Ok, I got my hands on a pretty good bench
I tried it on another brand of mobile powering with a AC/DC converter at 9V 2A+7805
The quiescent is about 5mA and the current measured at the adapter and the 7805's input leg is approx 350mA, the LED is OK
With the same settings, I changed power source to 9V battery and the measured current was 100mA (approx, the battery was near dead so I don't expect much from it) and the LED is OK only when the current was measured >100mA.
As the current falls, the LED starts blinking and dims
Then I hooked up my old mp3 (again, the internal battery is probably worn down but it is fine enough for testing).
With the AC/DC adapter, the current starts at around 140mA and dropping, pretty fast, to around 50-60mA. The LED, again, starts blinking at around <100mA
Then I changed power source to 9V. It's pretty much dead, so.... Current starts at around a bit more than 100mA and slowly dropping to 50mA. But the LED starts blinking only after significant current drop, something around 70-80mA but it is brighter than when the power source was AC/DC converter at around the same current
The mentioned AC/DC adapter/converter is a big box that you can adjust voltage and max current, I don't know what it is called...
What I don't understand is:
Why does it blink? I thought I hooked up the comparator as a SPST switch to ground...?? The LED should be just ON or OFF. I understand that when the current is lower than 100mA, the voltage at negative of the comparator starts to get lower than that of the positive. I also understand that there is a range when the power isn't completely out but dropping to 0, and said range is short enough (according to the datasheet, it is only 5mV). But it seems to blink even when the current is very low....
The second test case, is it because the battery is worn down? That the current output from the converter gets decreased over time. I set the charge mode to 500mA, so why doesn't it draw 500mA but instead only 140mA and then dropping?
Sorry if I sound confusing, I'll try explain better if you don't get what I meant
One more,
I forgot to put in a diode to prevent backward current. When will backward occurs? (The battery can't supply enough voltage/current so the current flows from the device instead?)
What happens during backward? Can you show a simple diagram of how things go? Like the battery changes into a cap instead and the device turns into power source or so (Just my imagination)
How can I know if the power is actually going backward? Not by the device's battery indicator please, I don't trust it
If the current actually goes backward, will the LED lid?