What are the indicators of fully charged lead acid cell?
Answers
The simple answer for determining that a lead acid cell is fully charged is if it will not take take significant charging current when charging at a normal charging voltage (~2.4V) or that the voltage becomes very high (~2.7V) when charging at a fixed current equal to its ~0.05 x Ah rating. Both these conditions indicate that there is little further lead sulfate discharge product left to charge. Usually, lead acid cells are overcharged for an extended period of time to insure they are fully charged and to fully mix the electrolyte concentration.
Without charging equipment, there are still ways to determine if a cell is mostly charged. If the cell has free liquid electrolyte and an access vent, the specific gravity can be measured. If it is greater than 1.28, the battery is probably reasonably charged. If the cell has been recently charged from a full discharge, however, the specific gravity measured at the top of the cell may not reflect the charge condition of the cell because the electrolyte may not be uniformly mixed leaving stronger specific gravity at the bottom and weaker electrolyte at the top. This gives a false impression that the cell is not very charged when in fact it may be approaching full charge.
The cell’s voltage can also be measured do determine its charge state. If it is high, >2.13, the cell is probably reasonably charged. However, again if the battery has been recharged without thoroughly mixing the electrolyte, there may be false high voltage readings until the cell stabilizes to a uniform condition.
Measuring resistance or conductance can tell if the cell is significantly discharged, but is not particularly sensitive in the upper charge regions. In other words, it is difficult to tell the difference between a fully charged cell and a cell that is 10% or more discharged.