A good friend of mine just bought a 2003 - 50th AE convertible 2 weeks ago. The car only has 3300 miles. When we test drove it, we noticed it had a fairly new battery, not the OEM one. The owner hardly ever drove it and I'm sure it just sat without a battery tender on it.
Well, my friend went to drive the car today and the battery was dead. He charged it up and drove it a little and the 'check engine' light came on. He went home and checked the OBD-2 codes. This is what came up; P1518 TAC Module Serial Data Circuit
B0503 and B0508 RH and LH DRL Relay Circuit

He was wondering if the battery just needs a good charging, or a new battery, or if there is a drain somewhere.

 

You wouldn't really expect a 6+ year old car ( even with only 3300 miles) to have the factory battery. You probably have a run down battery. I'd start by putting a Battery Tender style charger that shuts off or floats when the battery is fully charged on the battery. Could take a few days to fully charge if the battery is that low. Driving a car to try to charge a low battery is usually not a good idea and takes forever to charge the battery up anyway. I'd also clear all the codes because if there is a real issue, it will come back soon enough after the battery is charged. Once fully charged I'd disconnect the neg battery cable for a couple hours and then measure the battery voltage directly across the battery posts with a digital voltmeter. A good fully charged battery will measure close to 12.9 volts. Leave the battery disconnected for at leasy overnight. Next morning remeasure the voltage across the posts. If it is not at least 12.5 volts the battery is going bad as it does not hold a charge with no load attached. If the battery is still at 12.8 volts or more it should be OK. Connect the battery back up and wait a day or two. Then measure the voltage again directly across the posts with NO KEY on the ignition. If it measures less than 12.5 volts, you have a current drain.

There is a procedure defined in this post from Bill Curlee on how to find a current drain:

http://forums.corvetteforum.com/c5-t...n-long-30.html

Then scroll to post #590

Good luck and post back what you find.

 

The on board computers in a C5 are a constant drain. The fact that it has only 3300 miles on it (nice find) means it has done a lot of sitting.
If you plan to let it sit for more than 2-3 weeks, it is best to hook up a battery tender. I use the "Battery Tender Plus".
Hard to say if the current battery is shot or just needs a charge. I would prob get a new one if you think it is a few years old. C5s can have gremlins with marginal batterys. Good luck!

 

Check out the Schumacher brand battery tender. Fully encased, 1.5amp tender that you can mount directly adjacent to the battery. Then you can keep the battery charged up at all times unless you determine that it won't hold a charge by following the previous advice. But unless your Vette is a daily driver you should hook up some brand of tender, otherwise you'll be buying alot of batteries over the years. The Diehard Gold in my '01 is 7 plus years old!

 

From what I understand, you should never hook up a battery "tender" - type unit to charge up a completely dead battery. I'd say you will need a battery "charger" that puts out at least 5 amps, but 10 amps may be even better.

Once the battery is fully charged, clear the codes and see if they come back after a few driving cycles.

Good luck.