Internal resistance and rechargeable battery
Think of internal resistance as the weight that is being added to a battery over time as it ages that will not allow the battery to perform it to the full extent, for a battery to perform satisfactory it has to supply useful amount of power to the device that is being used as required by the device a new battery has low internal resistance and thus it will be able to supply the required power, but as a battery ages or if a battery is damaged it’s Internal resistance will increase over time, thus it might not be able to power the device.
A good example of internal resistance is if you use a old rechargeable battery which might have developed high internal resistance in a high drain device like digital camera, you might get just 6-7 shots even after fully charging the battery, you might notice that the time taken to re-charge the flash will be a lot longer, the problem here is that the battery is not able to supply the required power to the device, it has the capacity but its simply not able to deliver the required power to the device it’s like a tap to a bucket full of water but the tap is just half open, the water is there but you just cant get it as fast as you want.
The problem of internal resistance is particular important for modern digital equipment as these devices do not draw the power in a steady manner, they might require high power for a couple of seconds (high current in bursts) where a battery with high internal resistance will create a bottle neck, comparing this to older analog devices which mostly draw constant current thus its important that a rechargeable battery has low internal resistance with modern digital devices.
Do note that the batteries which have developed high internal resistance will function properly in low drain analog devices, because in low drain devices it will be able to deliver the power the device needs.