Lead-acid battery construction and rating

Construction

A 12 V battery has six cells in series, each of about 2.1 V.

 Terminal(+)                  Terminal(-)
   |                              |
 [+plates PbO2 | separator | -plates Pb] x n  (cell 1)
 ----- cell connector ----- cell 2 ... cell 6
 Container (polypropylene) filled with dilute H2SO4

• Positive plates: lead dioxide ($PbO_2$) on a lead-antimony or lead-calcium grid.
• Negative plates: spongy lead ($Pb$).
• Separators: microporous plastic sheets that prevent short circuits.
• Electrolyte: dilute sulphuric acid, with a specific gravity of about 1.26 to 1.28 when fully charged.
• Container, cell connectors and vent plugs (sealed maintenance-free types have none).

Chemical action

$$PbO_2 + Pb + 2H_2SO_4 \underset{\text{charge}}{\overset{\text{discharge}}{\rightleftharpoons}} 2PbSO_4 + 2H_2O$$

On discharge, both plates change to lead sulphate and the acid becomes weaker (water forms). On charging, the plates return to $PbO_2$ and $Pb$ and the acid becomes stronger.

Ratings

• Ampere-hour capacity: the charge (current × time) that a fully charged battery can deliver at a stated discharge rate before its voltage falls to a specified cut-off (10.5 V for a 12 V battery) at 27 degrees C. It is usually quoted at the 20-hour rate: a 60 Ah battery gives 3 A for 20 hours.
• Cold cranking amps (CCA): the current in amperes that a fully charged battery can supply for 30 seconds at -18 degrees C while keeping every cell above 1.2 V (7.2 V for a 12 V battery). It shows the battery's starting ability.

State of charge

• Hydrometer: measures the specific gravity of the electrolyte: 1.28 = fully charged, 1.20 = half charged, 1.12 = discharged.
• Open-circuit voltage: 12.7 V at full charge, 12.2 V at half charge.
• Load (high-rate discharge) test to check its condition in service.