Re: [Gumstix-users] measuring gumstix power

[hypo <hyp...@gm...>]

Ned,

thanks so much for your informative reply! from reading your reply, it looks
like LiIon is generally good but requires more protection circuitry. This
makes me want to lean towards the other alternatives, as I think the
charging mechanisms would be simpler. Did I get that right? Also, I think
most consumer electronics prefer LiIon because of favorable charge / weight
ratios, which for me is not such a big deal. do let me know if i'm barreling
down the wrong road. 

Also, do you just measure power by hooking up a multimeter in series with
the main supply to the gumstix? I feel doing so may skip quick instantaneous
spikes when the camera turns on or is crunching data.

thanks,
hypo



Ned Forrester wrote:
> 
> hypo wrote:
>> I'm trying to figure out which battery type i can / need to use: LiOn,
>> rechargable NiMh / NiCd, lead-acid. Does anyone have any experience
>> powering
>> the gumstix with these batteries? Any practical usage experience in
>> addition
>> to technical power profile compatibility is more than welcome: e.g. (just
>> guessing) lead acids leak or need to be ventilated etc. Any pointers to
>> useful literature is also more than welcome (google generates almost too
>> much info :)
> 
> Lead-acid definitely gases, even supposedly sealed ones.  The primary
> output of sealed lead acid is hydrogen, the oxygen seems to stay inside
> (this from measurements on a specific type of sealed lead-acid).
> 
> LiIon cannot release gas (except in a failure mode) because the cells
> are hermetically sealed.  They must be gas tight because the chemistry
> is destroyed by contact with oxygen.  However, LiIon is hazardous
> material to varying extent, depending on the size of the battery and how
> the cells and battery have been tested.  Generally, more elaborate
> over-charge protection is required for LiIon.  There are various LiIon
> chemistries and the ones with high energy density (LiCoO2) can burn
> violently.  The Hazmat regulations do not distinguish between the more
> and less volatile types.
> 
> I have not designed with NiCd or NiMH, so I know less about those.  I
> think that both types can out-gas.
> 
> Nominal lead-acid cell voltage is about 2V/cell, LiIon is about
> 3.6V/cell, NiCad is about 1.25V/cell, and I suspect that NiMH is about
> the same as NiCad, but I'm not sure.  Keep those voltage differences in
> mind when you compare AH ratings of cells, because the voltage-AH
> product tells you the energy provided by the cell.
> 
>> Also any heuristics on how current consumption maps to battery life? i
>> mean
>> if a battery is rated for H amp-hrs, do i simply divide by average /
>> steady-state current / peak current to get expected average, min battery
>> life?
> 
> Average current (mathematical average) is the right measure.  Each
> electron that flows consumes a corresponding chemical component.  For
> accuracy, this average needs to be taken over the entire battery life,
> accounting for both load variation and any change in load current with
> reducing battery voltage.  Lead-acid and LiIon have significant voltage
> reduction as the battery discharges.  NiCad and (I think) NiMH hold a
> much more steady voltage, and then suddenly collapse at end of
> discharge.  If the device has a DC-DC converter to make a regulated
> voltage, then the current will likely rise as the voltage declines
> (constant power), but the device has a linear regulator (as I is true of
> an un-adorned Gumstix), the the current will remain constant and less
> power will be wasted as the voltage declines.
> 
> Consider published battery capacity ratings to be averages for new
> batteries.  I have had good luck getting published output from LiIon
> batteries, but less so with lead-acid.  Any published rating is for the
> early life of the battery; capacity declines with calendar age and cycles.
> 
> -- 
> Ned Forrester                                       nfo...@wh...
> Oceanographic Systems Lab                                  508-289-2226
> Applied Ocean Physics and Engineering Dept.
> Woods Hole Oceanographic Institution          Woods Hole, MA 02543, USA
> http://www.whoi.edu/sbl/liteSite.do?litesiteid=7212
> http://www.whoi.edu/hpb/Site.do?id=1532
> http://www.whoi.edu/page.do?pid=10079
> 
> 
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