November 5, 2010
Rev1.4
53
Appendix A – Battery life calculation
When to replace the batteries
Compared to the cost of deployment, the cost of a new set of batteries is close to
negligible. We highly recommend that the batteries be replaced before every
deployment unless there is no doubt that they have sufficient capacity.
Battery voltage does not decrease linearly and is therefore not an accurate indicator of
battery capacity. However, if the logger software indicates that the batteries have less
than 12V remaining (6V for single channel loggers), the batteries should be replaced.
Predicting battery life
Battery life prediction is a complicated issue. In RBR loggers, battery life is a strong
function of the type of logger, the type and number of sensors attached, the sampling
scheme (sampling period and the use of: thresholding, real-time data collection and
averaging), and the temperature of the water during the deployment. Manufacturers’
data for the batteries do not provide information that can be directly related to way the
batteries are used in RBR Loggers.
The Ruskin Software calculates the expected battery usage (in mA Hours - mAh) for the
logger during setup, see Ruskin User Guide. The nominal capacity of the batteries is
1300mAh at room temperature. The software will warn the user if the expected battery
usage for a particular deployment scenario is above 1000mAh. The software will not
prevent a logger from being started even if a warning is given. In tests at RBR Ltd. of
multiple examples of various brands of batteries at 6°C, all examples were able to
provide the threshold capacity of 1000mAh. The user should view these predictions
and further de-rate the batteries if the expected deployment is in cold water. The
battery usage estimate is stored as part of the log files and may be used after
deployment to enable the user to obtain a characterization of how a particular logger
performs.
RBR continues to characterize battery life in its loggers and will continue to improve the
battery life calculator in future versions of the Ruskin Software.
Battery life will be reduced if the RS485 serial transmission protocol is employed. The
consumption for the RS485 driver is defined as 2.5mA nominal. Hence for every hour
of deployment, 2.5mAh must be added to the battery drain figures.
When in doubt, replace the batteries.
