Here is a simple solution to confirm whether a battery is charging or not. This tiny circuit can be hooked between the charger and the 9V or 12V battery. When the circuit is connected to the charger, LED remains off. But when the battery is connected, LED turns on. At first, the LED brilliantly lit indicating that current is flowing to the battery. Once the battery is fully charged LED turns dim. So it confirms
1. Battery is properly connected
2. Current is flowing from the charger to the battery.
You need the following components
1. 27 Ohms ½ W resistor – 1
2. 1K ¼ W resistor – 1
3. BC 557 PNP Transistor – 1
4. Red LED – 1
How it works?
There is a resistor R2 ( R Sense ) between the Emitter and the base of the PNP transistor. Its value is selected using the formula
0.65 / I in Amps
So we need a specific current drop across the R Sense resistor. Let us fix it as 24 mA to control the working of T1.
0.65 / 0.024 A = 27 Ohms
So when a current higher than 24 mA passes through the R Sense resistor, LED brightness increases.So the charging current will be around 70 mA. It is good for slow charging. But this is the theoretical value. You can expect a current between 80 mA and 100 mA due to tolerance of the resistor used.