[OT] More on Batteries

[Bradley Miller]

> > That would make a battery dead in 10 days. I find that hard
> > to believe.
>
>That would be 10% of the 'remaining charge'. i.e.
>
>12V - (12V * 10%) = 10.8V
>10.8V - (10.8V * 10%) = 9.72V
>
>Which means the battery would be effectively dead after 24 days when it has
>less than 1V of charge.  Is that believable for some battery types? Maybe.

Actually the voltage on NiCads is like 1.25 (not the full 1.5 of a dry 
cell).   (There are some "hotter" Nicads that give out a full 1.5, but 
we'll leave those out for now.)    Going by the voltage rate shown, they 
would be down do .91 volts in only 4 days . . . and .9 volts is where 
typical discharge cutoff voltage is.  (I raced RC cars for years.)   Now 
instead what the "loss rate per day" is referring to overall 
capacity.  Take a 1800 mah AA NiMH battery and apply a 10% loss per day on 
the overall capacity.   The cell may show 1.1-1.2 volts after x days, but 
it won't have any capacity (amp hours) left in it.

>However, what about alkaline batteries?  I'm looking at some Rayovac's that
>have Dec 2009 written on them.  What about lead-acid batteries?  I've had
>lead-acid batteries in cars that sat for over 6 months and were still able
>to crank over the engine.

Apples and oranges here.  Alkaline batteries are a different beast and 
don't have the same loss rate as a rechargeable type battery.   The car 
battery thing is another "capacity" issue.  Most passenger cars provide 
very little resistance for a modern battery.  Try sticking one of those 6 
month batteries in a high compression V8 with a lot of cubic inches.  ;-)

-- Bradley Miller