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I'm gonna make something that uses a DS1337, and runs on 5v worth of batteries.  Needs a backup battery to prevent time-reset when main ones run out.  Seems simple enough.  Anything not obvious about this that I should be aware of?
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I'd probably use a couple of diodes and a 3V lithium (or maybe a supercap) to keep it ticking when main power is out.

-j
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thx for the supercap idea.  

I read that with supercaps you don't have to have a full-charge detection circuit, like w/ batteries.  I might use a supercap, but if I use batteries, I'd like to find the utterly simplest, yet efficient, way to know when they've gone too low.  The simplest one I can find is here:  
http://www.4qdtec.com/batty.html   Does this seem like a good one?
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Are we talking about a low battery indicator for the RTC backup battery? I haven't read any datasheets or done any math, but my knee-jerk reaction is that the indicator circuit will use lots more power than the RTC.

-j
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my bad.... i was assuming that what i read about the backup-battery-related low-charge detection ckt was referring to one that watched the main batteries and enabled the bu batteries when necessary.  But, maybe it was for the bu batteries.  

I only care about watching the main batteries and determining the minimum circuitry it takes to do that.
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