Inverter Battery Losing Backup: One Weak Cell and a Replacement Decision
A customer brings a 12 V, 150 Ah tubular lead-acid inverter battery to a shop. It is three years old and its backup has fallen from about 4 hours to 45 minutes on a 200 W load. The customer has been topping up with tap water. Measurements: open-circuit voltage 12.6 V; voltage falls to 10.2 V within 30 minutes while supplying 20 A; specific gravity readings are 1.250 for five cells and 1.120 for one cell; the low cell's electrolyte level is below the plates. The shop can sell a new battery for ₹9,000 with a 36-month warranty, or offer a ₹1,500 service that includes replacing the weak cell with a used one and adding electrolyte. A local scrap dealer offers ₹800 for the old battery.
1. Interpret the specific gravity and voltage readings. What do they show about the battery's condition, and why is the open-circuit voltage misleading?
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A fully charged cell has a specific gravity around 1.265; 1.250 is a normal, slightly discharged value. A reading of 1.120 in one cell means that cell is deeply discharged and likely sulphated, with its active material hardened and its capacity lost. The battery is a series string, so its capacity is limited by the weakest cell: five healthy cells cannot carry the load once the sixth collapses. The open-circuit voltage of 12.6 V (about 2.1 V per cell) looks healthy, but it is a surface charge with no current flowing. Under a 20 A load the voltage falls to 10.2 V (1.70 V per cell), which indicates very high internal resistance in the weak cell. That load voltage, not the open-circuit reading, is the meaningful test. The low electrolyte level in that cell also suggests it has been overheated or overcharged.
2. Estimate the battery's expected and actual usable energy, and use the figures to judge whether the ₹1,500 repair is worthwhile.
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Rated energy = 150 Ah × 12 V = 1,800 Wh. For good life, a lead-acid battery is normally discharged to about 50% depth, giving roughly 900 Wh usable, so a 200 W load should run about 4.5 hours ideally and around 3.5–4 hours in practice, matching the original performance. The actual backup of 45 minutes at 200 W is about 150 Wh, roughly 8% of rated capacity. Replacing a single cell in a three-year-old battery is poor practice: the new or used cell will have different capacity, age and internal resistance from the other five, so it will be overcharged or undercharged and fail early. The ₹1,500 repair buys a short, unreliable life and cannot restore the sulphated plates. The honest recommendation is a new battery, with the old one sent through a proper take-back route rather than sold loose.
3. Advise the customer on correct maintenance and on disposing of the old battery safely and legally.
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Maintenance: use only distilled or demineralised water, never tap water, because its salts contaminate the electrolyte and increase self-discharge; top up to the marked level, not above the plates; keep terminals clean and lightly coated with petroleum jelly; avoid deep discharges below about 50%; charge fully after every outage; and check specific gravity and voltage periodically. Tap water use and a dry cell explain part of the present failure. Disposal: lead-acid batteries contain lead and sulphuric acid and are classified as hazardous waste. They must go to an authorised battery dealer, recycler or e-waste take-back point, never into household waste, open ground or a drain. The ₹800 scrap offer from an informal dealer may be convenient, but the customer should obtain a receipt or exchange credit from an authorised channel so the battery is tracked. A new battery purchase usually includes a buy-back, which is both safer and often better value.