A modern lead-acid battery assembly still reflects Gaston Planté’s original 1859 concept, of diluted sulfuric acid separating two lead sheets. Although it also benefits from Camille Faure’s later idea of pressing lead-oxide paste into a lead grid lattice for extra strength. We discuss the assembly of these components in terms of a more familiar version. And then we end with a description of how lead-acid battery chemistry works.
Basic Features of a Lead-Acid Battery Assembly
Each individual lead-acid battery cell comprises a separator between a positive lead-oxide plate, and a negative lead plate. This sub assembly is in a concentrated sulfuric acid / water solution, that acts as electrolyte. Although this electrolyte may be liquid, in an absorbent glass mat, or even a gel.
If we sample a liquid lead-acid electrolyte with a hydrometer to measure the relative density – our gel products are maintenance free – this reveals the following:
- If the reading is heading for 1.15, some sulfur is on the plates and the battery is going flat.
- If the reading is around 1.28 after we recharge it, the sulfur returned to the electrolyte.
If we tickled your interest in the chemistry inside a lead-acid battery assembly, please read on.
The Chemistry Inside a Lead-Acid Battery
The following is true of all lead-acid batteries, whether they are refillable, absorbent glass mat, or gel types:
- Discharging a lead-acid battery creates lead sulfate crystals at both terminals.
- This affects the atomic structure of the plates, both of which become lead sulfate
- The water ratio of the electrolyte increases, reducing performance as it goes flat.
- Recharging a battery converts the plates to lead or lead-oxide, renewing the electrolyte.
A small amount of sulfur remains on the plates after recharging, progressively coating them and weakening the electrolyte. If a battery technician tells you ‘your battery is sulfated you need a replacement’ then this is what they really mean.
More Information
Lead-Acid Battery Corrosion – How It Works
