How to Charge and Handle Lead-Acid Batteries Safely

Why this matters

Lead-acid batteries sit in more of the homes you service than any other big battery: the backup sump pump in the basement, the uninterruptible power supply (UPS) on a security or network cabinet, the starting battery on a standby generator, the deep-cycle battery on a boat lift or a gate. They look inert. Charging one makes hydrogen gas, which burns in air across a wide range, and the electrolyte inside is sulfuric acid, which blinds. The battery explosions that injure techs almost always happen at the terminals, at the moment a clamp goes on or comes off. This is the procedure that keeps the spark and the gas apart and the acid off you. Lithium batteries are a different chemistry with different failure modes; see related: Lithium-Ion Batteries in Tools, Vans and Customer Equipment.

What you are dealing with

Sulfuric acid. The electrolyte in a lead-acid battery is dilute sulfuric acid, roughly 30 to 40 percent by weight in a charged flooded battery. It causes chemical burns to skin and severe, permanent eye injury, and it eats cotton clothing.

Hydrogen. Charging splits water in the electrolyte into hydrogen and oxygen, slowly during normal charging and fast near full charge, during overcharge and during an equalizing charge. Hydrogen's lower flammable limit is about 4 percent in air, it has almost no ignition energy requirement (a small spark lights it), and it is invisible and odorless. It rises and collects at ceilings and inside battery boxes and enclosures. A battery that explodes typically blows its case apart and sprays acid.

Three construction types, because the type changes the procedure:

• Flooded (vented): removable caps, visible liquid electrolyte, vents freely during charging, needs water topped up.
• AGM (absorbed glass mat) and gel: both are valve-regulated lead-acid (VRLA) batteries, sealed with relief valves. They vent far less in normal use, but they do vent hydrogen when overcharged or overheated, and an overcharged VRLA battery can enter thermal runaway, heating, swelling and venting.

Lead. The plates and terminals are lead, and terminal corrosion is lead and lead sulfate. Handling the battery puts lead on your hands; wash before eating or smoking. See related: Lead Exposure for the Worker, Not the Occupant.

The procedure

1. Look at the battery before you touch it. Check for a cracked or bulging case, wet acid on the top or the floor, heavy white or blue-green corrosion, and heat (a battery warm to the touch when it has not been charging is wrong). Why: a damaged, swollen or hot battery is the one that fails during handling. What breaks if skipped: you put a clamp on a battery that is already venting, or lift one with a cracked case and acid runs down your arm. A swollen, hot or leaking battery is not charged; it is disconnected at the source (step 4) and replaced.

2. Put on splash goggles and a face shield, acid-resistant gloves and long sleeves. Goggles under the shield, not safety glasses alone, because a battery explosion and an acid splash come from below and around the frames. Neoprene, butyl or heavier nitrile gloves resist dilute sulfuric acid. Why: the face is where a terminal explosion goes. What breaks if skipped: an eye injury that a face shield would have stopped. In construction work, 29 CFR 1926.441 requires face shields, aprons and rubber gloves for workers handling acids or batteries; general industry rules reach battery charging under 29 CFR 1910.178(g) for industrial truck batteries, and the shop's hazard assessment under 29 CFR 1910.132(d) sets the PPE everywhere else.

3. Know where water is before acid is out. Find the nearest sink or hose, or carry a portable eyewash bottle, before you open caps or move a battery. 29 CFR 1926.441(a)(6) requires facilities for quick drenching of eyes and body within 25 feet of battery handling areas in construction; 29 CFR 1910.151(c) requires suitable drenching and flushing facilities wherever a worker's eyes or body may be exposed to corrosives in general industry. Why: an acid splash in the eye needs flushing within seconds, not after a search. What breaks if skipped: the minute spent finding water is the minute the injury deepens.

4. Remove power before touching terminals. On a sump backup or UPS, unplug the charger or controller from the wall and switch off any inverter. On a generator, set the controller to off and remove its fuse or disconnect per the manufacturer's manual so it cannot crank. Why: a live charger or a crank command makes the terminal spark you are trying to avoid. What breaks if skipped: a generator that starts while your hand is at the battery, or a spark from a charger still pushing current.

5. Ventilate the space and remove ignition sources. Open the door to the room, run a fan that moves air out of the space (with its motor away from the battery top), and keep flames and sparks away: a gas water heater or furnace pilot nearby, a grinder, a cigarette. Never charge inside a closed battery box or cabinet with its lid shut. Why: hydrogen from a battery that has been charging builds under lids and at the ceiling. What breaks if skipped: the first spark at the terminal finds a pocket of gas.

6. Disconnect negative first, connect negative last. Removing a battery from equipment, take the negative (ground) cable off first; installing, put it on last. Why: with the negative off, a wrench that touches the positive terminal and the frame at the same moment cannot complete a short circuit. What breaks if skipped: a wrench welding itself across the terminal and frame, with a spark at the battery top.

