driving time needed to charge battery.
 

I remember being told in the old days, that when the weather is cold, it takes 20 minutes of driving to recharge the battery. Does this still hold true? I don't drive the Z much in the winter, but I like to take it out when the weather is good for short drives.

Everyone is gonna tell you to buy a $15-25 battery tender. And I agree

I have a battery tender, but normally the car never sits more than a couple days without being driven. It is also in a heated garage. The only time I use the tender is if we had a snow storm and then it might sit for a week. It is just that sometimes I don't drive very far. The problem with using the tender is that there is no convenient outlet to plug it in. I have to get a ladder and run an extension cord up and over other cars.

This! ^

The battery get charged once the car is started from the alternator and depends on the level of charge of the battery, it may or may not be completely charged in 20 min. It takes a lot of amps to start the car and my guess is that 20 min. probably wouldn't replenish the amps it uses to start the car.

A lot of variables involved but 10-15 minutes is usually all it takes to recharge.

For the record the alternator does NOT "charge" the battery it only maintains the battery's charge. In the old days the alternator did charge the battery but these days it's not going to charge up a battery that is noticeably drained.

Get a battery tender and call it a day. In your case is there any chance you could just mount a retractable cord reel over where you park your Z? If so this solves the problem of having an extension cord draped across your garage.

20 min it just a cover all for all cars, be it an old school large displacement v8 that might take a bit if time to start or just a car with a cheap alternator/charging system. It probably takes a few hundred amps to start our car for a second or two. It should take more than 10min to make that back. If your dd for less than 10min there are other issues to worry about too, like engine not getting up to temp before you shut it off again. Alternators do charge batteries, they just do it slowly usually at only a couple amps, where a auto battery charger can do it at 6 or 9 amps, which is why if you have a dead battery and get a jump after 30min or an hour you can shut your car off and start it normally.