Low Voltage Relay – Monitor – Detector


low battery relayThese are low voltage detectors for 12V and 24V battery monitoring. When the applied nominal voltage drops below the trip set-point, the relay changes state. This can be used to switch off equipment or trigger an alarm etc. when the battery voltages drops too low.

Product Purchase 12V module. or 24V module.


Note: Refer to the module label to confirm the connection order.

Power Connection

The 5 screw terminals are from left to right, +ve, -ve (–), and 3 relay connections; typically C, NC, NO.

Apply the 12V or 24V power to the +ve and -ve (–) terminals and the module is operating.

The module is protected against reverse polarity connection of the supply.

Relay Output

The relay terminals provide a standard clean contact output, with common (C), normally-closed (NC) and normally-open (NO).

From common to normally-open or normally-closed the relay can switch any load up to 50 volts AC or DC and up to 2 Amps.

The normally-closed circuit is typically used for alarm monitoring or circuits that are active while the monitored voltage is good, or above the low trip setting. This can be used to power equipment while the monitored battery voltage is good, and shut the equipment off when the battery voltage drops too low.

The normally-open circuit is switched to common when the monitored voltage drops below the low trip setting. This can activate an alarm when a monitored battery voltage drops too low.

Both equipment power and an alarm can be connected at the same time when using the same power supply.

Setup and Use

The DIP-Switches set the trip voltage. The LED indicates normal operation and a low voltage tripped state.

Mounting:  Typically using industrial grade double sided sticky foam: see notes below.

Protection Fuse :  A fuse of maximum 5 Amp for the relay contact load will protect the circuit board.

Default DIP-Switch setting = 3, 5 and 8 on. This provides a low voltage trip at approximately 11.2V or 22.6V and a restore from trip at 12.5V or 25.2V. This should suit most SLA 12V battery systems.

The trip voltage you set depends on the application – is it an indication, activating an alarm or shutting off the load.  If shutting off the load, is it more important to save the batteries or keep the load running for as long as possible.  Most batteries don’t like being discharged very low.  The battery manufacturers data will probably suggest a minimum discharge cut-off.

Example:  AGM batteries may be at 45% capacity remaining at 12.0V (switches 8,5,4,3,1) and 30% capacity remaining at 11.8V (switches 8,5,4,2).

LED indicator

There is one LED indicator on the board.

  • Normal = brief pulses at 1 second rate
  • Tripped = fast flashing (about 2Hz)

DIP-Switch Setting

The switches 1 to 6 set the trip point voltage. 1 ON is the lowest voltage through to all 1-6 ON for the highest voltage.  The chart below provides an approximate indication. This is good for general setup, but see the setup procedure below if you require a more precise trip point.

Switch Voltage Voltage Note
1 9.8 19.9 trip
2 9.9 20 trip
3 10 20.3 trip
4 10.25 20.9 trip
5 10.9 22 trip
6 12 24.4 trip
7 invert relay invert relay .
8 12.6 25.2 restore
4+2 10.45 21.2 trip
4+2+1 10.5 21.4 trip

Note that the voltage steps are not even: You can have more than one switch turned on to increase the trip point voltage. ie. Switch-5 only gives you 10.9V or 22V, by adding Switch-3 you get 11.2V or 22.6V.

The more switches ON, the higher the trip voltage, with switch 1 providing the smallest increase and switch 6 the largest increase. switch 1 on/off has a small effect, switch 2 on/off has a slightly larger effect.  Add switches, fine tuning with switch 1.


This switch inverts the relay state.  This can be useful depending on your application to reduce power draw in the standby or tripped state.

Switch-7 OFF and the relay is normally off and switches on when the low voltage trips.  This reduces the current draw when everything is normal as may be desired for solar or low power systems.  This also matches the terminal functions for the relay; common, normally-open and normally-closed.  Switch-7 ON, means the relay is on when everything is normal and switches off when tripped.  This increases standby current draw, but is fail-safe and reduces the current draw when tripped – when less power is available.


Switch-8 affects the restore/reset voltage.  Switch-8 OFF, the restore voltage is about 1V (or 2V 24V version) higher than the trip voltage, providing some hysteresis and preventing the relay from chattering. With Switch-8 ON, the restore voltage is fixed at approximately 12.4V or 25V which can better suit SLA battery systems.

With Switch-8 OFF, the restore voltage runs out of head-room above a trip voltage of about 13.5V and the relay may cycle on/off at a 1 second rate.  A 12V battery system should not have a low voltage trip set this high, so it should not be a problem.


Most SLA 12V batteries are considered dead-flat at 10.8V.

Set-up Procedure and Reset Trick

You may find applying a variable voltage to fine tune the setting then adjusting the voltage to reset the relay is a bit slow.

Turn switch-8 and switches 1 to 6 off.  Apply the voltage at which you want the relay to trip on low voltage – wait 30 seconds or so for the reading to stabilize. Turn the switches on starting at 1 and work up in binary until the relay trips.  This is the setting. 

To reset the relay and check the setting again.  Switch the highest of 1-6 off and the lowest off,  then the highest back on.  This will reset the relay.  The lowest switch should then trip the relay again.

Counting up in binary. Switch 1 is the smallest increase, switch-6 is the largest increase:

1,  2,  1+2,  3,  3+1,  3+2,  3+2+1,  4,  4+1,  4+2,  4+2+1,  4+3,  4+3+1,  4+3+2,  4+3+2+1  and so on.

It sounds tricky, but is quite easy and reasonably quick, when you have done it a couple of times.


  • microcontroller based
  • supply voltage: 8 to 15V DC
  • standby current draw: typically <4mA
  • activated current draw: typically <25mA
  • response time: approx 2 seconds (depends on voltage swing)
  • reverse polarity protected
  • output relay: isolated changeover contacts
  • output relay: 50V AC/DC, 2A
  • Relay energized when normal (not tripped)

Note: earlier versions had a slightly higher standby current draw.

Relay current rating and Safety

We highly recommend fusing the load switched by the relay contacts 5A maximum.  This will protect the module in the event of a short circuit or fault.

Carrying 3 Amps, the volt drop terminal-terminal (C to NC) is about 140mV, resulting in a total power dissipation of about 0.43 watts of heat.  This is split between the relay and the pcb trace resistance.  The relay accounts for about half (70mV) of the volt drop and therefore represents a resistance of about 25mR.

The relay is rated to switch 2A.  We have tested it to carry 5A for short periods, but this exceeds the switching rating.  Switching currents of greater than the relay rated current will likely cause contact failure. Larger highly inductive or capacitive loads may not exceed the current rating, but can cause contact damage and reduced life.


Typical mounting would be by using double sided foam sticky tape. The foam must be thick enough so that the back of the board cannot contact metal surfaces when pressure is applied as happens when screwing in wires.


Low Battery Monitor for 12V or 24V (2 versions) available.

This is a voltage monitor relay typically used to signal low battery conditions for alarm and monitoring of batteries for solar and wind systems and for security and safety. The DIP switches allow the low voltage trip point to be adjusted. The relay can be invereted for lower power draw or fail-safe operation.

Microcontroller based 8-15VDC or 16-30VDC supply typically drawing 20-40mA, reverse polarity protected changeover contacts 50V AC/DC 3A rated contacts and LED status indicator.

see http://rcbeacon.com/blog/?page_id=2276  or more details.

We usually have a few of these in stock for immediate delivery.