A freshly, fully charged battery will read around 13.3~13.5 volts at it’s terminals. 
When you put this battery into a properly functioning and running motorcycle 
charging system, the measured battery voltage will read 14~14.5 volts. 
The reason for this is because the charging system is providing the charging power. 
The charging system’s voltage is designed to be greater than the battery ‘fully charged’ 
voltage. The reason for this is so as to cause current to flow from the charging system 
into the battery, thus maintaining the battery’s charge. If the charging system voltage 
drops too low (i.e. charging system malfunction), the current no longer flows into the 
battery, but instead flows out. This will soon lead to electrical system failure due to 
a discharged battery.

A battery, from the moment it no longer receives charge, begins to discharge. 
This process happens regardless as to whether the battery is connected to a circuit or not. 
A battery that sees no charging current loses about 1% of charge per day 
(aka self-discharge). The discharge curve is a non-linear one; 
therefore the 1% per day figure is an approximation, averaged over the ‘recoverable’ 
portion of the battery discharge curve.

A process that is a function of battery discharge is known as ‘plate sulphation’. 
Battery discharge cannot take place without plate sulphation, and vice-versa. 
Plate sulphation is a process where the plates inside the battery become coated
with a sulphate. A slightly discharged battery will have a very thin coat on the plates, 
whereas a heavily discharged battery will have a relatively thick coating. 
When a battery that is either slightly or greatly discharged is connected to a charging 
source, the voltage of the charging source must ‘break through’ the coating of sulphation 
on the battery plates, which then allows a charging current to begin to flow. 
If charging is allowed to continue, the sulphation process reverses and the battery returns 
to a fully charged state. In situations where a battery is so greatly discharged that 
normal charging voltage is insufficient to break through the sulphation, 
a greater charging voltage must be used. The ‘danger’ in using a greater charge voltage 
is that charge current is a function of charge voltage. 
Using too high of a charging voltage can result in too high of a charging current, 
which may cause the battery to suffer irreversible damage. In practice, 
what is commonly done to recover a heavily sulphated battery is to use a higher charging 
voltage source and to monitor the charge current. Once charging current begins to flow 
(sulphation has been cracked through), the charge voltage is reduced to a ‘normal’ level. 
This process just described is exactly what a ‘smart charger’ does. ‘Smart chargers’ 
have been on the market for a few years now. These chargers do all the ‘thinking’ and 
decide what is the proper course of action to take. Batteries too heavily sulphated for 
a ‘smart charger’ to recover, can sometimes be rescued by using the ‘manual’ process
described.

The other side of the coin is that once a battery’s charge is ‘topped-off’, 
the battery only needs the small amount of current necessary to replace the amount 
of charge lost due to ‘self-discharge’. If more current is forced into the battery 
than what it needs and if this excess current is maintained, the battery will soon overheat 
and ‘boil-away’. Once this happens, the plates inside the battery warp and you have 
now converted your battery into a door-stop. Smart chargers know when to reduce charge 
current and to what level so as to never ‘boil-over’ the battery. Smart chargers can be 
left connected to a battery indefinitely, with no damage. 

***Sam***