Battery Help

[itm]

Hello everyone
i need some help on a project which i am working on.
i designed a circuit were the load current is 130mA running off a bench power supply.
now i wand to convert this circuit so that it works off a battery with at least a 5 year life.
Is this posisble ?
This first thing that comes to mind is a step-up switch mode the MAX1771 which uses 100uA supply current and can deliver 12v @500mA from a 4.5v input.

http://www.maxim-ic.com/MAX1771

my main question is which current would i have to use to calculate the battery life?
Is it the load current of 130mA or the 100uA

Thanks in Advance
Isaac

 

[sdmays]

To answer your main question, the load current, from which you would calculate the battery life, is the 130mA. However, if you use a 4.5VDC source (as you suggested above), you would have to recalculate the load current.


Because energy can neither be created nor destroyed (except by God), the energy your battery provides has to equal the energy your circuit consumes. Of course, you also have to take into account the energy the conversion process will consume.

Punchline: PIn = POut + PConversionLoss. POut = (12 * 0.130)W = 1.56W. The Maxim text claims 90% efficiency over the range of IOutput (which is often dependent on the load and input voltage).
PConversionLoss ~= (100% - 90%) * POut = 156mW.
PIn = POut + PConversionLoss = 1.716W = VIn * IIn.
IIn = PIn / VIn = 1.716 / 4.5 = 381mA (nearly three times the current drawn by your circuit).


To calculate the necessary battery life, you need to use 381mA instead of 130mA or 110uA. By the way, I am guessing the 110uA the Maxim text claims is how much current the Max 1771 controller draws - not how much current is needed for the input of the conversion process.


Can you drive your circuit from a 12VDC battery? If so, you can eliminate conversion loss, therefore requiring less battery.

I hope this helps, and if I missed your point, please forgive me.

 

[melone]

Does the whole circuit need to be powered for 5 years, or does data need to be retained for 5 years? Does it have to be completely battery operated or can an external supply be used to recharge batteries? What is the final application (try to list as many factors as possible, i.e. temp, humidity, vibration, interaction with other systems, chemicals, etc.).