battery life questions

[caress]

not for a stompbox, but for a small LED project for my girlfriend.

i just need to make sure my math is correct and i'm a little confused as to the W to mAh conversions, etc.

i'm using 3V/225mAh coin cell batteries in a group of 3 for 9V, then paralleled to another group of 3 for a total of 9V/450mAh.
this will be powering an LED array which uses around 1.9W at 12V.  i'll be running it at 9v as well as at a pretty low brightness, so that should affect the wattage as well.  it'll be so dim you can barely tell it's on...

how long would this setup last me if running all the time?

[Cardboard Tube Samurai]

How long is a piece of string?

[caress]

umm depends on the piece of string?

[DiscoVlad]

Ohm's Law:
(V and I as before, R is Resistance in Ohms)

V=IR, I=V/R, R=V/I

Power Equations:
(P=Power in Watts, V = Voltage in Volts, I = Current in Amperes)

P=VI, V=P/I, I=P/V

These can be combined by substituting for V and I to express power in terms of resistance.
eg. P=I²R, P = V²/R.

so... you've got 1.9W @ 12V the current draw is thus:

I = 1.9W / 12V = Amps.

Divide this into the total capacity (mAh) of your battery pack, and there's your answer.

[caress]

all i had to do was fix the blown fuse in my multimeter and i just tested the current draw... 

the resistor limiting the voltage dropped the draw from 10 mA to .2mA!  so if it's running for 24 hours a day and my pack is 225mAh (without being paralleled) i should get:

.2mA * 24 hours = 4.8mA/day, so let's round up to 5mA
225mAh / 5mA = 45 days

right?

[DiscoVlad]

Not quite, but you're on the right track.

For a load of 0.2 mA, in one hour it uses 0.2 mAh of charge, so with a 225mAh battery it's 225/0.2 = 1125 hours = approx. 47 days.

This is based on the assumption that the cells have a linear discharge rate (they don't). To maximize  battery life, you could add a joule thief type circuit.

[brett]

Hi
my 2c says..
NiMH and LiPo batteries can be recharged and will last for years.
Can you chuck those silver oxide batteries? They are expensive and bad for the world.
cheers

[caress]

Hi
my 2c says..
NiMH and LiPo batteries can be recharged and will last for years.
Can you chuck those silver oxide batteries? They are expensive and bad for the world.
cheers

space was a consideration here and these fit the bill.  i could have spent more time looking, but they do the job...
i don't use any batteries in any of the pedals i build - for myself or for others - i gave them up about a year ago.  there are a lot of other things i do to conserve as well, if it makes you feel better...