!989 Covrvette sits for 5 or 6 days and battery is only 7 volts.

Are there any warning lights on the dashboard? The next obvious thing to check is the battery. A starter needs a lot of voltage to turn it. Vehicle batteries are considered 'Old' when they are more than 5 years old. If you know how to use a voltmeter or a multimeter you can test the battery.
With the ignition key Out the battery voltage should be somewhere between 13.6 and 14.2 volts. Get a helper to crank the engine and watch the voltage reading. If it drops below 10 volts the battery could be faulty.

You have to make sure battery has a full charge and that means testing. Your saying the starter motor won't even crank the engine? I'd have to check starter wiring circuits for voltage drop?Also, make sure battery connections are good. Engine cranking, your systems get battery voltage. Engine running, the vehicle system get generator voltage which can be several volts higher than battery voltage.
Use a digital multimeter across battery posts, everything off, what is the reading, the generic spec for a full charge is around 12.6 volts. Leave the meter leads hooked up, have helper turn key to crank, what is the reading, how far does it drop? The generic spec says it shouldn't drop below 9.6 volts, that spec may be subjective but, close.

Does the battery have a full charge, battery connections all clean and tight?
With engine cranking, the vehicle systems get battery voltage. With engine running, it gets alternator voltage, there can be several volt difference.
With everything off use digital multimeter across battery posts, the generic spec for a full charge is 12.6 volts. Leave the leads on the battery posts, have a helper crank it, the voltage shouldn't drop below 9.6 volts, that is a generic spec.
The next test, w/o jumpers, have a helper crank it while you check spark at the plugs--check proper fuel pressure and fuel injector pulse. Any applicable trouble codes?

Battery ? battery cables ? Test the starting system , voltage drop starting system ! go on you tube a check out voltage drop testing starting system .

Check grounds. Look at your cables. Could be corrosion.

I'd use digital multimeter across battery posts, everything off, the generic spec for a full charge is 12.6 volts. Leave the meter leads touching the battery posts, have helper turn key to crank, what is the reading, It shouldn't drop below about 9 volts. If it drops down to almost zero, I'd have to test the battery. If it is open top battery, where you add battery water, you could use digital multimeter to check each cell, 2.1 volts.
You could do other checks? Maybe something is making the engine hard to turn over?

Check and verify that your alternator is charging properly, if you hook a multimeter up to your battery you should see a voltage between 13-14 volts on the charging side, if this is the case your alternator is good, next i would check the resting voltage of your battery that should be tested with the car off and it should be between 12-12.5 volts, if all of this checks out i would start looking into the parasitic draw on your battery while your vehical sits overnight, make sure all your courtesy lights are going off, your glove compartment light any under hood lights things like that if any lights like this are remaining on you will have a dead battery day after day because the light or draw on the battery keeps draining it of its charge....

The proper voltage acceptable is 12.4 volts (across the battery) this test does not test the seperate cells.due to the excessive drop in voltage I think your battery has a bad cell in it.A good battery might drop to 11.8 but 6.5 your battery is no good.

, do a simple test on the charging system...check battery voltage. should be 12v or better. start engine & check volts at idle, no accessories on. volts should read 13.5 -14v. Turn on all accessories(including headlamps on high) volts should drop slightly but should not creep down. Definitely not below 12.5. If voltage drops lower than 12.5, the regulator (in the alternator) is not working. either change regulator or change entire alternator. Alternator drive belt should have sufficient tension so that alternator cannot be turned easily by hand. If it can, check belt tensioner.