Detect when battery is missing?

Question

I have a battery charging circuit that works fine except that I want to turn off the battery charger IC when the batteries are removed. When the charger IC is enabled and charging, it's outputting voltage into the battery/circuit. Anything I've tried so far doesn't work as even with batteries removed, most charger ICs still output voltage, even in a fault state (I have eval boards for BQ25306, MP2515, etc - they all do this).

Are there ways to detect when the actual battery is removed outside of mechanical/optical solutions?

Answer 1

The problem

... with current

It is difficult to determine the absence or presence of a battery by measuring the current because in both cases there is an open circuit. In the first case, it is a true open circuit (infinite resistance), while in the second case it is a virtual open circuit (an oppositely connected equivalent voltage source).

... with voltage

It is difficult to determine the absence or presence of a battery by measuring the voltage because the two voltage sources (the charger and the battery) are connected in parallel so in both cases there is the same voltage.

Solution

To solve the problem with voltage, we need to somehow decouple the two sources from each other. For example, we can do it by connecting a diode between the charger and the battery (with the anode to charger and the cathode to battery). Then, when the battery is removed, there will be no voltage after the diode.

To detect this high-impedance state, we can connect a pull-up resistor between it and a higher voltage source.

Operation

Charging battery

The diode is forward biased. The battery voltage is lower than the charger voltage, and it determines the indicator voltage VIND = 5 V.

^{simulate this circuit – Schematic created using CircuitLab}

Charged battery

The voltage across the diode is zero so it is off. The battery voltage is equal to the charger voltage, and determines the indicator voltage VIND = 6 V.

^{simulate this circuit}

No battery

The diode is backward biased. The indicator is connected through R2 to the additional V1 voltage. There is no current flowing, no voltage drop across R2, so the indicator voltage is VIND = 10 V.

^{simulate this circuit}

More experiments

• Try the circuit above without a diode; it might work depending on what is inside the charger output.

• Monitor the (change in) voltage at the charger output. It should change (increase), even slightly, when the battery is removed.

Answer 2

Monitor current. A fully charged battery left to Float on a charger will have a small amount of charge current flowing (battery self-discharge current). Detect when the current drops to zero amps. You know you have an open circuit or a missing battery.

Answer 3

Once per minute turn off the charger for very small time (like 1 sec) and check if battery is presented. Simple RC oscillator can do the timing.