--- title: "Lead-acid battery construction and rating" subject: "Automotive Technology" chapter: 9 key: "semester_5/automotive_technology:lead-acid-battery-construction-rating" source: https://www.bctengineer.com/q/semester_5/automotive_technology/lead-acid-battery-construction-rating site: BCT Engineering --- # Lead-acid battery construction and rating Automotive Technology · Chapter 9: Vehicle Electrical Systems - Asked in: year not given, 6 marks - Video: [How A Car Battery Works - basic working principle](https://www.youtube.com/watch?v=VnPRX5zQWLw) — The Engineering Mindset - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/automotive_technology/lead-acid-battery-construction-rating - Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-9 ## Question Describe the construction and the chemical action of a lead-acid automotive battery. Explain the terms ampere-hour capacity and cold cranking amps. How is the state of charge checked? ## Short answer A lead-acid battery has PbO₂ positive plates, Pb negative plates, and H₂SO₄ electrolyte; discharge converts both to PbSO₄, diluting the acid. Ampere-hour capacity is the constant current that discharges it to 10.5 V in 20 h at 27°C; CCA is the current sustained for 30 s at –18°C above 7.2 V. State of charge is checked by specific gravity (1.28 full) or open-circuit voltage (12.7 V full). ## Answer #### Construction A 12 V battery has **six cells** in series, each of about 2.1 V. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How many cells does a 12 V lead-acid battery have and what is each cell voltage? A. Three cells of 4.2 V B. Six cells in series of about 2.1 V each C. Twelve cells of 1 V D. Six cells in parallel of 12 V Answer: B. Six cells of about 2.1 V in series give about 12.6 V. (Key point: Cells) 2. What happens on discharge? A. Plates change to PbO2 and Pb and acid strengthens B. Only the negative plate changes C. Acid becomes stronger D. Both plates change to PbSO4 and acid becomes weaker as water forms Answer: D. On discharge PbO2 and Pb both become lead sulphate and sulphuric acid is used up, reducing specific gravity. (Key point: Chemical action) 3. A 60 Ah battery at the 20-hour rate supplies A. 3 A for 20 hours to a 10.5 V cut-off B. 30 A for 2 hours C. 60 A for 1 hour D. 12 A for 5 hours Answer: A. Capacity/20 h = 3 A to the cut-off voltage at 27 C. (Key point: Ah capacity) 4. Which pair is correct? A. CCA is current for 20 h at 27 C; full charge SG 1.12 B. CCA is capacity in Ah; full charge SG 1.0 C. CCA is voltage at 0 C; full charge SG 1.20 D. CCA is current for 30 s at -18 C with cells above 1.2 V; full charge SG is 1.28 Answer: D. CCA shows starting ability; a hydrometer reads SG 1.28 for full charge, 1.20 half, 1.12 discharged. 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