You put your key in the ignition and turn it. Your bike won't start. You need to find out why. You first have to determine whether you have a low battery, a dead battery or some other issue. Loose or dirty battery terminal connections need to be fixed right away. Clean, tight connections are essential.
If your lights come on, the bike won't turn over and you hear a buzzing sound (the starter relay) when you hit the starter button, you have a low battery voltage condition. This situation calls for charging the battery properly and then finding out whether it will remain alive or not with a load test.
Sometimes, your bike will crank over without starting and gradually slow down until it's dead and buzzing. This situation also calls for charging the battery properly and then finding out whether it will remain alive or not by load testing.
If your lights don't come on and nothing happens when you press the starter button, you have a dead battery, a blown main fuse or some other issue like a loose battery cable. The solution here is to install a known good and fully charged battery, verify the basics (good connections) and try again.
Motorcycle charging systems are basically low-tech and require proper care and feeding in order to work properly. The engine produces electricity, the regulator/rectifier manages electricity and the battery stores electricity. The first thing to know is that bike batteries can go bad and die from just a little bit of neglect. Lack of use is the kiss of death. Using a bike daily generally keeps the battery charged up, as long as it's not being ridden in constant short-trip mode. If your bike sits parked and unused for periods of time, the best thing you can do is to keep your battery juiced and maintained with an automatic charger. This is your best plan for having your bike start when you need it to. An automatic maintenance charger is cheap when compared to the cost, hassle and downtime of replacing a dead battery and will pay for itself in more ways than one. Don't cheap out- if you have a bike, you should have the appropriate charger for it. Not all chargers have an auto function, so check before use. You can easily kill a battery by overcharging it with a constant output charger.
Your battery is tapped into bigtime in order to start the bike. Cranking it over draws a lot of amps, so the battery has to be in good condition. The charging system gradually replenishes the juice used in start-up. On most motorcycles, there is a 3 winding stator coil pack and rotary permanent magnet (rotor) on the end of the crankshaft that spins when the engine runs that produces AC electricity. This juice then proceeds through the stator harness to the regulator/rectifier. Several things happen here. The AC electricity is changed into DC electricity by what's called a diode bridge in the rectifier. This component "rectifies" AC output into usable DC output so the bike can run. The voltage regulator then decides just how much juice to send back to the battery for recharging purposes and how much is syphoned off and lost as heat through the cooling fins on the regulator/rectifier body. Note: The charging system only works and puts out decent power when the bike is revved up to the 3,000 to 4,000 rpm range and above. Engine speed matters. At idle, the system is basically running at a loss because the stator output is minimal.
If you have a dead battery and replace it without checking the system vitals, expect it to go dead again. This replacement may get you running again temporarily, but you really need to find out why your other battery died in the first place so you don't end up stuck someplace with a dead bike. Testing a charging system is fairly straightforward. Your battery needs to be fully charged in order to test the charging system. You'll need a multi-meter in order to pin things down. The first thing to check is your DC charging voltage at the battery terminals. Set the meter to 20 volts DC and attach the positive and negative leads to the battery. This will give you your basic battery charge condition. A fully charged battery will have a free voltage reading of 12.6 to 12.8 volts. Start the bike, allow it to warm up and rev it up to 3,000 to 4,000rpms. Look at the DC voltage at the battery. You want to see about 13.5 to 14.5 DC volts when the revs are up. If it's above or below this range, you need to perform further tests. Shut the bike off and hook up the battery charger while you get ready.
If your DC voltage reading at the battery is above the 14.5 volt range, your voltage regulator might be defective. Too much electricity will roast that battery. Replace the regulator/rectifier and retest.
If your DC voltage is below the 13.5 volt range, your stator AC output needs to be tested. Generally, there are 3 wires coming off of the stator that go to the regulator/rectifier. Locate the stator harness (generally coming out of the left side engine cover) and disconnect the harness from the regulator/rectifier. You'll have 3 wires inside the gang connector, A, B and C. Set your multi-meter to 100 volts AC and check the readings between A to B, B to C, and C to A when the bike is revved up to 4,000rpms. Each of these 3 results should be the same, generally in the 50 to 80 volts AC range. If you find one or more winding with a low or zero reading, your stator is burned up and needs to be replaced. This replacement may fix things with no further work, but I have seen defective stators take out regulator/rectifiers because of the erratic voltage outputs. Testing the rectifier calls for an ohm test of the diode bridge to see if there are any bad diodes present. You'll need the service manual for your bike to identify the appropriate wires to test.
A bad stator or regulator/rectifier can also take out a battery. A well maintained battery will typically last about 5 years or so. If the battery is ever discharged or neglected, this lifespan can be shortened by quite a bit.
When all is said and done, a final DC voltage test should be done to verify the repairs were successful and you're good to go. If you're still feeling a little lost, it's best to get help from someone who knows the testing process. This will go a long way to taking the mystery out of charging systems and getting you back on the road.