GPS Logger IV – Power and Batteries


The prototype is at the stage of being able to successfully achieve GPS lock, log the received coordinates, waypoints and go to sleep. The power usage of the system needed to be analysed in order to specify the battery requirements. The current draw was monitored after the 3.3V regulator as the device was put into its various modes.

Looking at the current draw of the PIC, SD Card and GPS receiver

Initial Power Usage

The initial measurements showed that the prototype used ~110mA when running and 65mA when suspended with the GPS receiver running. When the SD card was writing a sector the current could peak to 120mA for a short period.

Operating Standby
Current (A) 0.110 0.065
Duty (Hours per Day) 8.000 16.000
Average Mode Current (A) 0.037 0.043
Average (A) 0.080
Days Hours
Run Time (h) 7.000 168.000
Required Battery Capacity (Ah) 13.440

Operating for 8 hours a day over a week would require a 3.3V, 13.4Ah battery, which would reduce the portability of the device significantly and be rather expensive. Steps needed to be taken to reduce the power usage.

The main loop of the code consisted of an infinite loop, calling functions to process any data that may, or may not be present and consisted of the following code fragment.

while(SysState != STATE_OFF)
{
	gps_process();		//process the GPS information
	button_process();	//process any button pushes
}

All of the data in the system arrives via interrupts and no polling is performed so continual looping is not required. The process functions were modified to return a boolean indicating if they needed to continue to process information, ie the button is down and it has to wait for it to be released. The main loop was then modified to put the PIC microcontroller into an idle mode if nothing else had to be done, shown in the code fragment below.

while(SysState != STATE_OFF)
{
	bool run_again;

	run_again = gps_process();	//process the GPS information
	run_again |= button_process();	//process any button pushes

	if(run_again == false)
	{
		//all data has been processed for now, go to sleep
		//any incoming interrupts will wake us back up again
		_asm sleep _endasm
	}
}

By making this change the operating current reduced from 110mA to 90mA, a reduction of 18%. The major current consumer in standby mode was the GPS receiver. Keeping the receiver running while the logger is off enables almost instant startup times as satellite lock is maintained. The SiRF protocol support ‘TricklePower’ modes that reduce its operating time and duty cycle while attempting to maintain lock. These modes were tried, however they only resulted in a reduction in current draw of 15-20mA. If the GPS receiver is used often, it is able to get lock in under 10 minutes from a cold start. For example the logger could be enabled while getting ready for a hike or travelling to the destination so it has achieved lock by the time the user is ready to start hiking. This processes allows the receiver to be switched off entirely, reducing the current draw to almost 0mA.

Operating Standby
Current (A) 0.090 200e-6
Duty (Hours per Day) 8.000 16.000
Average Mode Current (A) 0.030 0.000
Average (A) 0.030
Days Hours
Run Time (h) 7.000 168.000
Required Battery Capacity (Ah) 5.062

The final power usage calls for a 3.3V, 5.06Ah battery, which is easily obtainable. To put the capacity into perspective, my Nokia 6110 Navigator has a 3.7V, 0.9Ah LiPo battery which would need to be charged at least once a week with minimal usage (2 hours talk tops). This GPS logger is able to continuously operate for 56 hours before needing a charge, much longer than the 6110’s 3.5 hour specified talk time.

Battery Selection

Now that the requied battery size is known, a suitable rechargeable battery needs to be chosen. Due to space and weight requirements Lead Acid and SLA batteries are excluded and NiCads are ruled out due to better technologies being available. The main contenders for battery types are NiMH and Li-Ion as they are readily available. A large amount of information on battery specification is available in the Electropedia guide on How to Specify Batteries.

NiMH

Nickel Metal Hydride are a 1.2V (nominal) cell available in a range of shapes, sizes and capacities. The batteries are charged using a constant current source which is terminated when the rate of temperature increase of the cell exceeds a preset amount. To use these cells in this project there would need to either be 3 cells in series of the required capacity or a boost converter could be utilised to bring the operating voltage up to 3.3V. Using a boost converter the actual battery capacity becomes:

Cell Voltage 1.200
Operating Voltage 3.300
Converter Efficiency 85%
Actual Battery Capacity (Ah) 16.378

Due to losses in the converter, the capacity is slightly larger than simply 3*5.062. Looking around for available capacities, with a preference for AA size, the following batteries would be required to power the logger:

Available Batteries Capacity (Ah) Cost Cells Total Cost
AA 2900 mAh NiMH Accupower Battery 2.900 $       4.85 6 $    29.10
AA 1500 mAh NiMH Battery 1.500 $       1.99 11 $    21.89
AA 2000mAH Rechargeable Ni-MH Battery 2.000 $       4.75 9 $    42.75
Battery NiMH AA 2500mA Pack 4 2.500 $       5.45 7 $    38.15
Ni-Mh Rechargeable Battery 3300mAh Sub C Single 3.300 $       9.95 5 $    49.75
AA 2400MAH NI-MH – 4 PACK 2.400 $       4.74 7 $    33.16
D SIZE 9000MAH RECHARGEABLE BATTERY – NIPPLE 9.000 $    24.90 2 $    49.80

An average of 6 AA cells, or 2 D cells would weigh the project down and take up a large amount of space. The logger is meant to be as small and light as possible.

Li-Ion

Lithium Ion batteries are a 3.7V (nominal) cell that comes in the standard sizes and that can easily be made into custom shapes. These batteries are charged using constant current up to a pre-set cell voltage, then switched to constant voltage until a timeout. As these cells are above the desired operating voltage a buck regulator will be used to reduce the cell voltage to 3.3V. Some of the available Li-Ion cells are as follows:

Available Batteries Capacity (Ah) Cost Cells Total Cost
Li-Ion Battery for LG KU990 2.800 $       4.99 2 $       9.98
18650 Li-ion 3.7V 2800mAh Rechargeable Battery 2.800 $       3.15 2 $       6.30

Li-Ion cells will be used for the logger as they are cheap, readily available and only require 2 cells for 56 hours of operation.

Next Steps

A final circuit needs to be designed, allowing for battery charging from USB and voltage monitoring as well as fixing up some errors in the initial circuit. Once the circuit is designed, all components need to be purchased, a case picked and the PCB designed.

Continue to part V

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