Simple low-battery alarm







Having been inconvenienced by a flat car battery at home more than once, I would love a simple low battery alarm that could be direct-connected to the battery terminals, that would let out an ear-piercing sound if the battery was discharged below a
threshold voltage.
Would you consider publishing a suitable circuit/kit that would activate a suitable relay? Now I don't know what voltage would be a safe threshold voltage to activate the alarm-perhaps between 10.5V and 11.5V?
But I do know that having a time delay of approximately one minute would be necessary to prevent nuisance alarms because of starting the engine. (C. D., Auckland, NZ) • We published a couple of battery saver projects, one in July 2004 ("Versatile Micropower Battery Protector", by Peter Smith) and one in September 2013 ("LifeSaver for Lithium or SLA batteries", by Nicholas Vinen) but unfortunately, both work by disconnecting the load when the battery voltage gets low, which is not what you're asking for.
Also, neither comes close to handling the current that can be drawn from a car battery during starting.
Having said that, you could build the Battery LifeSaver and leave off Mosfet Ql, then connect a 5V piezo buzzer between the 5V output of REGl and the output (pin 6) of ICl. The piezo would then sound when the battery voltage drops below the set threshold.
A time delay can be accomplished by drastically increasing the value of the l0nF capacitor at pin 3 of ICl; you can probably fit an SMD ceramic capacitor of up to 100µF here, which would give a time delay of a minute or two.
For a 12V lead acid battery, a good alarm point would be 11.5V but a car battery can be expected to drop below that value while cranking the engine. Keep in mind that when the piezo sounds, to warn you that the battery voltage is low, its current draw will only speed up the battery discharging! 

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