For example, lead-acid batteries typically lose about 2% of their voltage per cell per hour when discharged at a constant rate. As a battery discharges, its voltage drops.
The actual voltage output of a lead acid battery will decrease as it nears empty. This is because as discharge progresses and more electrons are transferred from one plate to another, there is an increasing resistance to electron flow due to loss of active material on the electrode surfaces.
The actual process is dependent on the type of battery we are talking about. In a lead acid battery, The cell voltage will rise somewhat every time the discharge is stopped. This is due to the diffusion of the acid from the main body of electrolyte into the plates, resulting in an increased concentration in the plates.
The voltage across each cell during discharge will depend on a number of factors, including the type of electrolyte used, the size of the plates, and the rate at which discharge occurs. However, for a typical lead acid battery, the voltage will be around 2 volts per cell.
So many lead acid batteries are 'murdered' because they are left connected (accidentally) to a power 'drain'. No matter the size, lead acid batteries are relatively slow to charge. It may take around 8 - 12 hours to fully charge a battery from fully depleted. It's not possible to just dump a lot of current into them and charge them quickly.
Personally, I always make sure that anything connected to a lead acid battery is properly fused. The common rule of thumb is that a lead acid battery should not be discharged below 50% of capacity, or ideally not beyond 70% of capacity. This is because lead acid batteries age / wear out faster if you deep discharge them.
The final 20% of lead acid battery capacity can not be “fast” charged. The first 80% can be “Bulk Charged” by a smart three-stage charger quickly (particularly AGM batteries can handle a high bulk charging current), but then the “Absorption” phase begins and the charging current drops off dramatically.