I'm wanting to make a simple low battery indicator for 12V SLA battery – just an LED that lights when the voltage gets below a certain level. What sort of power would a circuit like this http://www.electroschematics.com/9010/12v-lead-acid-battery-low-voltage-indicator/ draw from the battery (when the battery is charged)? Is it worth considering a different circuit that uses less power?
12V Low battery indicator that has minimal power draw
batterieslow-powerpower
Related Solutions
What you want is a low pass filter.
A temporary dip is a high frequency event, while a real low battery is a low frequency event. If you pass this signal through a low pass filter, it will remove the high frequency events, and allow the low frequency one to pass.
Increasing the value of the capacitor and resistor increases the time constant, thus decreasing the frequency threshold.
Fortunately, there is. You can find diagrams of the discharging process of a battery type, search for "lead-acid discharge graph" or so. This returns graphs like this one:
In the datasheet of your battery you can find the exact graph for your battery.
Say you want a circuit to detect 80% discharged, just check in the graph, the voltage of a lead acid battery with 80% discharged is about 1.9V.
You can check this voltage with a multimeter, or use a comparator circuit or a microcontroller with ADC to make a dedicated device for this.
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Best Answer
That is exactly the wrong approach. What you want is an LED which is on when the battery is good. Even better, you want an LED which is not powered by the battery.
Using your approach, if for some reason the indicator is ignored then the LED will eventually completely discharge the battery, and with SLAs that is A Bad Thing. Granted, this can take a while. If the LED is the primary current drain, then a 5 mA LED current will waste .12 Ah per day, or about an Ah per week.
If you must do it with an LED which is on to indicate low charge, at least make the LED blink rather than be on steady. A flash rate of .2 seconds on every 2 seconds will draw 1/10 the average current as a steady LED, and will arguably be more noticeable.