One volt is a joule per coulomb, therefore over time the battery looses electrical energy (joule) and this is why the battery becomes discharged.
put simply it is because the electrical energy is being consumed.
It should decrease
Cutoff voltage is the point at which the battery is fully discharged. This is usually the point at which the device will shut itself off.
Alternator is defective.
End cell voltage refers to the voltage level of a battery cell after it has been fully charged or discharged during a charging or discharging cycle. It is used as a measure of the state of charge or state of health of the battery. Fluctuations in end cell voltage can help diagnose battery issues or determine when a battery needs to be replaced.
Either you won't get enough voltage, and the car will stop once your battery is discharged, or you'll get excessive voltage, and you could end up actually overloading the battery and causing it to rupture, starting a fire, etc.
A C cell is fully discharged at 0.9 volts, 60% of the nominal 1.5 v.
A battery float charger is intended only to keep a fully charged battery from losing its charge. It will NOT work to charge a battery that is discharged or damaged. Check the voltage of the battery to be charged. Do not use charger if the battery voltage is less than 9.6 volts.
Cutoff voltage is the point at which the battery is fully discharged. This is usually the point at which the device will shut itself off.
Often it's a problem with a loose connection or corroded battery terminal, but it can also be just a discharged battery. The question is too vague to answer in any kind of detail, but it would be good to start with the basics; test the battery. Use a volt meter to see what voltage is present at the battery, then start to go out from there. If you have voltage at the battery, test voltage at the connectors, then the relay, then the fuse box.
That is all it is suppose to read with the engine not running. With the engine running it should read 13.5-16 volts. If it does not the alternator is defective. A charged battery which is not being discharged will read 12.6 ~ 12.8 volts, depending on ambient temperature. This is known as the open circuit voltage. A battery with an open circuit voltage of only 12.12 volts is effectively 50% discharged (depending on temperature). If the battery is new, it will have discharged over time because it was not being used. I would recommend an overnight charge before puting it into service. With the engine running at around 1500 ~ 2000 rpm, you should expect a charging voltage of around 14.2 volts.
The device it powers will tell you this. That is the only way to know as attempting to use a meter on these batteries will still show full voltage when discharged and accidentally shorting them with the meter can cause a fire or explosion.
The reason is that internal voids within the insulation will discharge as the voltage rises; once discharged, they will not do so again at some higher voltage. if all voids become discharged, theoretically the plot of tan delta versus voltage would level off to a horizontal line. In effect, tip-up becomes "negative".