How can we say a battery,say Li-ion battery, is dead when it is 2.8V itself instead of 0V? How can the 2.8V not give current? What is happening? This question, is applicable to all other batteries also. Why do we say a battery is dead when it is not completely 0V, but when it is hanging some very around 2.8V for Lithium ion and 1.1V for the AA battery whose nominal voltage is 1.5V and others?
I have a Li-ion battery which I tried to charge. The maximum voltage mentioned in the battery is 4.4V max. And its nominal voltage is 3.85V. But, when I connect the battery to the USB Charger to charge the dead battery which was at 2.8V, the voltage line reads 4.6V. How is the 4.6V obtained and won't the battery get damaged as we are feeding 4.6V which is more than its maximum 4.4V voltage which is mentioned?
Short answer: Think of your dead battery as the same battery but with very very reduced capacity. If you leave it alone, it shows some voltage. But as soon as you plug it into a device to extract current from it, it gets empty within milliseconds. When you try to charge it, it appears to fill up quickly.
Discussion: There are many failure modes for each type of battery, some leave them at 0V, some cause them to have high internal resistance, some leave them with reduced charge capacity, some make them leaky. These failure modes can occur alone or simultaneously. The two indications that you mentioned, both seem to be caused by a battery having too little capacity.
But even for a good lithium ion battery, the amount of charge available in the battery is not proportional to it's voltage. An Li ion battery delivers most of its charge between 3.4 V and 4.2 V. Below this voltage, it does have some charge, and it will definitely supply current, but its voltage will fall towards zero very quickly if you draw current from it.
Inside the cell there are two voltage sources in series (one for each electrode). What you read at the terminals is the sum of those two voltage sources (or the difference, depending on how you define the sign).
- For an LCO Li-ion cell, at 2.8 V at the terminals, the internal voltages inside the cell are still all positive.
- At about 2.5 V at the terminals, one of the internal voltages in the cell drops to 0 V. That's when the cell is truly empty, but that's too close to disaster, so you don't want to go there.
- Below about 2.5 V at the terminals, that internal voltage source is negative. That damages the electrode irreparably
So, yes, you can discharge the cell to 0 V at the terminals. But that's the last time you get to use that cell. If you then try to recharge it, for some chemistries the cell will catch fire.