Is it my altenator?? on 2006 Lexus GS430

First i thought it was the battery but its fine. When i try to start the car i have to hold in the buttom and its a delay then when i it starts i can hear how it drains the battery..??
best regards Thor

Asked by for the 2006 Lexus GS430
It's usually bad contact and plunger in the starter solenoid that does this. Turn on the ignition switch to the on but not run position do you see an icon on the instrument panel representing a battery, if you do not see this battery icon the alternator is not seeing power to allow it to charge. Start the engine does the battery icon go out. A more complex but good diagnostic test is checking "voltage drop" on the battery ground and power cables. You can find a video on how to do a voltage drop test on youtube. Disconnect the plug at the back of the alternator on of the smaller wires should have switched power via the ignition switch (ignition key on the wire should have power, ignition key off should not have power) if at least one of the small wires has no power with the ignition key on you need to trace the wiring for an open cicuit. The larger wire at the alternator should have 12 volts all the time, Put a volt meter across the battery terminals with the engine off, at rest the battery voltage should be 12.6 volts if the battery is fully charged. Now get a friend to start the car while you watch the voltmeter, the voltage reading across the battery should read approximately 14.6 volts if the alternator is charging correctly.