
re: construct a power supply
14 mar 1999
h. l. miesner continues:
>...aa batteries may not be acceptable because of the current draw.
>assuming that the radio actually uses 300ma@6v...
that's 1.8 watts.
>the aa battery pack you propose would probably only last about 2.5hrs max.
>this doesn't really follow the formula that would be used for calculation
>but is an 'experienced' answer.
curious how you choose to ignore your own data...
your data says aas can supply 54ma@20hrs, at 1.5v, i presume.
that's 1.62 watthours. divide by 1.8 watts and we get 0.9 hours.
(watthours/watts = hours.) if the formulas don't work, let's fix 'em,
or find new formulas, or figure out how to apply them correctly, so
the world becomes a more predictable place. this kind of arithmetic
has been around for hundreds of years now and has been used correctly
by hundreds of thousands of people around the world. why shouldn't we
use it correctly here?
>...if you hook up a aaa battery to a 20ma load it should last
>about 21hrs, or, 20ma=.02ah times 21 hrs=.42ah
that makes no sense at all, h.l. amphours times hours equal amphours
squared, not amps or amphours. even the numbers are screwed up here.
0.020 does not equal 0.02 times 21, as your first equals sign says.
let's try this again, with unscrewed thinking:
hours = amphours/amps, ok? amphours = ampsxhours, ok? so the aa
battery stores 20max21h = 0.42 amphours of charge, in coulombs, ie
large bushels of electrons, and if we suck them out at a rate of
0.020 coulombs per second, ie amps, the battery runs out after
0.42ah/0.020a (cancel the a's and what's left?) = 21 hours...!
this confusion can further be lessened by including the voltage, ie
figuring out how much energy the battery stores in watthours, and
how much the load is in watts. watthours/watts = hours.
it's a lot easier to think this way than to figure out how to
manipulate all these screwed up units in your head. the units
helpfully give us clues. this is basic highschool physics, and
it's embarrassing to see pv people do it wrong over and over again,
and insist they are doing it right. that marks them as rank amateurs,
and raises red flags with electrical inspectors who say things like
"you'd better tear all that out and get it redone by a licensed
electrician (who knows what he's talking about.)" it makes utility
engineers wince, and say things like "we will not accept your
calculations. you need to find a licen$ed profe$$ional engineer
to redo that application if you want to hook that up to the grid."
you may be able to build a fine working pv system, like an auto
mechanic who can fix any car, but how far would an auto mechanic
get if he always used the word "steering wheel" for carburetor
and "spark plug" for radiator, and so on?
to make renewable energy grow from a cargo cult, we really need
to think more clearly, and speak the same language that hundreds of
thousands of engineers and people of science speak all over the world...
>battery size carbon alkaline
>
>aaa 20ma@21hrs 37.5@25hrs
>aa 54ma@20hrs 20ma@107hrs
nick

