12 V battery charger

A charger for the small 12 V starter battery of a motorcycle. It plugs into the mains and charges the battery off the bike or through a lead left on the bike. This is not the charger for the traction battery of an electric bike: see Charger.

What it does

A battery loses some charge every day even when nothing is switched on, so a bike that stands for a few months can have a flat, damaged battery. A charger puts the charge back slowly. A maintenance (smart or float) charger senses when the battery is full, drops to a very small current and starts again when the voltage falls, so it can stay connected through storage.

Signs it is worn or failing

• The battery gets hot, hisses or smells while charging: the charge rate is too high or the charger has failed. Switch off at once.
• The battery never reaches full charge on a good charger: the battery may be worn. Test it with Read a battery's state of charge from a chart.
• Damaged clips, cut leads or a cracked case: replace the charger.

When to replace it

Replace a charger that overheats the battery, has damaged leads, or does not have a setting for your battery type.

What to match when buying

• Battery type: flooded lead acid, sealed AGM or gel, or lithium. A lithium battery needs a charger made for lithium.
• Charge current: small. A common rule is no more than one tenth of the battery capacity in amps (for a 6 Ah battery, about 0.6 A). K-Light 125: 0.6 A for 10 to 15 hours. A large car charger can overheat and ruin a small motorcycle battery.
• An automatic (smart) charger that stops or floats when full is worth the extra cost.

Fitting notes

• Connect the clips with the charger switched off at the wall; switch off before removing them.
• Charge in open air, away from flames and sparks. Batteries give off explosive gas.
• Do not charge a frozen battery. Let it warm up first.
• Do not let a battery get hot on charge. If it does, stop and use a smaller charge current.
• How to charge step by step: Test and charge a 12 V battery. For storage: Prepare a bike for storage.

Reference (K-Light 125)