> 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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads