My Kawasaki ER-6 gives a buzzing sound when I hit the starter button and that’s it. Could this mean that the battery needs replacing, as it sounds like it’s struggling to kick in.
The buzzing noise you can hear is the starter relay, if there isn't enough power in the battery to turn the starter motor and engine then the relay will arc inside causing this buzzing sound. So the battery needs charging first off, then you will need to find out if it’s a duff battery or the charging system is failing or failed (ie, not charging the battery). Charge the battery (with an Optimiser-type charger) then once fully charged, take off the charger and leave to stand for an hour or two. Check the volts with a voltmeter. If you are getting 12.5-12.8 volts your battery is OK. If the battery is reading less than 12 volts then it will need replacing as a cell is probably on the way out or just not holding a charge. If you find the battery is giving higher than 12.8 volts after an hour standing, then the charging system is overcharging the battery and it will overheat and die.
If the battery is OK refit it to the bike, get the bike going and the engine warmed up then check the battery again, you should be getting 14.5 volts optimum charge, anything above this is overcharging. Then that’s going to be the regulator/rectifier. If you find the battery (in good condition) is giving less than 13 volts then the charging system will need investigating. The most common problems are a faulty regulator/rectifier, but it can also be a poor connection at the reg/rec connector block, a blown fuse, poor earth or earth connections, or high resistance in the generator to reg/rec plugconnection. In extreme cases if there is no power at all, but the battery is good, then the generator may have failed.