Wow thanks so much everyone for the comments and suggestions.

I need to run @ 5V (well ... 4.75V minimum), and a DC/DC to go from 9V
to 5V is simpler and cheaper than doing something from 4x AAA (no space
for 4x AA) which will be > 5V initially.  2x AA is doable space-wise,
but I will have to do some calculations / cost analysis to see if this
is better than 2x 9V.  The attraction of the 2x 9V is that my client can
decide yes/no on the extra space later, or can tell users they can run
with 1 or 2 9V, their choice.  At the least I want to present two
options and he can decide.

The power consumption is very peak-y, pulses of up to 50mA for <2msec to
run an IR LED, and 10msec PWM of 20mA for a red LED, and some other
stuff that is <5msec and <2mA.

Its a real balancing act all this, power vs. cost vs. physical size vs.
complexity.

Thanks to everyone for for the diode tip.  Duh of course...!  and the
small voltage drop is no problem for the DC/DC.

Best!
Jesse



Russell McMahon wrote:
>>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.
>
>
> Not "by the book".
> Entirely doable. I'd say it was probably always safe *IF* both batteries
> were always new.
>
> HOWEVER Changing one battery and not the other - eg replacing one battery at
> a time when flat while the other was still in place - WOULD lead to
> discharge between batteries. This could produce significant energy losses in
> the good battery and heating in both. Probably up to an amp or so worst case
> and several watts dissipation.
>
> A sure fire safe way (ie no fires :-) ) , at the loss of some capacity,
> would be to place a diode in series with each battery. This would take about
> 0.6v off the available voltage. For an eg 6 volts endpoint for your
> equipment, instead of 9 to 6 volts you'd get 8.4 to 6 volts at the terminals
> or a useable battery voltage of 9 to 5.6v. I'd guess maybe 10 capacity loss
> depending on acceptable endpoint voltage and current drain.
>
> Using small Schottky diodes would reduce the energy losses. By how much
> depends on current characteristics.
>
> Knowing the usage pattern would help greatly in predicting what minimum
> series R would make things safe.
> eg is this run 24/7 for 2 months or intermittently. What is the average
> drain and what is the peak drain (max instantaneous when LEDs on).
> Capacitors may make the mean battery current approximate the peak etc.
>
> 5 ohms in series with both would give a MAXIMUM current of 9/5 = 2A and far
> far less inter-battery in practice.
>
> Adding a resistor in series with ONE battery only would reduce losses and
> give similar protection (think about it)
>
> I assume you are specifying Alkaline batteries to achieve this life time. If
> not, you should, as you could then use one battery.
>
> If 2 batteries in parallel suit your need the acceptable minimum terminal
> voltage must be about 6 volts. If an inverter is not used it could be
> considered - quite possibly cost precludes.
>
> AA cells have over FOUR  times the energy capacity of a 9v cell or over
> twice the capacity of 2 x 9v cell (PP3 type). Maybe the cost of 2 x battery
> snaps versus an AA cell holder etc makes a difference. IF you can get by on
> 4v endpoint then 4 x AA absolutely walk all over 2 x PP3. At 6v endpoint = 6
> AA cells The AA holder becomes larger and annoying but is even more superior
> to 2 x 9V. If 2 x 9V giver 2 months operation then 6 x AA give 4 months plus
> !!!
>
> Even the dreaded 4 or 6 AAA have equivalent capacity to 2 x 9v PP3 !!!!
> ( 4 x AAA have less volume than 2 x PP3 and 6 x AAA are perhaps slightly
> larger)
>
> 4 x AA have slightly more volume than 2 x PP3. If you can live with the 4v
> end-point (3.6v to absolute last gasp) of 4 x AA I'd really really try to
> use them. Battery snapped holders are available. The 4 month+ life is
> presumably attractive.
>
>
>
>         Russell McMahon
>
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