Applying load when testing battery voltage

You need to put a load on the battery to see if it has any charge left.

Without a load, it may show an acceptable voltage, but when you actually try to use it the voltage drops because the battery is nearly dead.

So to see if a battery is really usable you must measure the voltage when the battery is connected to a load. Like this:

Dead Battery, no load, 1.4 Volts

Dead Battery, load of 100 Ohms, 1.0 Volts

Good Battery no load, 1.5 Volts

Good Battery, load of 100 Ohms, 1.4 Volts

Those numbers are just representative - do NOT use them to actually measure your batteries. Check the unloaded voltage of a good battery, then check the voltage of a good battery under a typical load. Use that typical load to test other batteries. That is to say, figure out the equivalent resistance for the load and use a resistor of that value in your test.

Quite a lot of battery chemistries will, if left alone, raise their terminal voltage. But there may be no capacity behind it and it will drop as soon as you try to use it. So a load is connected to the battery to verify that it is actually useful.

As typical Alkaline and other batteries go bad or get weak, they develop greater internal resistance. With no load or very little load you could say that there is a voltage divider formed by the internal resistance and the high resistance external "load". The high external resistance will show a high or full voltage drop. With a good external load, low resistance, the internal resistance of the battery will experience a greater voltage drop, meaning you'll see lower voltage externally.