From: Phil Seakins
Subject: Re: 9V bat ->2AA + switcher ?
Organization: Phil Seakins
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Date: Thu, 05 Dec 2002 03:30:42 +1000
NNTP-Posting-Date: Thu, 05 Dec 2002 04:30:50 EST
On 4 Dec 2002 08:27:01 -0800, email@example.com (Steve Sousa) wrote:
>I've been trying to figure if it would be worth to change a power
>supply from a 9V bat to a couple of AAA bats + a switching regulator,
>but i'm getting confused, I don't know if i could actually get a
>longer running time.
>The data i have is:
>Type Vol. Cap(mAh)
>AA 1.5 2850
>AAA 1.5 1250
>P3 9.0 625
>The load takes about 3mA. The discharge curve to .8 Volts per cell
>gives the following running times:
>AA 1000 h
>AAA 500 h
>P3 370 h
>BUT the current cut-out for the 9V bat is at 7.2 which gives a running
>time of about 170 h. The current configuration is not regulated, but
>when regulated, it should be at about 7.5 volts.
>Lets say the switcher has an efficiency of 80%. How would using a
>single cell or 2 or three in series influence the running time?
>My first guess is that if i use a single bat at 80% i would get (for
>AAA) 1000mAh, but if i put two in series, with the same switching
>efficiency i still get that figure because i can only extract the same
>1000mAh but at a higher voltage. Another aproach would be to start
>with 1250mAh at double the voltage i get half the capacity?? I don't
>think that is correct, because what matters is the total amount of
>energy used??? It's certainly very simple, but i'm confused, i don't
>know how to calculate this, the switcher confuses me.
When you put two batteries in series, for a given current you are
doubling the power.
You are comparing oranges with apples. You shouldn't be comparing
current, you should be comparing power. Eg., say you're putting 3V at
10ma into a switcher with 80% efficiency. That's 30mW in and 24mW out.
If the output voltage were 9v the current would be only 2.6mA. (Hope
my calcs are correct). A single cell input (1.5V) with the same
efficiency and output load would have a current drain of 60mW.
The relationship between current and voltage in these circumstances is
inverse. In fact, it's the same relationship as AC via step-up or