Shop Inverter Battery: One Low Cell and Short Backup
A shop owner in Nagaon runs a 12 V, 100 Ah lead-acid battery with a 200 W inverter for emergency lighting. The battery is supposed to give about 4 hours of backup at 80% depth of discharge. After eight months it gives only 25 minutes. The inverter efficiency is about 85%, and the load is 200 W. After a night's rest, the battery open-circuit voltage is 12.1 V. The charger's float voltage is 13.8 V and its boost voltage is 14.4 V, but the charger is not temperature-compensated. Specific gravity readings are 1.250, 1.245, 1.255, 1.150, 1.240 and 1.250. One cell's electrolyte is below the top of the plates; the others are at the correct level. The owner added distilled water to the low cell and wants to know whether to add battery acid, equalise the battery, or replace it.
1. Estimate the DC load current and the expected backup time at 80% DoD, and compare with the observed 25 minutes. Show the steps.
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DC power drawn from the battery: 200/0.85 = 235.3 W. At about 12 V, current I = 235.3/12 ≈ 19.6 A.
Usable capacity at 80% DoD: 0.8 × 100 = 80 Ah. Ideal time: 80/19.6 ≈ 4.1 h. Peukert losses at this rate will reduce that, but a healthy battery should still give roughly 3–4 h, not 25 min.
Observed: 25 min = 0.417 h, so delivered charge is roughly 19.6 × 0.417 ≈ 8.2 Ah, about 8% of capacity. This is not normal end-of-life drift; it suggests a failed cell, severe sulfation, or a high-resistance connection or battery.
2. Interpret the specific gravity and open-circuit voltage. What does the 1.150 cell tell you, and what is the likely effect of the low electrolyte and the non-temperature-compensated charger?
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OCV 12.1 V after rest corresponds to roughly 50–60% state of charge, so the battery is not being held fully charged. The six SG readings cluster around 1.240–1.255, near full charge for good cells, but one cell at 1.150 is far out of line. A single low cell usually means that cell is weak, sulfated or has a partial short - not that the whole battery needs acid.
Low electrolyte exposing plates causes sulfation and permanent capacity loss. Topping up with distilled water is correct, but only to the specified level. The non-temperature-compensated charger at 14.4 V boost can overcharge in Assam's heat, causing water loss and grid corrosion, or undercharge in cool conditions. That helps explain the low electrolyte but not the single bad cell.
3. Decide between adding acid, equalisation and replacement. Give a step-by-step recommendation with safety and testing.
Read the worked answer
Do not add acid. Acid is only added if electrolyte was spilled; adding it to a low cell raises SG without restoring capacity and worsens stratification.
Steps:
- Top up all cells with distilled water to the correct level; never above the top of the plates.
- Give an equalisation charge at about 14.4–14.8 V for 6–8 h, with ventilation and temperature monitoring; stop if it gets hot or gasses excessively.
- Recheck SG. The low cell should rise toward the others; a difference of more than about 0.030–0.050 after equalisation indicates a failed cell.
- Load-test or perform a timed capacity test at a known load; also check charger output and connections.
If the low cell still reads about 1.150 and the capacity test gives only a few amp-hours, replace the battery. Use PPE, remove sparks and flames, and vent the area.