RBR Technical Note: Battery Capacity

Below is a discussion of some of the factors affecting the battery life of your RBR logger. Along with introducing some general principles, there is also some discussion of some typical deployment examples. Other examples may be worked out as further exercises. Remember that the battery capacity has to be derated if the loggers are deployed in lower temperature situations. Please feel free to download the latest version of the RBR Logger software and use the battery/memory usage calculator built in to the deployment settings window (this may be used with, or without, a logger connected to the computer).

All of the submersible loggers use 123A size 3V lithium cells. These are standard camera batteries, and may be obtained from retail photographic or hardware outlets. Battery capacity is measured in mAh (which stands for “milli-Ampere hour”). The capacity of the 123A cell is nominally 1300 mAh at 20 ^oC. The manufacturers derate this to 1000 mAh at 0 ^oC and 650 mAh at -20 ^oC.

Equivalents Chart

Duracell Notes on Lithium Batteries

Duracell Batteries: Care and Handling

Duracell Batteries: Disposal

Duracell Batteries: Safety

TR-1050 and DR-1050

The TR-1050 single channel temperature logger is designed to run for a year and collect 2,400,000 readings using just one set of batteries (based on 8MB memory). The power used to take each sample is 6 mA. Sample duration is 0.3 seconds. This gives 6 x 0.3 or 1.8 mA-seconds per sample. For each sample the battery drain is therefore 1.8/3600 or 0.0005 mAh. The sleep current of the logger also needs to be considered. The TR-1050 sleep current is 40 µA. For any elapsed time (T, measured in hours) and number of samples (N), the total battery drain is, therefore: 0.0005 x N + 0.040 x T. For 600,000 samples logged over one year the total mAh required is:
Total mAh = (0.0005 x 600,000) + (0.040 x 24 x 365)
= 300 + 350
= 650 mAh

In practice these figures are conservative. In tests at room temperature the TR-1050 is able to collect a complete set of data on one set of batteries. For a typical deployment lasting for a year or less, and calling for a full set of readings there should be no need to be concerned about battery life.

A caveat to these comments is that some of the loggers have been found to exhibit anomalous sleep currents during the first minute of sleep. In any critical long-term deployments, contact RBR for further advice. We are continuing R&D on methods to reduce battery consumption and permit even greater amounts of data to be collected on one set of batteries.

TDR-2050

The TDR-1050 two channel temperature logger is the first of a new generation of data loggers which has considerably reduced power consumption. It is designed to run for a year and collect 1,200,000 pairs of readings using just one set of batteries (based on 8MB memory). The power used to take each sample is just 2 mA. Sample duration is 0.6 seconds. This gives 2 x 0.6 or 1.2 mA-seconds per sample. For each sample the battery drain is therefore 1.2/3600 or 0.00033 mAh. The sleep current of the logger also needs to be considered. The TDR-1050 sleep current is 20 µA. For any elapsed time (T, measured in hours) and number of samples (N), the total battery drain is, therefore: 0.0003 x N + 0.020 x T. For 1,200,000 samples logged over one year the total mAh required is:

Total mAh = (0.00033 x 1,200,000) + (0.020 x 24 x 365)

= 396 + 175

= 571 mAh

In practice these figures are conservative. In tests at room temperature the TDR-1050 is able to collect more than one complete sets of data on one set of batteries. For a typical deployment lasting for a year or less, and calling for a full set of readings there should be no need to be concerned about battery life. Because of the low consumption it is possible with careful planning to consider deployments of up to three years with one set of batteries.

XR-420 Series Loggers

The battery capacity for a logger from the XR-420 series may be evaluated in a similar way. There are, however, a number of considerations depending on the model of logger being considered, the mode of use, and the number and type of sensors attached.

The maximum number of samples is 2,400,000 (based on 8MB memory) divided by the number of channels. So, for a two channel instrument, this is 1,200,000, and for a 3 channel instrument this is 800,000 and so on. The sleep current in all models is 30 µA, lower than that of the TR-1050.

The total sample time and current depends upon the model. Here are the figures for the main models.

The addition of extra sensors to the XR-420 loggers is encouraged, but the additional current consumption of the sensor must be considered. Turbidity and fluorescence sensors typically add a current of 5.5 mA and 17 mA respectively. These figures include 2mA for the interface cards. In the case of pH and DO there is no additional drain to the battery capacity apart from 2 mA for the interface card.

Finally, if averaging is enabled, then the total sample time is equal to the averaging time in seconds. Recalling that N = number of samples and T = total operating time in hours, some solutions to the following basic equation may be obtained:

Total battery drain = (drain per sample) x N + (sleep current ) x T

Examples

Example 1: XR-420 CTD

Q: How much battery drain is there if one logged 200,000 samples from a XR-420 CTD unit over one year?

A: For 200,000 samples over one year the capacity required is therefore:

= (0.0023 x 200,000) + (0.030 x 12 x 365)

= 460 + 132 = 592 mAh

Example 2: XR-420 CT, with averaging

Q: Let us suppose that the unit is programmed to average for 30 seconds and take a reading every 15 minutes. How long will the batteries last?

A: Each sample takes 30 x 11 mA seconds, or 330/3600 mAh = 0.0917 mAh. A full solution to this example may be found by solving the equation:

Total Battery Drain = (drain per sample) x N + (sleep current) x N x (time between samples)

where N = number of samples

Using the battery capacity and working backwards:

Battery capacity (1300 mAh) = (0.0917mAh x N) + (0.03mA x N x (15 / 60)hours )

N = 1300 / ( 0.0917 + 0.0075) = 13,104 samples

At a 15 minute sample rate, this corresponds to 13,104 samples/(4 samples/hour * 24 hours/day) days, or about 136 days.

Example 3: Profiling with an XR-620 CTD++

Q: What happens when you add two extra channels: a fluorometer; and a turbidity sensor?

A: In that case the total current used is 34.5 mA. For profiling, the battery capacity is therefore 1300/34.5 hours, or 37 hours. For extended battery lifetime and more efficient memory usage, the thresholding mode may be used to ensure that only data acquired when in the water are stored in memory. In December 2002 loggers were introduced with power saving software to further enhance battery life.