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As can be seen below from an actual measured device on continuously this rough calculation is quite close.
2 X AA Alkaline Battery Discharge @ 3Hz Power Save
3.2
3
2.8
2.6
2.4
2.2
Min Level
2
0
1
2
3
4
5
6
7
8
9
10
It must be remembered that this calculation is for when the device is continuously on and not sent to deep sleep.
So if the above device was used with a handheld and woken up 5 times a day for 5 minutes each time the total
usage in a 24 hour period would be
5 X 5 = 25 mins = 0.416 hours
So we are only using a fraction of the daily 24 hours so battery life will be 24 / 0.416 times the continuously on
life. i.e.
24 / 0.416 = 57.6
So
57.6 X 23.1 days = 1330.56 days
Or
44.3 months
Or
Over 3 and a half years.
Note that the above rough guide does not take into account the battery usage as the device periodically wakes
from deep sleep to check whether it should wake up properly. If the device wake check interval is set to 5
seconds then we can modify the battery life from above by multiplying by a factor of 0.6 (We have only
calculated the factor for a 5 second wake check interval.)
3.6 years X 0.6 = 2.1 years
Accurate Guide
The following shows how to more accurately calculate battery life. This does rely on an estimate of usable
battery capacity which is not as high as manufacturers state their battery capacity to be except for batteries
where the cuttoff voltage above the minimum voltage for the device.
To estimate usable capacity you will need to refer to manufacturers discharge graphs and find a curve closest to
around 30mA and estimate the capacity as Amps X Time where time is the point where the battery falls below
where the device would be supplied with 2.1V. This will be 1.05 in the case of 1.5V cells as we use two in series.
TXInterval = TXInterval Parameter / 1000
SampleTime = SampleTime Parameter/ 1000
LoadImp = Impedance of strain gauge in Ohms
HoursUsage = How many hours the device is NOT asleep in a 24 hour period
BattAH = Usable battery capacity in Amp Hours (Ah)
WakeInt = WakeChkInt parameter
FixedMeasurementCurrent = 45
Internally the device will adjust SampleTime if it is too long for the TXInterval so we need to take this into
account for our calculations:
91
Mantracourt Electronics Limited T24 Technical Manual
Mode
11 12
13
14
15
16
17
18
19 20
21
22
23
Days
Min Level
24
25
26
27 28
29
30
31
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