Ah, but power-capacity wise, one gets:
550mAh * 7.25V ("nominal" mid-discharge) = ~4W
2500mAh * 1.25V ("nominal" mid-discharge) = ~3.2W

I need 4.7-5V to run things.  (A part that is not a PIC requires 5V)

The the DC/DC cost & efficiency may be different for 6->9.25V -> 5V vs.
1->3V -> 5V.  And with diodes (see posts in other thread) I can use one
9V or two 9V, user's choice.

But I still may go with 2x (or 3x AAA), a decision based on cost, size,
and my client's whim.  :)

J


Michael Rigby-Jones wrote:
>>-----Original Message-----
>>From: Jesse Lackey [mailto:jesse@CELESTIALAUDIO.COM]
>>Sent: 16 February 2004 05:42
>>To: PICLIST@MITVMA.MIT.EDU
>>Subject: [EE:] Using two 9V in parallel for greater capacity... ok?
>>
>>
>>Hello all,
>>
>>I have a project that involves blinking LEDs that the client
>>would like to have run for 2 months per battery or battery set.
>>
>>2 9V wired in parallel would do it and is preferable to 4 AA.
>>
>>Is this a reasonable thing to do?  Is it dangerous at all if
>>both are fresh of the same kind?  How about if not?  Would a
>>5ohm resistor in series with each before being wired in
>>parallel alleviate any dangers from users who don't follow
>>directions?  (its a consumer product).  The current is pretty
>>low so the loss due to a small resistor is not a problem.
>>
>
>
> Out of interest why would 2x PP3 style batteries be preffered to the AA's?
> An alkaline PP3 has a capacity of around 550mAh, an akaline AA cell has a
> capacity of 2500mAh.
>
> If you need 9v for whatever reason a simple, low cost voltage doubler (e.g.
> MAX1044) could be used to provide 9v from just three AA cells and still
> provide more capacity than 2x9v batteries.
>
> Regards
>
> Mike
>
>
>
>
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