A lead-acid bank lasts when it is kept topped up with distilled water, kept clean at the terminals, checked cell by cell with a hydrometer, equalized now and then, and never left discharged. Measure its true capacity by discharging it through a known load until it falls to 10.5 V for a 12 V battery, then dividing the ampere-hours delivered by its rating, because a battery that reads full on a voltmeter can still be failing.
Part of Battery Building and Maintenance
The routine checks that extend a lead-acid battery’s life, and how to test, diagnose and recondition one that is failing.
Low Water, Sulfation And Corroded Terminals Are The Failures The Corpus Names
The carrier names the two recurring causes: a low electrolyte level and sulfation from being left discharged. Energy Storage & Batteries adds corroded terminals. A battery stored discharged sulfates, with the sulfation becoming permanent within weeks, so charge it fully before storing and top it up monthly. The remedy for every item below is regular inspection.
| Task | How often | Detail |
|---|---|---|
| Electrolyte level | Monthly | Distilled water only (Water Level Maintenance) |
| Terminals | Monthly | Clean white or green crust with baking soda and water, then grease |
| Specific gravity | Monthly | Each cell separately (Specific Gravity Control) |
| Equalization | Monthly to every two months | Flooded cells only (Equalization Charging) |
| Capacity test | When in doubt | Discharge test below |
Symptoms Point To Particular Faults
- Charges quickly, dies quickly: the energy-storage article gives this as the sign of sulfation (Desulfation Techniques).
- One cell low in gravity or voltage after a full charge: the lead-acid article says cells below 1.200 after a full charge indicate sulfation damage; a cell reading near zero has an internal short.
- 12.6 V at rest but collapses under load: damaged plates (Cell Voltage gives the example of 12.5 V falling to 10 V under a 10 A load). The energy-storage article’s load test uses a 55 W headlight bulb for 30 seconds: voltage staying above 11.5 V is serviceable, below 10 V means internal damage.
- Warm case on charge, or gassing from the start: overcharge, or a short; reduce the charge voltage.
- Brown sediment, case swollen, white crystals leaking at the terminals: beyond recovery (the salvage article puts a swollen, cracked, leaking case in that category).
What You Need
- A voltmeter, a hydrometer and a thermometer
- Distilled water, baking soda, grease or petroleum jelly
- A known load — a resistor or bulbs that draw a steady current (Handmade Resistors)
- Insulated tools, eye protection and gloves
Method: A Discharge Capacity Test
Step 1: Charge the battery fully and let it rest.
Step 2: Connect the load and record the current, voltage and time. Keep the current steady.
Step 3: Stop when the battery reaches 1.75 V per cell (10.5 V for six cells; Rechargeable Batteries). Going lower hardens sulfate.
Step 4: Capacity = current × hours. Compare with the rating, and recharge at once.
Worked Example: A Test On A 100 Ah Battery
Discharge at C/20: 100 ÷ 20 = 5 A. At about 12 V the load resistance is 12 ÷ 5 = 2.4 Ω, dissipating 5 × 12 = 60 W. If the battery reaches 10.5 V after 16 hours, it delivered 5 × 16 = 80 Ah, which is 80 ÷ 100 = 80% of its rating. A lower result shows lost plate material or sulfation. The test is the real measure: the metering article puts a resting 12.0 V at roughly 50%, whereas two other corpus tables put the same voltage at about 25%, so resting voltage cannot settle a capacity question.
Common Mistakes
- Topping up with tap water — minerals poison the plates
- Testing to zero volts — it ruins the battery being tested
- Equalizing a bank with a dead cell — the dead cell overheats
- Skipping the monthly checks because the battery “seems fine” — the first sign is usually a failed night
Safety
Maintenance work sits between acid, hydrogen and a short
Cleaning terminals puts a wet tool and a spanner within reach of both poles, and a metal tool across them welds in place with enough current to rupture a cell. A battery that has just been charged holds hydrogen at the top of each cell and its room. Wear splash goggles and gloves, remove rings and watches, use insulated tools, and keep sparks and flames well away (Acid Handling, Hydrogen Gas Hazard).
What’s Next
- Learn next: Water Level Maintenance — the commonest chore
- Combine with: Desulfation Techniques — trying to bring back a sulfated bank