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Is there such a thing as a switch that will close the circuit if the open voltage drops below 12 volts dc.?

I am using a LiPo battery to power a bike light and the battery can be permanently damaged if the voltage drops below 12 volts.  Is there a switch I can wire in parallel which will cut power if the voltage drops too far.  

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framistan6 years ago
If you use a COMPARATOR circuit such as the LM339 IC.... you can set whatever trip point you want. The output of a comparator does not put out a VOLTAGE. The output of a comparator LM339 is either GROUNDED or OPEN depending upon the conditions at the inputs A and B. A small resistor of value 10k ohms can be connected to the output pin of the comparator, and up to the voltage rail. This will "pull-up" the output to Vcc whenever the comparator is NOT grounded at the output pin.
MOSFET.
Agreed. fire the relay using a P channel mosfet to eliminate an always on condition

If you need to avoid chatter at the threshold, consider adding a threshold detector circuit with hysteresis.
kelseymh6 years ago
There are minimum-voltage relays which you can use for this. Wire the coil as a "short" across your battery, and then daisy-chain the NO contacts to your supply and load. If the voltage drops below the relay's minimum, the contacts will open.

There's a more complicated setup you could use with a pair of batteries to hand the circuit off to a backup, until the primary gets charged back up.

However, note that the relay coil itself will be drawing some (small) current, and hence draining the battery all the time. I think the comparator-transistor idea (from iceng) is probably a better choice if you really want zero draw once the voltage goes below threshold.
iceng kelseymh6 years ago
Thank you, but your relay will be able to remove its own load too,
if there are enough contacts on the relay and you use a momentary
button switch to engage.

I still worry about the Open Voltage remark from someone who does not
realize it needs to be in Series. It could mean that the voltage falls below
12VDC under load ?

A
iceng6 years ago
Use a comparator IC driving a MOSFET transistor that takes and disconnects
the battery when below 12VDC and its own loading.
But you say OPEN Voltage !!
Does this mean when the light is used it is OK to be below 12V ???

A