It is well known that a fully discharged lead–acid battery at the beginning of charging can tolerate a relatively high charging current without overheating or excessive gas formation (the current in amperes may be up to 80% of the battery capacity in ampere-hours). As the charge progresses, the current must be reduced to prevent battery failure.
The law describing the change in maximum charging current depending on the battery’s state of charge is known as the ampere-hour law, formulated by Woodbridge in 1935.
The charging current of a battery (in amperes) must not exceed the amount of charge still missing (in ampere-hours) until full capacity is reached.
Mathematically, this law is expressed by an exponential function, illustrated in the diagram. This graph shows that 90% of the charge delivered by the battery can be restored in 2–3 hours, while the total charging time (including a 15% overcharge) does not exceed four hours.
Woodbridge’s method was developed during the automotive boom, when the mass production of batteries and the issues of their operation became so relevant that scientific research was required. Woodbridge’s Law was created specifically for lead–acid batteries, and there is no evidence supporting its applicability to other types of storage batteries. In other words, using this method for other battery types is not scientifically justified.