Battery is boiling, swollen or leaking

The battery case bulges, it is hot to touch, it hisses or bubbles, it smells of rotten eggs or acid, or acid is leaking onto the frame. This is a failed battery and often a sign of overcharging, a charger set too high, an internal short, or physical damage. A lead acid battery that boils gives off hydrogen, which explodes, and sulphuric acid, which burns. A swollen lithium battery can catch fire. Electric motorcycle traction packs are a different and more dangerous case: see Battery pack is dropped, dented, swollen or wet.

Is it safe to ride?

No. Stop now. Switch off the engine and the key.

Quick checks first

• Do not charge it, jump it or try to start the bike.
• No flames, cigarettes or sparks nearby.
• Wear glasses and gloves before touching it. Let a hot battery cool outdoors.
• If acid touches your skin, wash it off with plenty of water (baking soda helps neutralise it). If it gets in your eyes, rinse with clean water for a long time and get medical help.
• If a lithium battery is smoking, move away and keep people back.

Diagnose it step by step

1Is the battery smoking, very hot or (lithium) giving off smoke or flames?

• Yes: move away. Do not touch it. Use a fire extinguisher only if it is safe, and call for help.
• No: go to step 2.

2Once cool, remove it: negative (−) first, then positive (+), keep it upright. Is the case swollen, cracked or leaking?

• Yes: it must be replaced. Take it to a battery recycler. Clean acid off the tray and frame with baking soda and water, then rinse. Replace a 12 V battery.
• No (it only got hot): go to step 3.

3Was it on a charger, and was the charger the right type and rate? (K-Light 125: 0.6 A standard rate)

• No, a car or fast charger was used, or the wrong type for lithium: the charger caused it. Replace the battery and use a suitable charger next time. Test and charge a 12 V battery.
• It happened on the bike: go to step 4.

4Before fitting a new battery, does the charging voltage stay in range? (K-Light 125: 14.5 ± 0.5 V at 5000 rpm)

• No, too high: fix the regulator first. Battery overcharges and bulbs blow, Test the regulator rectifier.
• Yes: the battery had an internal fault or was very old. Fit the correct new battery.

5Flooded battery: is the vent tube blocked, kinked or missing?

• Yes: gas could not escape and acid was pushed out. Route a new vent tube with no kinks, away from the chain.

Causes, most likely first

Reference values (K-Light 125)