Every battery has 2 terminals Positive and Negative. Current always flows from positive terminal of the battery to the negative terminal of the battery. Only correct connection with respect to the polarity will ensure correct functionality of the circuit. If the circuit is reversed then the circuit will not function properly and also results in damaging the circuit. Therefore it is very important to ensure correct polarity during the connection. In case if the polarity of the circuit is unknown, it must be determined first before it is used.
To check the polarity of battery, simple polarity tester circuit
Here terminal 1 and 2 are connected with the terminals of the battery with unknown polarity.
Case 1:- If terminal 1 is connected to positive ,terminal 2 is connected to negative terminal, then diode D1 will be forward biased and D4 will be negatively biased. As a result current starts flowing from D1 passing through the LED D5 and resistor and leaving from D3. Here LED D5 will glow showing that current is passing through it.
Case 2:- terminal2 is connected to the positive terminal of the battery and terminal 1 is connected to the negative terminal of the battery, then diode D4 will be forward biased and D1 will be negatively biased. As a result current starts flowing from D4 passing through the LED D6 and resistor and leaving from D2. Here LED D6 will glow showing that current is passing through it.