battery monitor

Sounds like it does exactly what it's meant to

NT

Nigh on impossible. Depends so much on the current draw and the past history of use that they might even recover from being "dead" if left overnight in a warm room or conversely leak and destroy your kit.

Duracell have managed to do that to me once too often to ever get bought again. Panasonic and EverReady are now my batteries of choice.

I keep batteries that are no good in high current applications and run them to flat in my mouse or keyboard. Mostly I use rechargables but there are some devices where they don't produce enough terminal voltage at low currents to make LCD displays on phones and clocks visible!

The circuit may have been designed to also work reasonably well with NiCd/NiMH rechargeable batteries. These batteries tend to "fall off a cliff" when discharged - their voltage remains almost the same until they're nearly discharged, and then drops rapidly. And, these batteries can be damaged if you discharge them deeply, so it pays to have a fairly conservative "low battery" warning.

Alkaline batteries have a more gradual discharge curve, so you could build one with a lower threshold, and it'd probably be reliable when used with alkaline batteries. But, if you did this, it wouldn't warn you of the upcoming going-flat of rechargeable batteries.

It's also quite possible that the "lasts for another half hour" behavior is exactly what the manufacturer intended: warn the customer that the batteries don't have enough remaining charge to cover a reasonable amount of additional use (e.g. a half-hour run or walk). "Dude, you really want to change these batteries before you leave home, because if you don't, they won't last until you return."

My son had a digital camera that you could set for alkaline or NiCd. I think it used that for the low battery indicator. The alkaline ones seemed to last a bit longer than a NiCd charge. Possibly because of a bit more voltage range on the alkaline.

Where are you gonna put these 6 parts? How are you going to protect them against physical damage? What's the current drain when not in use?

How tight are your shorts that you don't have room to carry a spare battery? "Is that a battery in your pocket, or are you just happy to see me?"

Often, the schematic is the least complex part of the solution.