Battery Charging Fault Indicator Circuit

The article explains a battery status indicator circuit which can be also used as a battery charging fault indicator circuit. The idea was requested by Mr. Faizan.

The Design

The idea presented here takes care of all the parameters required for charging a battery ideally and safely.

Referring to the shown battery charging fault indicator circuit, the design may be understood with the help of the following points:

The IC LM3915 which is a dot/bar LED display driver IC forms the main charging indicator module of the circuit.It's pin5 is the sensing input, the rising battery voltage is sensed at this pin and the IC responds to it by producing a proportionately sequencing LED illumination across its 10 outputs, as shown with the 10 connected LEDs.

A LM317 IC can also be seen attached at the input of the circuit, it's wired as a constant current generator so that the circuit is able to produce error free indications and operations regardless of the input current level. Rx is selected suitably in order to enable this correctly.

Circuit Diagram

When power is switched ON, the 100uF/25V capacitor across the pin5 preset of the IC momentarily grounds pin5 so that all the outputs of the IC begin by staying shut off.

This is important to make sure that the TIP122 is able to initiate the charging process and the BC557 is inhibited from an accidental switch ON due to the initial surge transients.

As soon as the 100uF is charged up, pin5 is allowed to sense the actual voltage that's been utilized by the battery while it's been charged, which should be normally anywhere around 3 to 3.3V for a fully discharged 3.7V Li-ion battery.

Here each LED may be set to indicate an increment of 0.42V, which implies that the illumination of the 10th LED indicates 4.2V which may be assumed to be the battery full charge level indication.

This also implies that during power ON, 7 LEDs must be illuminated to indicate a correct battery discharge level and charging process.

Less that 7 LEDs illuminated would indicate a badly discharged battery or a damaged battery consuming excess current than the specified range.

With all the LEDs lighting up during power switch ON would imply either the battery is fully charged or the battery is not accepting charge and is faulty.

Under normal conditions, around 7/8 LEDs should be illuminated at power switch ON and as the battery voltage increases  due to charging, the LEDs should also sequence by illuminating the 8th, 9th and the 10th LED sequentially.

Once the 10th LED is illuminated, a low logic is sent to the base of the TIP122 which is now inhibited from a base bias and the charging voltage to the battery is thus cut off, switching off the charging voltage to the battery.

The low logic from the 10th pin is also sent to the base of the shown BC557 which conducts and connects pin5 of the IC directly to the 5V supply making sure that the 10th LED becomes latched and the situation is locked until power is switched OFF and ON for further actions.

How to the set up the circuit

It's the simplest part in the design.

Initially do no connect any battery across the shown points.

Apply a precise 4.2V at the input.

Now begin adjusting the pin5 preset such that the LEDs light up sequentially and the 10th LED just illuminates brightly.

Seal the peset once this is confirmed.

Your battery charging fault indicator circuit is all set now for the proposed battery fault indications and also charge level indications.

Battery Fault indicator Circuit using a Flashing LED.

The following update shows a simpler design that may be used for indicating a battery charging malfunction through a flashing LED

Initially both the opamp outputs may be assumed to be low, if the battery is discharged below 11V, this will be indicated with a fast blinking of the LED. C1 must be set for achieving this fast blinking.

The lower opamps is set using pin5 preset such that when the connected 12V battery reaches around 12.5V, its output pin goes high, once this happens the BC547 triggers and adds a high value capacitor C2 in parallel with C1 slowing down the flashing rate significantly and indicating that the battery has entered the next upper charging phase and also that the battery is good and is accepting the charge well.

As the battery continues to get charged and acquires a voltage level of around 14V, the upper opamp which is set using pin3 preset to trigger at this point triggers and renders a high across the connected LED stopping its flashing and illuminating it to solid.

Once this happens the user may assume the battery to have reached the optimal charging level and may remove it from the charger.

How the Battery fault is Indicated

1) If the LED blink rapidly would initially indicate that the connected battery is over discharged, however this condition should improve and the LED should transit into a slow flashing after an hour or so depending upon the sate of the battery. If this does not happen, the battery may be assumed not accepting the charge due to internal damage or short circuit.

2) If the LED lights up solid when power is switched ON would clearly indicate a faulty battery which may be completely inactive internally and unable to accept any current.

The above explained battery charging fault indicator circuit can be upgraded for an automatic over charge cut off  through some modification as shown in the following diagram:

While setting up the two presets make sure the 100K link remains disconnected in the upper opamp.

After setting up the thresholds, the 100k link can be reconnected into position.

The circuit will not initiate until a battery is connected, so make sure the battery to be charged is first connected and then power is switched ON.

For a 3.7V battery, the 4.7V zener must be replaced with two

A little in-depth investigation shows that in the above circuit C2 will not have a discharge path through the connected BC547 and therefore it won't help to slow down the oscillations while the lower opamp is in the activated state.

The correct implementation of the above concept could probably be done by using an optocoupler as shown in the following figure.

Here instead of targeting the frequency determining capacitor C2 the resistor counterpart is selected for the intended control of the frequency and LED blinking rate:

Schematic for Blinking LED Fault Indicator

Now it looks much better.