AGM Batteries are used widely in Caravans, Motorhomes and camper vans for providing power to 12 V appliances. They can last many years as long as they are not discharged below 50% too often. Monitoring the battery voltage is a good way of keeping an eye on the State of Charge (SOC) of the battery. One should stop discharging the battery below 12 V - at which voltage the typical Lead Acid Battery is at 50% capacity.
 
This Battery Monitor Buzzer starts beeping as the battery voltage drops down to less than 50% and then with increasing persistence as the battery voltage drops down even further - it is hard to ignore. 
 
The cost of this unit is easily recovered in extending the life of your 12 V Lead Acid Batteries from 1 or 2 years to 6 or 7 years.
 
It can also be connected to a bank of parallel 12 V batteries as used in renewable energy systems.
 
This unit is Made in Yorkshire and uses high-precision components to measure the voltage accurately - unlike many cheap imported devices that are mass-produced. It is supplied with user instructions and technical support is just an e-mail away.
 
This Battery Monitor Buzzer is supplied without an enclosure. The user has to just connect the wires to the 12V battery +ve and -ve terminals. Product Size: Length = 70 mm  Width = 45 mm 
 
Please note: This device is not for use with Lithium Batteries as they have a different relationship between voltage and capacity.  It should not be connected across a 24 V battery bank.