2000 Boxster S Battery
 

Think my battery is done. It's old and not holding charge. Leaving the door open for a few minutes drains it. Can I go with a larger battery? Would the alternator be able to keep up? Any other downsides? I think the metal clamp that holds the battery can be slid sideways to accommodate a bigger one?

I would not mess with putting in a bigger battery. If your charging system is working properly, and the car is driven regularly, the specified battery will last. If the car is not driven frequently, then you need to get a battery tender.

Pep Boys has a Bosch battery for around $100.

Get an optima. Smaller but lasts way longer and more powerful.

The major parts stores, like O'Reilly, will test your batter & charging system for free. I have a generic O'Reilly battery in my 2000 S and it works fine. In fact, I think the old Walmart battery that came with the car was actually still good when I replaced it.

After I bought the new battery I was golden for about a month, then the new battery went dead too. You'll want to check your car's voltage. With the motor running a little beyond idle you should have around 14.3 volts measured across the battery terminals. If it's less you could have a weak or bad alternator. O'Reilly sells remanufactured alternators. They're different between manual and Tiptronic cars. Replacing the alternator is a feasible DIY but it's tougher than a "regular" car.

There are some internet threads about replacing only the voltage regulator and I tried that first but it didn't help my car. It's just as much work to replace the voltage regulator (it's mounted on the alternator body) as it is to replace the whole unit. I wish I'd have just put the whole unit in the first time because it's kind of a hard job to pull the alternator out.

Thanks guys. I just got the Bosch from Pepboys. I'll test the voltage with the car running tomorrow.