Replace a 12 V battery

Home mechanic (basic tools) · Time: 20 to 40 min, plus first charge (estimate)

Fit a new battery when the old one is swollen, leaking, cracked, or will not hold a charge after a full charge. Before you buy, test the charging system if you can: a faulty regulator will kill a new battery too.

Safety

• Glasses and gloves on. Battery acid burns.
• Key OFF. Do not unplug the battery while the engine or ignition is on: the voltage spike can damage the regulator and the ECU (engine computer) (K-Light 125 manual).
• Remove the negative (−) cable first and fit it last, so a spanner touching the frame cannot cause a short.
• A swollen, leaking or hot battery may spill acid or, if lithium, catch fire. Lift it out upright, keep it outside, and take it to a battery recycler. Do not put it in household rubbish.

You need

• The correct battery. Check the old label: size code, voltage, capacity (Ah), terminal positions and type. K-Light 125: YTX7A-BS, 12 V 6 Ah.
• Check that the new battery fits the tray and the cables reach the terminals with + and − on the correct sides.
• Ask before changing type. A sealed (AGM) battery should not be replaced with an old style flooded battery on a bike made for sealed ones (K-Light 125 manual). A lithium battery is only suitable if your bike maker and the battery maker say so, and it needs a lithium charger.
• A new "dry" or acid pack battery must be filled and charged before use. A factory activated battery may still need a charge: check its voltage.

Steps

1. Key OFF. Put the bike on the centre stand or another firm stand.
2. Remove the cover. K-Light 125: left side cover.
3. Undo the negative (−) cable first, then the positive (+).
4. Remove the vent tube if there is one, then the strap or holder, then lift the battery out upright.
5. Clean the tray and the cable ends. White or green powder: scrub with a little baking soda in water, rinse with clean water and dry. See Clean and tighten battery terminals.
6. Check the cables for cracks, melted insulation or loose crimps. Replace damaged cables.
7. Prepare the new battery as its instructions say (fill if needed, then charge). Measure it: it should read the "charged" value. K-Light 125: about 13.1 V. Use Test and charge a 12 V battery.
8. Put the battery in the tray the right way round. Fit the holder.
9. Flooded battery: fit the vent tube with no kinks, routed away from the chain and frame. Acid from a blocked or misrouted tube rots the chain and frame.
10. Connect the positive (+) first, then the negative (−). Tighten firmly; do not crack the terminal.
11. Put a thin film of grease on both terminals, never on the case.
12. Refit the cover. Turn the key ON and check that the lights and dashboard work.

Check your work

• The engine cranks strongly.
• The terminals cannot be turned by hand.
• No cable rubs on the frame or can touch the other terminal.

If it did not fix the problem

• New battery flat again within days: A new battery goes flat quickly.
• Battery boils or swells again: Battery overcharges and bulbs blow.
• Still slow to crank: Engine turns over slowly.

Reference values (K-Light 125)