Technical Note No. 98/7 - Power Management

Technical Note No. 98/7 - Power Management

DOBIE fitted with 256k RAM data storage has two independent power supplies: a rack of 8 D-cells (Eveready alkaline No. E95, or equivalent; ~18 Ahr) mounted in the main battery compartment and a single 9V cell (Eveready alkaline No. 522, or equivalent; ~500 mAhr) mounted in a small compartment on the back of the microprocessor housing (the black box). The 9V cell is a backup supply for securing data in RAM should the main supply fail or be drained. Note that flashcard data storage is not volatile. DOBIE fitted with flashcard memory does not need and is not fitted with the 9V backup battery. It only has the main power supply.

It is important to be able to accurately estimate power endurance. The following discussion is intended for that purpose.

Very roughly, the following currents are drawn:

• 50 mA when sampling or processing data;
• 0.5 mA between bursts or when DOBIE is asleep.

You can use those figures to roughly estimate power endurance given your particular sampling parameters, figuring on about an 18 Ahr main power supply (fresh batteries). Better yet (better because the estimation will be more accurate), let DOBIE do it for you.

DOBIE computes power endurance for the current sampling parameter settings and reports it at the SETUP MENU (FULL or ABBREVIATED).

Here are some example endurances:

N	Δ t (secs)	IBB (min)	E (days)
1024	0.2	30	77
2048	0.1	30	77
2048	0.2	30	43
2048	0.1	60	144

• E is endurance, which is the time, in days, to expend 80% of the main battery supply (i.e. 14.4 Ahr) when collecting data in burst mode.
• N is the number of points per burst.
• Δ t is the sample interval in seconds.
• IBB is the interval between bursts in minutes.

The following considerations apply to DOBIE with 256k RAM data storage. Data stored on flashcard is not volatile and will not be lost if the power supply is drained. By all means run DOBIE fitted with flashcard data storage until the power supply is totally exhausted!

Plan your experiment so that DOBIE is recovered and data are downloaded before the endurance is exceeded. Alternatively, specify the total number of bursts in the ABBREVIATED or FULL SETUP MENU (as appropriate) so that DOBIE terminates logging and enters sleep mode before the endurance is exceeded.

Before the start of every burst, DOBIE checks the voltage of the main supply. If the voltage falls below 9.0 V then DOBIE halts sampling and enters sleep mode. The main supply will continue to be drawn down in sleep mode, and DOBIE will stay alive right down to about 4 V (at which point the backup battery will take over - see below). When the OPENING SCREEN is displayed (by, for instance, the user connecting the comms cable between the PC and DOBIE and typing a CTRL-C) for the first time after the critical low battery condition a warning message is displayed:

You must download the data from RAM immediately, or, if that is not possible, put DOBIE to sleep ( by typing 6 RETURN at the TASK SELECTION MENU) and leave that way until you are able to download the data.

Should the main supply become fully depleted or fail entirely, the 9V backup supply will automatically maintain the data in RAM memory. The backup supply is sufficient to maintain data in RAM for around 20 days with DOBIE in sleep mode. The data can be downloaded from RAM in the normal way when ready with DOBIE powered by the backup supply.

Should DOBIE put itself to sleep because of a low main supply the "SLEEP" LED mounted on the endcap will flash red when the magnet is held to the SWITCH, thus alerting the user that the instrument is awaiting data download.

Data will not be lost from the flashcard memory if the battery supply goes down. In that case, simply replace the batteries, boot DOBIE, and download the data.

MOG
June, 1998