Guest Blogger: Street Rod 101
Question: What is the difference between a fast and a slow charge on a lead acid
Answer: To answer this question, a brief description of charging and discharging of a lead acid battery is necessary. On a lead acid battery, there is a positive plate made of lead dioxide (PbO2) and a negative plate made of pure lead (Pb).
When placed in a solution of sulfuric acid and water (H2SO4), and during
discharging, the lead dioxide (PbO2) chemically changes to lead sulfate
(PbSO4). The pure lead (Pb) also changes to lead sulfate (PbSO4).
And finally, the H2SO4 changes to water. So when the battery is fully
discharged, the original two dissimilar metals are now the same and the liquid
has changed to water. During charging, the plates chemically change back to lead dioxide and pure lead and the water changes back to H2SO4.
Now refer to the graphic below. Positive battery plates (PbO2) are shown on the right side and a cross section of the plates are shown for a fast and a slow charge on the left. During a fast charge, the lead sulfate chemically changes back to lead
dioxide. However, as graphically shown, only the outer surface of the plate changes back to lead dioxide. The material in the center of the plate is still lead sulfate. During a slow charge, the lead sulfate has changed the outside as well as the inside
material of the plate back to lead dioxide. Using this graphic, it shows that a
slow charge (say between 2-5 amps for 24 hours) means that the entire plate has been change back to lead dioxide. However, during a fast charge (say 35 amps for 30 minutes), only the outer surface of the plate has been change back to lead dioxide. The same basic principle is true of the negative plate on the battery as well as the sulfuric acid solution of the battery.
The exact time and charge rate for a battery, either slow or fast charge, depends on the size of the battery, the ampere hours, and the condition. Always follow the manufacturer's recommendations for charging rates.