The battery is good and charges while riding, but it goes flat when the bike is parked for a few days. Something keeps taking current with the key off. A clock, alarm or ECU memory takes a tiny amount by design. More than that means a fault: a stuck brake light switch, a faulty key switch, a leaking regulator diode, an accessory wired straight to the battery, or a damaged wire touching metal. The K-Light 125 manual allows a key-off leakage of 1 mA or less, and names the brake light, the regulator, the battery and the ignition switch wiring (PDF page 36).
Is it safe to ride?
With care. The bike may not start after parking. A damaged wire that drains the battery can also overheat.
Quick checks first
- Brake light lit with the key OFF?
- Any accessory (alarm, tracker, USB socket, extra light) wired directly to the battery?
- Does the key turn fully to OFF? Is any dashboard light on?
- Any warm wire, burnt smell or melted insulation?
- Is the battery itself good (holds charge off the bike for a day)?
Diagnose it step by step
1Does the battery hold its charge for a day when disconnected from the bike?
- No: the battery is faulty, not the bike. Test and charge a 12 V battery.
- Yes: go to step 2.
2Is the brake light on with the key OFF?
- Yes: Test and adjust the brake light switches.
- No: go to step 3.
3Measure the key-off current. Is it above the limit? (K-Light 125: 1 mA or less)
- No: there is no drain. Look at short trips or charging instead: Battery goes flat after the bike is parked.
- Yes: go to step 4. Use Find what is draining the battery.
4Does the current drop when you disconnect an accessory?
- Yes: rewire it through a switched, fused feed. Wire an accessory safely.
- No: go to step 5.
5Does pulling one fuse drop the current?
- Yes: follow that circuit with the wiring diagram, halving it until you find the fault. Find a short circuit.
- No: go to step 6.
6Does unplugging the regulator drop the current?
- Yes: the regulator leaks current. Test the regulator rectifier.
- No: test the key switch (Test the ignition switch) and the unfused wires from the battery to the starter relay.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Brake light switch stuck on | Brake light lit with key off | Test and adjust the brake light switches |
| Accessory wired straight to battery | Current drops when unplugged | Wire an accessory safely |
| Leaking regulator | Current drops with regulator unplugged | Test the regulator rectifier |
| Faulty key switch | Continuity in OFF | Test the ignition switch |
| Damaged wire touching metal | Current drops on one fuse; chafed wire | Find a short circuit |
Reference values (K-Light 125)
| Item | Value | PDF page |
|---|---|---|
| Key-off leakage current | 1 mA or less | 36 |