Introduction

Mr. Lung wrote to me, asking for some help with a battery checker circuit. After some discussion, we arrived at the the low parts count, high performance "Battery Low" circuit. Mr. Lung prototype it and found it to work to his satisfaction.  The "Battery Low" checker lights the red LED if the battery is too low. A second circuit was devised, the "Battery Good" circuit, which lights it green LED when the battery voltage is high enough.

Why two circuits? Because different applications have different needs.

According to Mr. Lung, the lowest safe voltage across a lead acid battery while in long term storage is 2.0 volts per cell, or 6.0 volts for a 6 volt battery. Gates Energy Products, the load must be disconnected from a lead acid cell when the voltage reads 1.8 volts or less, in order to avoid damage. For a 6 volt battery with three cells, this critical voltage is 3 x 1.8 volts = 5.4 volts.  Choose your threshold depending upon your application. If there is a good chance that the battery will not be recharged soon after running down, be safe and go with the 2.0 volt per cell threshold.

These circuits can also be adjusted for other battery voltages and other types of battery. This circuit should work well with Nickel Cadmium (ni-cd or nicad), Nickel Metal Hydride (NIMH) or other rechargeable as well as primary cells types such as Carbon-Zinc and Alkaline. Just make sure that you don't exceed the ratings of the parts for your particular application.

The practical minimum threshold voltage for the Battery Low circuit is about 3.5 volts while the practical minimum voltage for the Battery Good circuit is about 4.5 volts.

Battery Low Battery Checker Circuit



As specified by Mr. Lung, this circuit indicates when the battery voltage is too low.  His purpose for the checker is to detect when a rechargeable lead acid cell has run down to its safest discharge voltage.

Circuit operation:
The TL431 has an internal threshold of 2.5 volts. Pot R2 is adjusted so that when the battery voltage equals the desired threshold, the wiper of R2 will be 1.25 volts. When the voltage across the battery is above the preset threshold, the voltage at  the wiper of R2 is above 2.5 volts, the TL431 conducts, shunting current away from the LED, keeping the LED off.

When voltage drops below the preset threshold, the voltage on the wiper of R2 drops below 2.5 volts, and the TL431 conducts less current, only up to about 1 milliamp, and the anode voltage rises until the LED conducts.

I tested this with a 6 volt threshold rather than a 5.4 volt threshold. When adjusted for a 6 volt threshold, the LED turns on at 6.0 volts and then slowly fades out as the battery voltage decreases, until it reaches about 2.5 volts, when the LED is too dim to see. The LED is off above 6.0 volts, but (for reasons I have not investigated) the LED comes back on at voltages above 8.35 volts. The "come back on voltage" is proportional to the threshold setting, thus, if the circuit were adjusted to turn on the LED when the voltage dropped below 12 volts, the LED would also come on at voltages above
The minimum voltage across the TL431 is 2.5 volts, which would still leave enough to illuminate D2 (which requires about 1.8 volts), so D1 was added to provide an additional 0.7 volt drop in series with D2 so that D2 will not illuminate when the TL431 is sinking the maximum current.

The luminance of the LED can be adjusted by changing the value of R1.

The push-button is in series with the circuit so that it will not add to the drain on the battery when the battery voltage is not being checked. Unless actually checking the voltage, this circuit draws no power.

Mr. Lung, who identified the need for this circuit, built and tested the prototypes, and has experienced  using the circuit in an actual application, can be reached at (this email address is a image).



Battery Good Battery Checker Circuit

In some applications, it is desirable to just simple have an indication of when the battery voltage exceeds a preset threshold, in other words, to indicate when the battery is "good." If, when pushing the button, the LED does not come on, then the only action indicated is recharging.  The circuit shown above does just this.

To calibrate this circuit for a 6 volt lead acid cell:
1. Connect the circuit to a power supply set to just above 6.0 volts
2. Hold down the button.
3. Adjust R2 to the point that the LED just goes off.

During the adjustment procedure, the wiper of pot R1 is set to provide 2.5 volts to the reference input of the TL431 when the battery voltage is at the preset threshold voltage. When the battery is below the threshold voltage, the reference of the TL431 is below 2.5 volts, and the TL431 conducts a minimal current of less than 1 milliamp.  R3, a 1k resistor across D1 in series with R4, drops only 1 volt at 1 milliamp, and so it does not allow enough voltage across D1 to allow the LED to draw current while in this state.

When the threshold voltage is exceeded, the reference voltage on the TL431 exceeds 2.5 volts and the cathode of the TL431 sinks current to ground, thus causing the LED (D1) to illuminate.  The current to the cathode, and consequently through the LED results in a voltage drop across R4. Q1 conducts as the voltage across R4 exceeds one base-emitter drop (about 0.6 volts), and current through Q1 causes Q2 to draw current through R2, thus changing the voltage on the reference input of the TL431, causing the current through the cathode to be regulated.

When there is sufficient voltage on the wiper of pot R1, the current through the LED (D1), is regulated at about I =  0.6/R4.

In the case of R4 being 47 Ohms as shown in the schematic, the current through the LED is about 0.6/47 = 13 milliamps. Since the luminance of the LED in nearly proportional to the current through it, the luminance can be controlled by changing the value of R4.

There is a slight temperature dependence to this current, of about  0.3% per degree C, but the current, and therefore the luminance of the LED is largely independent of the battery voltage.

I set this circuit up with a threshold at 6.0 volts. The LED came on sharply at 6.0 volts and remained on at constant luminance to the maximum test input voltage of 24 volts,  and it went off sharply at  6.0 volts and remained off through zero volts.

C1 can be eliminated, but it is good practice to use it because it stops the circuit from oscillating at high frequencies.


Construction

The Battery Low checker circuit was built by merely
soldering the parts together as shown above.