I want to put a volt meter in a light using 2S Li-ion. I remember a long time back, someone on CPF had come up with a tiny one and they used it in a Maglite body. I would love to find the smallest one possible, even if it's just something that would only light if the voltage dropped. I have seen some with led readouts, but they are just too big at 30mm or so.
Anyone have any suggestions on anything very small? (less than 20mm)
I’d say you want a 2s lipo alarm. Unfortunately most of the modern ones are built for connecting to the balance connector of a battery pack, you won’t have that (a tap between two two cells). It looks like this one is widely available though: Doohan 2S Audible Voltage Lipo Alarm
Just remove any buzzers / LEDs you don’t want.
The first place I looked was at Hobbyking but it looks like they only carry the balance tap ones these days.
For the smallest one possible you could have someone make you one using a dedicated voltage monitor chip. They can light up an indicator led when the voltage drop to the level the chip is preset for. 3.0v Voltage Monitor
Thats whats so nice about them. Tiny, easy & uses very little current (0.5 uA typ). Also don’t need to be able to program chips. Just connect the voltage monitor chip, indicator led & resistor (aim for <50mA led current).
Haven’t ordered one yet since the attiny can do low voltage detection.
I like the readout style over the lights. It's larger, but if I can fit it in, I will use the readout style. I don't really need a low voltage warning, I just think a readout of voltage would be kind of different.