The problem with using a fixed point as a "needs charging" or "battery
level" meter is that the battery voltage peak point will slowly decline as
the battery ages (as it chemistry changes), and it varies from unit to 
unit..

The ideal batterycharge guage is the scheme devised for "smart batteries",
It maintains a history about the battery, and therefore is able to determine
PRECISELY the charge level, battery quality, battery lifetime.

Intel and others developed the specification. Battery control and monitoring
is done by multibus I2C. Available thru Google.

--Bob
 

Mike Hord wrote:

>>Thos two aims may be less appropriate than it may appear.
>>You need to decide what you are trying to measure.
>>If it's available energy in the cell at constant current then the
>>above is OK. But that is less useful in most applications than other
>>measures.
>>    
>>
>
>I guess maybe I'm confused.  Doing a constant load current drain
>gives me two pieces of data:
>1.  Battery life at that current and
>2.  Joules available at that current (with some math involved).
>
>  
>
>>Consider, if a load draws constant current, such as eg a linear
>>regulator supplying a relatively constant load, then time to end point
>>at a given current is the key measure. This may be modified by various
>>pulsing or on off duty cycles but the fundamental measure is how long
>>you can operate the given load for. In which case mAH is what you want
>>and Joules isn't relevant.
>>    
>>
>
>Okay, so my constant current load tests have given me some idea
>as to how many mAH I will get at various loads.  I can fiddle and fart
>around with the curves I have (in their simplest form, distance to flat)
>and get a good idea of battery life.
>
>  
>
>>If you are operating into a switching converter then Joules is
>>relevant as the converter uses all the available energy until endpoint
>>is reached. But it doesn't draw constant current, and if the converter
>>efficiency varies as Vin changes, as it usually does quite
>>significantly in practice, then unless you model the converter
>>performance with you load you won't get an accurate result.
>>    
>>
>
>Again, my data provides me with an available Joules measure, from
>which I can calculate battery life in certain situations.
>
>  
>
>>Whether this approach is acceptable depends on how expert your
>>intended user is.
>>    
>>
>
>Intended user (so far) is you and me.  ;-)  So the users range from
>fairly inexpert to far more expert, BUT generally capable of using
>a device with fairly powerful capabilities as far as programmable
>load simulation goes.
>
>My thought on this matter was that by collecting constant current
>data at, say, 5 discharge rates, it would be possible to characterize
>the life cycle of a particular "species" of battery in such a way that
>it would be possible to calculate its lifespan based upon the load
>cycles/fluctuation.  
>
>Mike H.
>
>  
>


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