RockBLOCK 9603 - Power Consumption

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This document is intended to provide users with an introduction to the power consumption characteristics of the RockBLOCK 9603, to assist in determining battery size for a given application.

Functional Background

The RockBLOCK 9603 contains a supercapacitor which acts as an energy reservoir and serves to buffer the highly pulsed nature of its internal circuits from the user connections. In many ways this supercapacitor can be regarded as a battery, in that it needs to be charged up before it is effective. However, once charged, that energy is available for the normal operations and is therefore not wasted. If the device is put into its sleep mode, that energy remains available for some time – typically many hours

Modes of Operation

The unit has four operating states, each of which consumes different amounts of current.

(a) Charging: When the device is first powered or after a long period of disuse (typically more than a week)
(b) Idle: Powered-up, but not executing any commands.
(c) Active: Transmitting, or other command driven activity.
(d) Sleep: A low power mode to conserve energy. No activity is possible in this mode.

Charging Mode

• Time taken to 90% charge – approximately 30 seconds.
• Maximum current consumption during charging: approximately 500 mA.
• Battery capacity required for charging phase: 12.5 A-s (amp – seconds).

Idle Mode

The current consumption in Idle mode depends upon the quality of the view of the sky. With an un-obscured view, the average current consumption is approximately 40mA, but this can increase to nearly 50mA if the view is very poor.

In this mode, Ring Alerts can be received. If Ring Alerts aren’t required, then there is little reason to leave the unit in this state.

Active Mode

Active mode is entered whenever any AT command is sent to the unit. Invariably, executing a command will increase the current consumption, although the amount of increase depends on several factors; the particular command, the view of the sky and the length of time to complete the command (which can vary).

In this summary, only a SBD transmission is considered. Typically, a successful SBD transmission has an average current consumption of between 45 and 50 mA (averaged over a 60 second period).

The time to complete the transmission attempt is variable, typically 20-30 seconds, but in this assessment, a fixed time of 1 minute is considered, being the time from exiting sleep mode, allowing about 10 seconds for the unit to initialise, then carry out the SBD transmission, then a further time to allow the current to drop to its original level.

Accordingly, the battery capacity required to carry out a successful SBD transmission is approximately 2.8 A-s. Note: it is possible that under good “view of sky” conditions, and issuing commands immediately after exiting sleep mode, the time taken could be shorter and hence the battery capacity necessary could be less. In testing, the minimum usage achieved was 1.5 A-s.

An attempted transmission which fails, results in an average current of between 60 and 65 mA, giving a required battery capacity of 3.8 A-s.

Since there is a marked difference in capacity usage between a successful and unsuccessful transmission, there is an inclination towards determining the signal level prior to making the transmission attempt (using the “at+csq” command) to assess the likelihood in achieving a successful result. There are two downsides to this approach: (a) carrying out the signal strength measurement takes time (and therefore battery capacity), and (b) in poor signal conditions, the signal can change quickly from 100% to 0% and therefore the signal strength result can be irrelevant.

The guidance on this approach would be to adopt the signal strength measurement immediately prior to attempting a transmission where the view of sky is known to be poor and the chance of failed transmissions is very high, but otherwise to just attempt an immediate transmission.

Sleep Mode

In sleep mode, all the active circuits are turned off, leaving just the supercapacitor in a charged state ready for immediate use.

The steady state current consumption in sleep mode is typically about 73 uA, resulting in a battery capacity usage of 6.3 A-s for 24 hours of sleep mode.

However, it takes nearly 24 hours for the sleep mode current to drop to this steady value. After an hour in sleep mode, the current will only have dropped to about 100uA.

Therefore, since most applications will call for transmissions to be made more often than once in 24 hours, it is suggested that the sleep mode current is generally assumed to be 100uA.

The capacity usage then for one hour in sleep mode becomes = 0.36 A-s

Since the supercapacitor has a very good electrical efficiency, it is recommended that the unit is left powered-up and in sleep mode when not in use.

The only exception to this guidance would be when it is anticipated that there are extended period of disuse. If the period of disuse exceeds about 2 days, then it is more efficient to completely power down the unit.

Capacity used to re-charge the supercap = 12.5 A-s

Capacity used for 2 days sleep mode = 2 x 6.3 = 12.6 A-s (approximate)

Transition: Active mode – Sleep Mode

In active mode, the unit typically draws upwards of 40 mA and the steady state sleep mode current is approximately 73uA.

Note, however, that from a current consumption perspective, the transition from active mode to sleep mode is not instantaneous. After sleep mode is entered, there is a gradual decline in the current from its active level to the sleep level.

The battery capacity consumed during this transition phase (lasting one hour) has been measured to be about 4 A-s.

Consequently, in a SBD transmission cycle, consisting of sleep – active – sleep stages, not only must the capacity usage of the active mode be accounted for, but also the capacity usage of the transition phase.

Example Usage

Typically, the most energy efficient way of operating is to maintain the unit in sleep mode for most of the time, then exit from sleep mode immediately prior to executing a command to carry out the SBD transmission. Immediately after the SBD attempt has completed, return the unit to sleep mode. If a transmission fails, it is recommended to return the unit to sleep mode, then repeat the transmission attempt after 1 minute. (An immediate re-try is likely to fail as the satellite locations will not have changed much in that time). Your own retry strategy will need to consider your application’s requirements, known environmental conditions and other factors.

(1) Transmitting every hour:

24 transmissions per day = 24 * (2.8 + 4) = 163.2 A-s
There are no sleep mode periods since the transition period time is one hour and these are already accounted for.
Capacity use per month = 4896 A-s (= 1.4 AHr)
Capacity use per year = 59600 A-s (= 16.5 AHr)

(2) Transmitting every 6 hours:

4 transmissions per day = 4 * (2.8 + 4) = 27.2 A-s
20 hours of sleep mode = 7.2 A-s (ignoring the accounted transition periods)
Capacity use per day = 34.4 A-s
Capacity use per month = 1032 A-s (= 0.29 AHr)
Capacity use per year = 12600 A-s (= 3.5 AHr)

(3) Transmitting every 12 hours:

2 transmissions per day = 2 * (2.8 + 4) = 13.6 A-s
22 hours of sleep mode = 7.92 A-s (ignoring the accounted transition periods)
Capacity use per day = 21.5 A-s
Capacity use per month = 646 A-s (= 0.18 AHr)
Capacity use per year = 7850 A-s (= 2.2 AHr)

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Note

The above calculations assume all transmissions are successful. Whilst this is unlikely to be the case in practice, the fact that some transmissions will be shorter will make up for the repeated attempts where there are failures.