12 V battery

The 12 V battery is a small box, usually under the seat or a side cover, with a red positive (+) and a black negative (-) terminal. Most commuter bikes now use a sealed lead-acid (AGM, "maintenance-free") battery; older ones use a flooded battery with filler caps; some use lithium. Electric motorcycles also have a small 12 V auxiliary battery, separate from the traction pack.

What it does

The battery powers the starter and, on most bikes, the ignition, lights and horn. The regulator rectifier recharges it while the engine runs. A lead-acid battery left flat grows hard sulphate on its plates and loses capacity. Heat, overcharging and deep discharge shorten its life.

Signs it is worn or failing

• Engine turns over slowly or starter relay clicks.
• Battery goes flat after parking or a new battery dies quickly.
• Battery is boiling, swollen or leaking: stop using it.
• Corroded terminals.

When to replace it

When it will not hold charge after a full slow charge, or is swollen, cracked or leaking. A good voltage at rest is not proof of capacity; a battery can read well and still sag under load. Check charging too, so the new battery is not killed: Test the charging system.

What to match when buying

• Match the code, voltage, capacity (Ah), size, terminal position and type. In a code like YTX7A-BS: Y is the maker (Yuasa), T means sealed AGM construction, X is the product family, and BS means terminals and packaging, with acid supplied in a separate pack. Other makers' batteries with the same letters after the first are made to fit the same place.
• Do not fit a flooded battery where a sealed one was fitted, as the K-Light manual warns. Lithium needs a compatible charging system and charger.
• Check the date code; an old battery that sat uncharged for months may already be weak.

Fitting notes

• Remove negative first and fit positive first. Acid burns skin and eyes; charging gives off explosive gas, so keep sparks away.
• Home jobs: Test and charge a 12 V battery, Replace a 12 V battery, Clean and tighten battery terminals.
• Never jump start from a running car: Jump start a motorcycle safely.
• Do not charge a frozen battery. Let it thaw first.
• A battery sold dry must be filled and charged as its maker says: Fill and activate a new dry-charged battery. A pre-filled one that sat on a shelf for months should be charged before use.

Reference (K-Light 125)