Are all battery tenders worthless or am I doing something wrong?

[dodgestang]

I've had two already trying to keep my 96 GTS charged.

If I drive it everyday....no problems.
After 1 day it will start
After 2 days it will start, but will idle funny like it is struggling with voltage for the first few seconds
If I leave it sit 3 days, it will open the doors, but won't have enough juice to start (just clickclickclickclickclick)
If I leave it sit 4 days, I can't even open the doors.

If I put a hot jump on it, it fires right up.

So.....I hooked up a battery tender.....sometimes, just sometimes, it will start on day 3, never of day 4.
So.....I bought a different, newer, better tender. Here's the kicker, I hooked it up on the car after it sat for 3 days. The battery was already below 12v on my meter....I plugged the tender in....and after 10 seconds the tender says "Fully charged"

Car obviously doesn't start. WTF is up with these tenders? Do I have less of a voltage drain and more like a bad ground?