It is called a deep cycle battery
A rechargable deep cycle battery can be fully discharged without damaging the battery. Any auto battery cannot be discharged without damaging the battery. Any dry cell rechargable battery can be fully discharged without damaging the battery.
There is no usual. Depends on the quality of the battery and if it was ever fully discharged. 3-4 years is normal. Each time the battery is fully discharged, the life of the battery is shortened.
It is always good to charge a battery to its full capacity before it is fully charged.
Sulfation can happen when battery is not fully charged and remains discharged
By fully charging and fully discharging some batteries the expected charge time can be increased. Some rechargable batteries will last longer per charge if they are fully discharged before you recharge them again. Recharging batteries before they are fully discharged may cause a battery memory effect that will allow the battery to run for a shorter period between recharges.
Usually from being discharged in below freezing weather. The water will freeze in a dead battery, expand, & swell the battery. This won't occur with a fully charged battery.
A C cell is fully discharged at 0.9 volts, 60% of the nominal 1.5 v.
An alternator is designed to keep a fully charged battery at peak level. It is not designed to charge a dead battery. Installing a new alternator on a car with a dead battery can in some cases ruin the new alternator. Fully charge the dead battery before starting the engine.
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.
More than likely the battery is discharged. It can also be a defective starter. Fully charge the battery and see what you have.
It depends on what type of battery it is. Ni-CD battery do need to be discharged completely before recharging it (I would recommend getting a discharger), Ni-MH batteries can be discharged but don't need to be, and Li-PO batteries should absolutely not be discharged completely, because they catch fire and explode when fully discharged (Ni-CD batteries have a memory effect, while Ni-MH and Li-PO batteries do not).
I recommend you use an automatic charger but if you do not have one and the battery is fully discharged then click the link and insert the battery capacity in mAh and the charge rate in mA to determine the amount of hours to charge the battery.