Estimating power consumption, time left for shooting on portable power
if we use some "gauge" system we can do a pretty good estimate. There is no open standard for this of course, but to explain in simple words how it works :
> Each battery pack would have a chip that counts power getting > in when charged, and power out when getting discharged (think > like putting fuel in a car).
Too complicated for most cases, typical batteries have a very well known curve they follow when discharging, which can be used to calculate the remaining capacity from the voltage.
> This information can be made available trough i2c. The chip > managing this must known which kind of chemistry is the > battery pack.
> And also adjust to the efficiency of the battery over time (a > battery becomes less and less efficient over time, and 100% in > doesn't mean 100%).
IIC interface for voltage levels at the battery and storing simple information about the battery pack in an eeprom are a good idea for custom battery packages.
> There are specific chips for this task available. They are > cheap (like 0.5$).
> So, if the camera has a somewhat constant power need,
Which it doesn't :) We will make sure to power down everything not required during pauses where no recording or viewing happens.
> we could estimate the time left using so called intelligent > battery pack.
> The pack must have the chip for this to work, it's the memory > of past operations ofr this particular pack (averaged discharge > rate, power left, etc...)
> But do we want to make custom battery packs, I'm sure we don't > at this stage.
Correct, but we can keep this in mind for later.
> This, or provide 2 threshold levels for voltage (warning > and shutdown), li-po for example cannot go bellow a defined > voltage, else they are destroyed, or heavily impacted on > performance.
> Having some discharge curves preloaded in camera can help as > well (at least for lipo and to a lesser extend nimh).
> Or of course reverse engineer existing battery packs, who -I > bet- work exactly the same, with the exactly same chips inside.
Here a chip that does exactly that, for 2-4 cells batteries which are in our intended ballpark : http://www.ti.com/product/bq34z100