7. Charge with the charger off while you connect. Connect the charger clamps to the battery, positive to positive and negative to negative, with the charger unplugged; then plug it in or switch it on. Reverse to disconnect: off first, then clamps. Use a charger matched to the battery type (flooded, AGM or gel settings differ) and voltage. Why: the spark that ignites hydrogen happens when current starts or stops at a clamp. What breaks if skipped: this is the exact moment most battery explosions happen.

8. Watch it while it charges. A battery that gets hot, swells, hisses, smells of rotten eggs (hydrogen sulfide from overcharging) or bubbles violently gets unplugged at the wall, not at the clamps, and left to cool. Why: those signs mean overcharge or internal failure, the path to thermal runaway in a VRLA battery. What breaks if skipped: the battery vents or ruptures while you are elsewhere in the house.

9. Top up flooded batteries with distilled water only, after charging. Electrolyte expands during charging, so filling before charging makes it overflow acid. The exception: if plates are exposed, add just enough distilled water to cover them before charging. Never add acid. Why: overfilling sprays acid out of the vents. What breaks if skipped: acid on the battery top, the floor and you.

10. Neutralize spills with baking soda, and clean terminals the same way. Sprinkle sodium bicarbonate on spilled acid until fizzing stops, then wipe up with wet rags, wearing your gloves and eye protection. Why: neutralized residue is safe to wipe; raw acid is not. What breaks if skipped: acid left on the floor eats a customer's floor and burns the next person.

11. Move and dispose of it correctly. Carry the battery upright with both hands or a carrier strap, terminals covered, secured upright in the truck. A spent lead-acid battery is recycled, either through the federal spent lead-acid battery provisions at 40 CFR Part 266 Subpart G or as universal waste under 40 CFR Part 273, and most states require retailers to take used ones back. See related: Universal Waste Explained for a Service Shop.

Worked example: a backup sump that failed during an outage

A plumber is called because the backup sump pump did not run during a 6-hour power outage and the basement took water. The system is a 12-volt backup pump with a controller-charger plugged into a basement outlet, and a flooded deep-cycle battery in a plastic box on a shelf about 4 feet above the floor. A gas water heater with a standing pilot is about 6 feet away. The customer says the basement water was 2 inches deep at its worst and has since been pumped out by the primary pump, which is on its own circuit.

Electrical first. Water touched the floor but not the shelf, and the outlet is a GFCI that has tripped. Before touching the controller, the plumber confirms the basement floor is dry where he will stand and the controller and its cord never sat in water; a controller that was submerged is replaced, not tested.

Step 1 findings. The battery box lid is closed. Opening it, the plumber finds the top wet, the caps crusted white, and the case slightly bulged on one side. The battery is not hot. A bulged, leaking case fails step 1: it is not charged.

Steps 2 to 5. Goggles, face shield and neoprene gloves on. He fills a bucket at the laundry sink about 15 feet away before going further. He unplugs the controller from the outlet. He opens the basement door and runs a fan at the bottom of the stairs blowing out, and asks the customer to let him turn the water heater to pilot off for the 20 minutes he will be at the battery, because the box has been closed over a charging battery and he does not know what is under the lid.

Steps 6 and 10. Negative cable off first, then positive. He neutralizes the acid on the battery top and in the box with baking soda, wipes it, and lifts the battery out upright with a carrier strap into a plastic tub for the truck.

Installing the new battery, steps 6 and 7. A new flooded deep-cycle battery of the type the controller lists goes in the box, positive cable on first, negative last. With the clamps connected, he plugs the controller back into the GFCI outlet after resetting it.

Restoring the backup, with its check. Before closing up, he reads the controller's status display, which shows charging, and checks that the box has its vent openings clear rather than sealed. He lifts the backup pump's float by hand for a few seconds with the controller unplugged from the wall, so the backup runs on battery, confirming it pumps, then plugs it back in. He relights the water heater pilot following the appliance's labeled instructions only after the fan has run and he is done at the battery.

What would change the call. If the battery had been hot or hissing, he would have unplugged at the wall, left the room ventilated, kept the customer out of the basement, and waited for it to cool before moving it. If it had been an AGM battery, step 9 does not apply, but steps 6 and 7 are identical. If acid had gotten in his eye, he flushes immediately with the bucket water for at least 15 minutes, continuously, and goes to emergency care, calling 911 if vision is affected or the pain is severe.

How to tell you did it right

• The old battery left upright, covered and secured, headed for recycling.
• The new battery's controller shows charging, and the backup ran on battery when tested.
• No acid residue on the shelf, floor or box, and no ignition source near the battery that was not there before.
• The pilot relit after the work, not during it.

References

• OSHA 29 CFR 1926.441, batteries and battery charging (construction); 29 CFR 1910.178(g) and 29 CFR 1910.151(c), general industry
• OSHA 29 CFR 1910.132(d), hazard assessment and PPE selection
• EPA 40 CFR Part 266 Subpart G and 40 CFR Part 273, spent lead-acid battery management
• See related: Lithium-Ion Batteries in Tools, Vans and Customer Equipment; Universal Waste Explained for a Service Shop
• See related: Acid and caustic handling in the library's chemical safety cards