A charging lead-acid battery vents hydrogen, which is explosive in air at low concentrations. And the electrolyte is sulfuric acid, which causes chemical burns and permanent eye injury. Both risks are manageable, but neither is theoretical.
Ventilation and ignition sources
Charge outdoors or in a well-ventilated space, never in a sealed room, a closed cupboard or a small unventilated garage. Hydrogen is lighter than air and collects at ceiling level, so a low, enclosed space is the worst case.
Keep every ignition source away from the battery while it is charging and for a while afterwards. That means no smoking, no open flame, no grinding or welding nearby, and no switching things on and off close to the battery. A single spark at the terminal is enough — and the most common source of that spark is someone disconnecting a clamp while the charger is still running.
Connection order matters
Switch the charger off and unplug it before touching either clamp. Connect and disconnect in an order that keeps sparks away from the battery: when removing a battery from a vehicle take the negative terminal off first and put it back on last, so a slipped spanner cannot short the positive terminal to the chassis.
Take off rings, watches and metal bracelets first. A wedding ring bridging a terminal to metal can heat to burning temperature almost instantly, because a car battery will happily deliver hundreds of amps into a short.
Never rest tools on top of a battery.
Protecting yourself from the acid
Wear splash-proof goggles, not ordinary glasses, and acid-resistant gloves. Cover your arms and skip the clothes you care about, since acid holes show up in fabric a day or two later.
Keep baking soda within reach. It neutralises spilled electrolyte, and a paste of it also cleans terminal corrosion. Know where the nearest running water is before you start.
Flush with plenty of clean running water immediately and keep flushing for at least fifteen minutes. For any eye contact, get medical attention — keep flushing on the way. Do not try to neutralise acid on skin or in eyes with baking soda; use water.
Things not to do
- Do not charge a frozen battery. A deeply discharged battery can freeze, and charging one in that state risks rupturing the case. Let it warm up first.
- Do not open sealed AGM or gel batteries. They are not designed to be topped up, and the vent caps are not user-serviceable.
- Do not use tap water to top up cells. Distilled water only, and only on flooded batteries with removable caps. Top up after charging, not before, and only to the marked level.
- Do not leave a charging battery unattended for long periods, particularly with any charger that lacks automatic shutoff.
- Do not apply desulfation or equalisation routines to sealed batteries unless the manufacturer explicitly permits it. Overcharging a sealed battery drives it into thermal runaway.
- Do not attempt any of this on lithium cells. High-voltage or pulse reconditioning on lithium-ion is a fire and explosion risk. Different chemistry, different rules.
Disposal
A battery that cannot be recovered should not go in household waste. Lead-acid batteries are one of the most widely recycled products there is, and most auto parts shops, scrap merchants and municipal recycling points take them free of charge. Many also pay a small core charge or credit toward a replacement, so returning it costs you nothing and often saves you a little.
This page covers general precautions and is not a substitute for the safety instructions supplied with your battery, your charger or your vehicle. Where those differ from anything here, follow the manufacturer. If you are unsure at any point, a workshop can do this for a modest fee.