DIY battery alarm
Willem Bijl designs and builds a battery monitor
How to design and build a low voltage battery alarm
With regard to my misadventure detailed on the previous pages, in retrospect it should have been possible to prevent that outcome by simply having an alarm that would warn me of a dying battery before it was dead and too late to start the engine.
The minimum level of acceptable discharge is 11.5V. That would be the limit to start the engine and get the battery recharged.
The easiest solution would be to get a second battery dedicated to engine starting, but sadly there’s no space for that in my yacht Flotaxe.
So I set out to design a low-voltage alarm connected to the onboard 12V system that would warn me the very moment that the voltage dropped below 11.6V, and then cut out once the voltage has increased again.
It may look daunting to the uninitiated, but that shouldn’t stop anyone from copying it and building it as a kit.
It’s actually very simple with just a few components – and it works!
Basic version
A stable reference voltage is created with a Zener diode (D1) of a nominal value of 5.6V with a resistor (R1) of 4700Ω [4.7k Ohms] in series.
The resulting current is only 1.2mA which causes the Zener voltage to be slightly lower, around 5.5V.
This stabilised voltage goes to the inverting input (pin 2) of the op-amp 741 (IC1 on the diagram).
Simultaneously the voltage divider R2 and R3 creates a ‘comparison’ voltage, which is measured at the non-inverting input (pin 3) of IC1.
As long as the supply voltage is still 11.6V (or higher), the voltage between R2 and R3 is above 5.52V so the alarm is ‘off’.
When the voltage of R2/R3 drops to 5.48V this is lower than the reference voltage of 5.5V so IC1 flips over and the alarm goes ‘on’ (via pin 6). Note: pins 7 & 4 supply the operating power to IC1.
Flawed design
When my prototype was finished it had the disadvantage of ‘jittering’ or ‘chirping’ a weak sound of the buzzer when the critical voltage was approached due to its sensitivity – it responds to variations of 0.01V.
Enhanced version
I altered the design so that once a triggering low voltage is reached, however brief, the alarm must stay ‘on’ until the voltage has increased by at least 0.8V, an increase that could be caused by the engine being switched on (recharging) or by me switching something off (reducing current demand).
I introduced hysteresis (laziness) by inserting a capacitor (C1) and two Schottky diodes (D2 & D3).
These diodes (D2 & D3) have a voltage drop of about 0.4V when a current passes. This current charges capacitor C1. The 2 diodes are parallel and in opposition.
When the main boat electricity is switched on, supply voltage goes through R2 and D2 and charges C1 positive.
Example:
Assume supply voltage (Vs) is 13.4V (full battery), the node R1/R2 voltage is 6.4V: [(10k/(10k+11k)) x 13.4V].
Due to the forward voltage drop of D2 (0.4V), the capacitor C1 charges only up to 6.0V.
When the supply voltage drops, C1 remains at first at 6V, as the charge cannot drain away.
Only when the node R1/R2 drops below 5.6V, then C1 will overcome the forward voltage of D3 (0.4V) and begin to discharge via R3 to 0 (–).
A small rise in Vs voltage will stop more discharge, but the alarm will stay ‘on’.
A further rise, like starting the engine, will pass D2 and recharge capacitor C1 and only then will the alarm switch to ‘off’.
Alarm
The alarm is triggered from pin 6 of the op-amp 741 (IC1), which is either saturated +, or saturated –; that is about 10.5V or 1.5V.
In the prototype, this 1.5V was enough to allow the buzzer to make a constant weak sound. So, instead, the output of pin 6 was fed to one side of a setting potentiometer VR1.
The setting slider adjusts the current from + Vs to the Base of PNP Transistor (Q1) from either a few µA to more than 200µA, which controls the current through the transistor Q1 to the load, from 0mA to 25mA.
That, in turn, sounds the buzzer (BZ1) and lights up the warning LED (D4).
When the main power is switched on, the unit will sound for ¼ second during the charging of C1, which proves it is working OK, but if the battery is lowish it will sound for seconds as a pre-warning!
When the onboard battery is switched off, the fast-dropping Vs voltage triggers the alarm for ¼ second.