Watt the F#ck?

"fred_kneeman" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Heya group,

Cobbled together a home-made light that I *think* put out 5 watts for 1
hour - for $ 30!!! Want to share the recipe... but I'm all muzzy about
the maths.

Ok: the "mystery globe" runs off a 6 volt, 4.5 AmpH (SLA) battery, and
has a useful burn time of about an hour. If it's a 6v globe (I think
that, for reasons I don't understand, means it's probably actually 4.8v
or 5.2v or something) that means it's a 4 or 5 so watts, yeah?

OK......for ease of calculations, lets assume that the wattage of your globe
is 4 Watts (I am assuming that you used the garden stake one for the globe
and the torch one to source the battery??)

As Power = Current x Voltage (P = I x V), then this means that for a 6V
battery, I = P/V = 2/3 Amp.

Now in an ideal world with no internal resistances or conductor resistance,
your 4.5Ah battery would be able to run the 4W globe for 6.75 hours when
fully charged. A reasonable rule of thumb is say about 75% of the rated
figure so lets err on the conservative side and say that you should be able
to get about 5 hours from your battery.

If you are only getting 1 hour from your battery then I would say that
either the battery wasn't fully charged, the battery is nearly stuffed, or
the globe is much more than 4W.

Quoted message said:

Please help. Any donations/equations gladly accepted.

[censored]
p

ps: light made from
- http://www.radioparts.com.au/images/12654700.jpg
-

http://www.arlec.com.au/SearchResult.asp?Brand=8?&ServCat=8&SubCat=27&Product=242

Quoted message said:

pps: Power (=watts) = Amps x Volts. AmpH means Amps x hours, not
Amps/hours. Resistance = Amps / Volts, or is it the other way round?

It is the other way around (R = V/I)

Ride On,

Gags