battery life will be achieved. This
takes into account other factors like cooler temperatures, the fact that not all batteries have full capacity, batteries have less capacity
when supplying larger output currents and the fact that some sensors do not give accurate readings when the voltage drops below some minimum
Assume that a logger, like the WL16 water level logger, powered with 9 volt batteries is
recording at one sample per minute (60 seconds) with a 3 second warm-up time, it is powering a single 4-20mA 2-wire sensor who's output current
is assumed to be the maximum of 20mA. 3 seconds out of every 60, the current draw is the 20mA sensor current plus the 7mA current drawn
by the logger itself, or 27mA. The other 57 seconds out of 60, the logger is drawing the 0.07mA standby current. Thus the average
current is 3/60ths times 27mA, plus 57/60ths times 0.07mA. (3/60)*27mA + (57/60)*0.07mA = 1.42mA. The batteries have a capacity of
625mA-Hrs. The calculated battery life is (625mA-Hrs)/(1.42mA) = 441 hours or about 18 days. We can expect to get 80% of that or
about 15 days. This number is increased if the sensor is not supplying the full 20mA all the time but without more information, we must
assume the worst case.
A logger running on 9 volts is powering three 2-wire sensors for a warm-up time of 3 seconds with a one hour sample rate or 3600 seconds. The
amount of current during the time the sensors are powered up is the sum of the 3 sensor currents plus the 7mA for the logger. Two of the
sensors may be operating at the full scale output, while one is known to put out only 10mA maximum. During the warm-up time, the total
current draw is 20mA + 20mA + 10mA + 7mA or 57mA. The current during warm-up is drawn for only 3 seconds in every 3600, the rest of the time
the logger draws the standby current. The total average logger current being drawn from the batteries is: (3/3600)*57mA + (3597/3600*0.07mA)
= 0.117mA. The battery life is (625mA-Hrs)/(0.117mA) = 5322 hours or 221 days. We can expect 80% of that or 177 days. Note here
that 57mA is a large amount of current to draw from a 9 volt battery which reduces is power capacity, so 80% might be a realistic number. Note
too that this example has a much longer battery life than the previous example. That is because 3 seconds in 3600 is a much smaller ratio
than 3 seconds in 60.
A logger, like the GL500 multichannel data logger, is powered by a 2200mA-Hr battery, it has
a 3 second warm-up time, a 10 minute recording interval (600 seconds), and is powering a 4 sensor weather station. The wind speed sensor is
a 2-wire sensor and is assumed to operate at 10mA or less most of the time with only occasional exposure to higher wind speeds. Wind direction
is also a 2-wire sensor, but can output the full 20mA current at any time, prevailing winds often cause the sensor to output the full current, so
we use that number. The temperature sensor is a 2-wire sensor installed in a hot climate, so we must assume that it will be operating near
the full output most of the time as a worst case, 20mA. The humidity sensor is a three wire sensor with a current draw of 3mA plus the output
current. This is a dry location with humidity averaging 50% or less so the output current is half way between 4 and 20 or 12mA, current = 12mA
+ 3mA = 15mA.
Total current: 10mA(spd)+20mA(dir)+20mA(tmp)+15mA(Hum)+7mA(logger) = 72mA.
Average current: (3/600)*72mA + (597/600)*0.07mA = 0.429mA
Calculated battery life: 2200mA-Hrs / 0.429mA = 5128 hours
Expected battery life: 80% * 5128 hours = 4103 hours or 170 days
Several assumptions were made in this example, be conservative when making estimates.