>>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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>
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