You could use such a scheme to provide a LED blinking when the battery voltage is low:



The LED blinking could be controlled like that:
Every 5 seconds, when the micro wakes up, the micro checks the "battery low voltage detector". If the battery voltage is low, he emits a "high" to the gate of BSS138, which results in a rapid discharge of the two 22µ caps, making the red LED brightly flash for a few milliseconds. After this brief period the micro emits "low" to the gate again and goes to sleep again.

In the sleep period of 5 seconds the two 22µF caps are charged up to the battery voltage again, via the 22k resistor. Charging time constant is about 1sec.

If the micro detects a high battery voltage, on the other hand, he goes on emitting a "low" to the gate of BSS138, preventing the LED from flashing. The two 22µF caps stay charged up all the time and because X5R ceramics is used for these caps, leakage current through them is negligible.

The advantage of this scheme is, that the LED flash current isn't delivered by the battery itself, but by the two 22µF reservoir caps. By this, the rather high LED flash current (~20mA) doesn't stress the battery directly. Only about 140µA peak current is drawn from the battery. The average current is even much less, only about 30µA, and is only drawn if the LED blinking is active. More, as the reservoir caps are charged during the sleep period, when no additional supply current is drawn, the reservoir caps can be charged up to the higher voltage of an unstressed battery.

Alternatively to the BSS138 a 1/4 of 74LVC4066 could be used.

To allow the blinking down to very low battery voltages, a LED with very low turn-on voltage should be selected. Such a LED can be found under the red shining LEDs. Some decades ago red LEDs showing a turn-on voltage of down to 1,6V could easily be found. Today, many red LEDs show more than 1,8V, though.

Kai Klaas