Battery Life Calculation

If you are going to connect the two battries in sereis , find the AH rating of one battery and devide it over 0.015 amp and that is the estimated battery life in hours.

Thanks. Any thoughts on alkaline vs lithium vs whatever button battery. Which type gives the longes amp-hour ratings in the smallest size? Any link suggestions for this information?

Get on the web site for the battery manufacturers is the only way, they have graphs for different applications (e.g motorised toy (high current), radio (low current) etc) and different usage cycles.... continuous 30 minutes per day etc.

Unfortunately it's not quite as simple as suggested...
They quote Ah for the most favourable case and down to a very flat battery.
At 15mA you may be close to that case, so it will be a reasonable (but probably optomistic) first estimate.

e.g An alkaline battery is 1.5v ...right? No... wrong! To get any decent life out of it you will need to run it down below 1v, maybe down to as low as 0.8, so you need to check any circuitry will function at the lower voltage.

If it's just a led, resistor and switch you are fine...but if there is any electronics you need to check that when it is off..it really is drawing zero (or very very low) current.

I've seen commercial products claim 3 years battery life because they forgot this (quiescent) current... They had egg on their faces when all their units faild after 6months (tee hee... the unit I designed lasts a year, as spec'd)

Del

Have a look at the you-beaut circuit posted here: http://cr4.globalspec.com/thread/23137/Contribute-a-simplest-led-circuit

Your batteries will last a lot longer using something like this, rather than connecting them directly.

Won't- I have experimented extensively- all driver circuits consume power to work- the led power is approx 1/2- the best I have found is to use a lithium ion cell from a cell phone(3.7v)- use a dmm on current in series to get 20-30mA thru your 1/or more leds, adjusting a 100 ohm trimpot- insert a fixed resistor in line for this value- will last far longer than any driver circuit!.

Indeed, I have determined that a simple series resistance in my LED flashlight consumes only about 1/10 a watt. It is hard to get a control circuit to consume so little power.

If you just need to indicate that power is available, why not flash the LED say for 1/10th of a second each second, that way current usage is approximately 1/10 of the 15 ma you quoted.

There are LEDs which flash down to a specific voltage without any external components available for many years.

Some specialized circuits are around that reduce the current used to even less than I have suggested....but as you have given so little information as to what you are doing with this LED, it is hard to be more specific.....

Remember also that the bigger the battery, (generally anyway) the more power you have available and tiny LR44 flat cells will only have a small fraction of what is in an AA or even an AAA cell......

Thanks. The circuit is a tilt sensor to detect any tilted or incorrectly placed equipment during shipping. We added the blinking LED because it was easier to detect and of course less energy usage. It shows up better blinking in a dark warehouse than a solid light. This doesn't eliminate the root cause of the spillage which is "ID-10-T" error. Then again that might involve more cost than looking for a blinking light and / or redesigning the tilt sensative equipment.