Help needed - can't seem to charge battery

Patient is an '02 C4 cab, haven't driven it during the past week. Car is kept in garage, but recent cold temps may have contributed to weak battery (3 years old?). Tried to enter car yesterday morning and windows wouldn't crack open, no interior lights illuminated. Managed to open passenger door and connected my Porsche Charge-o-Mat battery tender. Tried again last night - same results. This morning I tried connecting it to my Audi S8 and got the same RED light indicator, suggesting charger is working properly. Reconnected to P-car and it will sit for another day. Any ideas what might be going on? Any help or advice is greatly appreciated!

Often battery tenders are unable to charge a completely dead battery. If your car had been left unlocked, it would not be unusual for it to go dead in a week. I would take the battery to your nearest parts store to have it checked for a dead cell, etc. You may also have the remainder of a replacement warranty. My guess is that the battery has simply seen the end of its useful days and needs replaced. Lots of options re size and makers have been regularly discussed on this forum. GL!

If your battery goes dead and it goes below about 24 degrees F, the battery is also gone. It will never charge again. You need a new battery - sorry. The root cause of why the battery drained needs to be found - after you have a new battery - or it will recur. There are a few good videos if you are handy with a multi-meter, then you'll need the electrical cirucit diagram to pinpoint the offending parasitic electrical drain. Or else you can take it in. My C4 cab has such a drain - it takes about 5 days to drain it to the point where it will only click instead of start. I haven't rooted out the problem yet (too cold), but I hear that often it is aftermarket security systems. GL

Thanks, guys - looks like a new battery it is. Is now the time I get to break into my frunk from the passenger side wheel well - pull a cable to release the hood?

No need, open the fuse box in drivers side foot well. Once the cover is removed, there is a fuse that I believe is red and can be pulled out just slightly, be careful not to pull to much as it isn't designed to be removed. This is where you can connect jumper cables to give you enough power to open the frunck. Use the positive on the fuse and then ground the negative cable.. I may have the positive and negative reversed as I don't remember which one grounds the car.. This should also be documented in your owners manual, much easier than what you mentioned earlier. Good luck

Positive on the "fuse" and neg on the loop on the body that the door latches to.

Skiracer - please let me know if you have any difficulty jumping from the fuse box. I've haven't done it yet - new (to me) car.

DON"T "JUMP" from the fues box. It is just to open the frunk to get to the battery or the engine compartment, to get to the jump start lug under the + cover on the box near the starter. Please

Thanks, everyone - it worked like a charm. The POS terminal slid right out and the donor car wasn't running (unlike when needed to jump the battery). New battery installed (84 mo premium from NAPA - $120) and she fires right up! I'll have to chase the power drain source when warmer weather arrives. fbp111 - reading my manual from cover-to-cover has been on my list for a long time - can't put it off much longer w/ this crowd!

Skiracer - had a simlar experience with my 01 cab with the battery drain. Mine ended up being one of the smartop addons. Not sure if you have either of the two for your cab, but the one that goes under the drivers seat (don't remember if that for the remote to open the top, or if its to open while driving) was the culprit for me. It basically kept trying to open/close the top and the car overriding the requests because the sensors were saying things weren't in the correct place for the process to start. You may want to disconnect/remove and see if that makes a diff. Other easy thing to try is replacing the ignition switch as that is also a common thing that goes wrong.