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When a battery is discharged and not immediately charged, the soluble lead sulfate that forms as
a part of the reaction converts to an insoluble form and this is eventually terminal for the battery.
In addition, stratification of the electrolyte can occur with the more dense electrolyte (e.g. higher
specific gravity) at the bottom. This can cause what is referred to as local action which damages
the plates since the chemical reaction is not uniform throughout the battery.
Not all the lead sulfate is returned to the base material when a battery is charged. Some of the
lead sulfate that is not recombined sloughs off the plates and settles to the bottom of the case. In
extreme situations this sediment can build up to the point where it shorts out the plates.
The open circuit voltage of a battery is a function of the specific gravity of the electrolyte corrected
for temperature. A check of the battery voltage does not indicate its capacity to deliver current to
a load. The ability of the battery to deliver the required current is a function of the amount of plate
area available for the chemical reaction. This, as can be seen from the above discussions, is
dependent on a number of factors which include the specific gravity of the electrolyte, the surface
area of the plates, the rate at which the reaction can diffuse into the plate structure, the rate at
which mixing of the electrolyte occurs and the internal resistance of the battery due in part to the
amount of sulfate built up on the plates in insoluble form.
Assuming all is right with the battery, most of the charging current goes toward forcing the
chemical reactions from right to left. The remaining current not involved in the reaction goes to
heat in the battery.
If the battery is badly sulfated, the active material is isolated from the electrolyte and the chemical
reaction cannot occur at a rate equal to the charging current and excessive heating occurs. In
addition to the excessive heating, the generation of hydrogen gas is accelerated and dangerous
levels of hydrogen can result.
The point of this discussion is that even though the alternator is pumping current into the battery,
it may be only generating heat and gassing of the cells. At the end of the charge, you still have a
dead battery.
Now what does this tell us. With a voltmeter, you can tell if the charging system is operating at
the point for a trickle charge. With an ammeter, you can tell the alternator is delivering current to
the battery. What you can’t tell is whether the current is charging the battery or just making gas
and heat. The only way to really tell the condition of a battery is with a load discharge after the
electrolyte has been equalized by topping up the cells, mixing thoroughly, and applying an
equalizing charge to the battery to bring all cells up to the desired specific gravity.
During a load test the battery voltage is monitored during the discharge to determine the capacity
of the battery. If an amp-hour meter is connected during the test discharge and the subsequent
charge cycle, the efficiency of the battery can be determined by a ratio the two values. This
should be above 90 percent for a good battery.
Battery Charging. When a battery is partially discharged, the charging current can be high. As
the battery comes up to charge, the charging current must be reduced to prevent excessive
heating of the battery and gassing (Hydrogen) from the cells. There are several ways to do this
but typical automotive charging systems use the constant potential method. If the battery charger
(in this case the alternator) is held at a constant voltage, the difference in voltage between the
alternator and the battery will become less as the battery reaches full charge. As this difference
in potential decreases, the current delivered to the battery also decreases. To prevent the
formation of insoluble lead sulfate on the plates of the battery, it is preferred to have a slight
trickle charge or discharge on the battery at all times. For a battery that is going to be inactive for
a period of time, the most important thing is that it is fully charged prior to being inactivated and
that it receives a periodic charge either with a battery maintainer or with a charger .