One Cell Down on a 100 Ah Battery
A customer brings in a 12 V, 100 Ah flooded lead-acid battery from a truck. It has been on a 4 A charger for 12 hours. After standing for 2 hours, the open-circuit voltage is 12.1 V. You check the six cells with a hydrometer; the electrolyte temperature is 35 C. Readings are: cell 1 1.250, cell 2 1.255, cell 3 1.245, cell 4 1.150, cell 5 1.250, cell 6 1.248. A load test at 300 A gives 8.9 V at 10 s and 8.4 V at 15 s; cell 4 begins to gas and bubble more than the others. The battery has no date code, and the truck's charging system was tested last week at 14.2 V with no AC ripple. The customer wants to know if an equalising charge will save it. The battery case is not swollen, and the terminals are clean. Workshop temperature is 25 C.
1. Correct the hydrometer readings to 25 C and decide whether the battery has a failed cell. Show your reasoning.
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Hydrometer readings are temperature-dependent. The usual correction for a lead-acid hydrometer is about +0.0007 per degree C above 25 C. The electrolyte is 35 C, which is 10 C above 25 C, so add 10 x 0.0007 = 0.007 to each reading:
- Cell 1: 1.250 + 0.007 = 1.257
- Cell 2: 1.255 + 0.007 = 1.262
- Cell 3: 1.245 + 0.007 = 1.252
- Cell 4: 1.150 + 0.007 = 1.157
- Cell 5: 1.250 + 0.007 = 1.257
- Cell 6: 1.248 + 0.007 = 1.255
The five healthy cells sit around 1.252 to 1.262. Cell 4 is about 0.095 to 0.105 lower. A serviceable battery should have all cells within roughly 0.050 of each other, so cell 4 is failed or severely sulphated. The open-circuit voltage of 12.1 V suggests only about 40-50% state of charge, but that alone would not explain one cell being so low. The load test is also a fail: 8.4 V at 15 s is below the usual 9.6 V minimum for a 12 V battery at that load. Cell 4 gassing more than the others during charge or test is a further sign of an internal fault, not a normal state.
An equalising charge is intended for electrolyte stratification or mild sulphation in a battery with balanced cells. It will not repair a cell with a 0.1 specific-gravity deficit and may overheat the battery or drive thermal runaway. The correct recommendation is replacement, not equalising.
2. The customer says the truck charges at 14.2 V, so the battery must be good. Explain why that does not prove the battery is serviceable, and what you would tell him.
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A charging voltage of 14.2 V only shows that the alternator is raising system voltage. It does not measure the battery's capacity, its cranking ability, or the balance between cells. A battery can accept charge and still have a shorted, sulphated or high-resistance cell; the alternator will simply keep charging the remaining healthy cells and overheat the failed one.
The evidence against this battery is the corrected specific gravity spread, the low open-circuit voltage after a long charge and rest, and the failed load test. The 14.2 V reading also does not rule out a charging system that is slightly low for the battery type, but that is a separate check. I would explain that the battery is a storage device, and the load test and hydrometer readings show it cannot deliver its rated capacity. Replacing it is more economical than repeated charging and the risk of a no-start. I would also check the battery hold-down, terminal condition, charging voltage at the battery posts and the vehicle's key-off draw after fitting the new battery.
3. State the safety precautions for handling the electrolyte and the failed battery, and how you would dispose of it.
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Wear a face shield, acid-resistant gloves and an apron. Work in a ventilated area away from sparks, flames and smoking. Keep the battery upright; do not tip it or carry it by the terminals. Remove rings and watches, and use insulated tools to avoid shorting the terminals. If electrolyte contacts skin or eyes, flush immediately with plenty of water for at least 15 minutes and seek medical help. Neutralise small spills with sodium bicarbonate or a commercial acid spill absorbent, not with a strong alkali, and place the residue in a labelled plastic container.
The failed battery is hazardous waste and must go to an authorised lead-acid battery recycler or the supplier's take-back scheme. Do not put it in general waste, do not store it where children can reach it, and do not leave it on a concrete floor for long periods if the case is damaged. Transport it upright and secured, with the terminals protected against short circuits.