Care & Maintenance of Lead Acid Batteries

Electrolyte of Lead Acid Battery

Care of a flooded lead acid battery cell means keeping electrolyte strength, cell voltage and water level within the maker’s limits so the plates stay as lead dioxide and sponge lead. The electrolyte is sulfuric acid in distilled water. Concentrated sulfuric acid has a specific gravity of about 1.84. Stationary cells are often filled near 1.215 to 1.250. Some makers still specify 1.2 to 1.23. Use the figure on the battery nameplate. Cell type and climate change that number.

Chemical Action of Lead Acid Battery

The cells are recharged by reversing the discharge current in the battery. Connect the positive terminal of a DC source to the battery positive and the negative terminal of that source to the battery negative.

(Note: DC stands for “Direct Current”, also called “DC Current“)

A rectifier-type battery charger of suitable capacity is used as the DC source for charging the battery. Charging current (the reverse of discharge current) converts the positive plates toward lead peroxide and the negative plates toward sponge lead.

When a load is connected across the terminals, discharge current flows through the load and the battery begins to discharge.

During discharge the acidity of the electrolyte falls and lead sulfate forms on both plates. Water in the electrolyte increases and the specific gravity falls.

If discharge continued until both plates were fully converted to lead sulfate, the electrodes would be electrically similar and there would be no potential difference left. In service the cell is taken off load long before that point.

Cells are taken off discharge at a set minimum cell voltage and specific gravity. A flooded lead acid battery cell on charge often sits near 2.2 V with a specific gravity around 1.215 to 1.250. Open-circuit voltage of a full cell is typically about 2.05 to 2.15 V. Many makers stop discharge near 1.75 to 1.80 V per cell. A specific gravity of 1.1 is a deep discharge. Use the maker’s voltage and gravity limits rather than one pair of numbers for every cell.

Maintenance of Lead Acid Battery

Sulfation hardens when a cell sits discharged or undercharged. Hard lead sulfate then converts poorly on the next charge, so specific gravity stays low and capacity falls. Overcharge is a different failure: it gases the cell, loses water and corrodes the positive grid.

Sulfated plates look paler and feel gritty. Those cells gas early on charge and deliver less ampere-hour capacity.

Long-standing sulfation is hard to reverse. A controlled equalizing charge at the maker’s current is the usual recovery step for flooded cells. Do not treat a long low-rate charge as a cure for every aged cell.

Cell terminals corrode, especially at bolted connections. Check bolt tightness and coat the joints with petroleum jelly. Replace a cell whose posts or links are already eaten away.

Specific gravity can fall permanently as a cell ages. Common causes are:

1. Sediment at the bottom of the cell container.
2. Loss of acid as spray during charging.
3. Poor recovery after a short between plates is cleared.
4. Heavy sulfation on the plates.

If gravity is low because acid was lost by spray or spill, and the plates are not sulfated or shorted, some makers allow a small sulfuric-acid addition to restore the nameplate gravity. For stationary vented cells the usual practice is to top up with water only. Do not add acid to hide sulfation or a short.

A short between plates can come from treeing or from buckling. Treeing starts when heavy gassing loosens active material on the plates.

Loose particles fall into the electrolyte and can pile on the negative plate until they bridge to the positive plate. An ebonite scaling stick can break that bridge.

The stick is worked through the space between the plates to lift out loose material or a tree.

If the short is from buckled plates, add a separator or lift the plates and straighten them. Follow the maker before you strip a cell.

After the short is cleared, restore gravity with a controlled recharge at the maker’s current. A high-current charge after a short can overheat the cell. Use the rate in the maintenance manual.

Maintaining Battery Lead Acid Battery Room

Charging releases acid spray and hydrogen during the charging of the battery. Those gases collect in a closed room. Give the room volume and mechanical ventilation so hydrogen cannot reach an explosive mixture.

Hydrogen can ignite. Keep naked flames and smoking out of the battery room. Fit at least one exhaust fan sized for the room so hydrogen and moisture do not collect.

Keep the room above about 10^oC unless the maker states a different minimum. Capacity falls as temperature falls. Coat walls, ceilings, doors, window frames, ventilators and metalwork with an anti-acid paint on a set interval. Run wiring in metal conduit. Use flameproof lighting fittings.

Install switching devices including electrical fuses and plug sockets outside the battery room so a switching spark cannot ignite hydrogen. Finish the floor, preferably with ceramic tiles. Wash the floor and walls on a set interval.

There are some safety measures to be taken during handling storage batteries in the substation
1	Do not smoke inside the room.
2	Do not bring a flame inside the room.
3	Do not generate any spark inside the room.
4	Wear splash-proof goggles and rubber gloves while working with the battery.
5	During preparing electrolytes always add the concentrated acid little by little to the water.
6	Never pour water into the concentrated acid.

Post those points on the door or another place that is easy to see in the battery room.
The following rules apply to operation, control and emergency-lighting service of the storage battery.

1. Do not leave the battery idle for a long time. Idle flooded cells sulfate and lose capacity.
2. Do not charge at a very high current. A high rate raises temperature, increases gassing, loses water and can overflow electrolyte from the cells.
3. After a complete discharge, recharge at once before returning the battery to float. Standing discharged lets a sulfate film form on the plates.
4. Charge at the maker’s normal rate so the cells do not gas at once or rise above about 40 ^oC unless the maker states a different limit. Watch the charge. If gassing starts and temperature reaches that limit, reduce the rate. If temperature still climbs after the rate is reduced, the battery is near full charge.
5. Check the voltage of each cell before you stop the charge. Compare the readings with the previous record so a lagging cell is visible.
6. If the electrolyte level falls, add distilled water up to the mark on the cell. That replaces water lost to electrolysis and evaporation.
7. Do not fill above the marked line. Over-filling overflows on gassing and can soften the cover seal so the cell leaks.
8. Do not trust a hydrometer reading just after topping up. Added water floats until it mixes. That can take days or weeks unless an equalize charge is applied first.
9. Some flooded-cell manuals call for a controlled overcharge every few months after a discharge to the allowed limit. Follow the maker’s rate. Equalize when cell voltage or gravity leaves the maker’s band, not on a fixed calendar for every site.
10. Correct every hydrometer reading for temperature so all values refer to the same reference, usually 25 °C. Rinse the hydrometer with distilled water so it does not contaminate the next cell. Many stationary cells sit near 1.180 to 1.240. A lower gravity cuts capacity. A higher gravity attacks the plates. Use the nameplate band.
11. A substation battery usually sits on float. The charger holds a constant voltage across the battery so the cells stay near full charge. In normal load the charger feeds the dc board and also covers battery losses. When many switchgear operations coincide, the battery and the charger share the demand.
12. On float, record voltage, specific gravity and temperature of the pilot cells each day. Record the same readings on every cell at least once a month.
13. Do not dump and refill the cells on a fixed three-year cycle unless the maker says so. Normal loss is water. Replace electrolyte only after a spill or a maker-approved overhaul.
14. A pilot cell is one cell chosen to stand for the bank. Keep the same pilot for a month, then rotate to another cell so one cell is not over-sampled.
15. This is a short account of maintenance of the substation battery. Follow the maintenance manual supplied with the cells when the two differ.