Batteries
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Battery FAQs
Gumstix has roots in robotics, where the power problem is typically solved somewhere else in the system. Here we examine ways to address the power issue to make a self-powered unit. One target is to have a computer that can be conveniently carried -- a wearable computer.
Please contribute photos if you've implemented any of these! It would be interesting to see how people solve the enclosure issue.
Low-power modes and soft power
verdex and battery backup
One change on verdex vs basic/connex is that verdex 24-pin connector exposes a backup-battery connection which can be used to power the RTC while the verdex is "off". To understand how to use this, refer to the docs for the TPS65020 PMIC which is used on verdex.
Additional information
This is currently just a placeholder. There should be tricks for lowering current draw by powering off unused devices, putting the cpu in powersave states, etc. We should also have a nice way to go from off (or "nearly" off) to full on with a button or some external event like bluetooth activity.
- Turn off bluetooth by dropping GPIO7 to hold the bluetooth module in reset, cutting its power draw and effectively disabling it:
- /etc/init.d/S30bluetooth stop
- echo clear > /proc/gpio/GPIO7
- echo "AF0 out" >/proc/gpio/GPIO12 (?) to cut the GPIO12 clock--NOT currently stopped in S30bluetooth
- PCMCIA: load the kernel modules to set all the GPIOs to the right states (this reduces initial draw). With no card and modules loaded, draw is minimal.
- Turn off wifi ?
- Turn off audiostix ?
- GPS:
- The u-blox LEA-4H GPS receiver module has a separate VBAT backup battery voltage input, but the GPSstix does not break out VBAT separately and does not provide control of VCC/VDDIO power to the GPS module
- The LEA-4H supports a power-conservation mode called FixNOW (see the ANTARIS 4 GPS Modules System Integration Manual on the u-blox Web site):
- FixNOW Sleep Mode lowers current consumption from 39mA to 80uA
- FixNOW operations can be used to automatically wake up the receiver at a selected interval, or to wait until software requests a fix
- Future GPS power control design considerations:
- If VBAT power is removed from the LEA-4H, the almanac and ephemeris data will be lost, the RTC will not run, and the receiver will not be able to perform rapid hot/warm starts (unless AGPS data can be supplied)
- If VBAT power is not available, the FixNOW timed modes will not be possible since the RTC will not run
Intel has a guide about power saving with the PXA255; see: Documentation/PXA255/PXA255 Power Consumption Guide [27878601.pdf Intel® PXA255 and PXA26x Applications Processors Power Consumption During Power-up, Sleep, and Idle].
- Unused GPIO pins should be set as outputs (how does this apply to the gumstix GPIO setup? Which pins in particular should we be thinking about?)
- Sleep state optionally uses a slow clock for better power savings
- Sleep state registers PGSR{0,1,2} can automatically keep GPIO in a special state during sleep
- During sleep, drive as many GPIO as output as possible; for pins that are normally inputs:
- Avoid driving pins to 0 if they have pull-up resistors
- Avoid driving pins to 1 if they have pull-down resistors
Cpufreq can ostensibly be used to change the cpu frequency, but the peripherals need to be disabled first or the results are "indeterminate." Sometimes this means crashing the cpu.
- UARTs
- IRDA
- USB
- SSP
- AC97
It should be possibly to tell the bootloader to start the cpu in a slower speed which it would use until the next boot.
Battery Types
AA/AAA Alkaline
Three 1.5v cells should be able to power the Gumstix for a few hours.
You can improve the discharge characteristics by adding a supercap in parallel with the batteries. Cooper's P-series are rated for 5v and carried at Digikey. (Digikey part 283-2516-ND is discontinued, check 283-2787-ND)
AA/AAA NiMH
With NiMH, you will be better off with four 1.2 V cells. I have not seen any discussion of building a circuit to charge these cells in-place, so people must be typically removing them to do the charging. Four 2400mAh batteries will run the Gumstix for about 6 hours.
NiMH have the worst internal resistance of all the rechargables, so you might need a supercap added here as well.
Lithium Rechargables
Traditional rechargable lithium and lithium polymer both have similar discharge characteristics and both can use the same charging circuit. Lithium polymer cells currently provide the best performance-to-weight ratio. They have a lighter cover instead of the tough outer shell that traditional lithium needs.
You definitely want to be able to charge a lithium battery in place. Take care in choosing and assembling your charging unit. Rechargable batteries of all kinds can catch fire or explode when mismanaged (sometimes they do this even when you do everything right--ask Dell about exploding batteries).
People have expressed interest in seeing Gumstix integrate a lithium charger on a board like the GPSstix. It would be very convenient to have the board trickle-charging its battery whenever you are using the USB data connection.
A couple of interesting batteries that will fit behind the Audiostix:
http://www.mouser.com/search/ProductDetail.aspx?R=UBC433475/PCM
http://www.mouser.com/search/productdetail.aspx?R=66380712099 (with terminals for user-replacement and external charging; wait for mouser to link to the right spec sheet)
Be sure to choose a battery with a built-in undervolt protection circuit or plan to provide your own protection circuit. An unprotected Lipo battery will fail if the voltage goes below 3.0v (measured under no load).
A convenient charging circuit like this one can handle a single cell and uses the Max1555:
http://www.sparkfun.com/commerce/product_info.php?products_id=726
Here's a side view of a gumstix with the audiostix, lipo board, and a small (470mAh) battery. This setup runs for about 4 hours on a charge.
http://www.xmission.com/~bmidgley/gumstix-lipo-side.jpg
Batteries for Robotics
All gumstix models except robostix actually regulate down to 3.3v internally but they will generate too much heat if you feed them much over 4v.
Robostix is the exception and needs a solid 5v to 6v. If you do have to put together a power system for a robotics project, these Tenergy 7.2v batteries generally work well when used with a regulator:
http://www.all-battery.com/index.asp?PageAction=VIEWPROD
