battery issue, only <32*F

[megaladon6]

this is in mjy DD, 1997 VW jetta TDI. i originally changed the battery to an optima a few years ago. I kept having dead/weak battery issues, but only after it sat for a few hours in freezing weather. i replaced the battery with a VW batt last year and things were good. Now that it's 1yr old and the weather has returned to freezing, it's happening again.
i tested the batt/charging system multiple times with the snap-on amp/load tester and a mid-tronics unit. that's on both batteries BTW. all tests come out fine, but i can't test it full cold.
oh, and if the batt is dead and i jump start it and run for say 30sec, then i can turn it off and restart with no problems.
any ideas?
thanks!

[Denvervet]

Battery drain somewhere. Suspect the cold weather is just exposing the problem rather than creating it.

[megaladon6]

i did try looking for that when it first started, no luck. plus it can sit for days, if it's warm. i do not understand this one.

[Denvervet]

Find a shop good with electricals and they can find if there is a drain somewhere. I went through this with my son's Saab. Changed batteries, alternators, etc...and learned once again not to just keep throwing parts and rather get diagnosed. His issue turned out to be the radio...which worked just fine but replacing it took care of the problem

[megaladon6]

i BELIEVE i have fixed it. i looked for drains, even disconnected a few possibles and to no avail. but the alternator started having issues at startup. the field wouldn't turn on at first, then it would either gradually come on, or it would suddenly come on and load the alt enough to squeak the belt, sometimes a few times before staying on. so i threw money at it and got a new alternator. it went in friday, sat saturday but started GREAT today in 20*F. tomorrow is the real test--as i have to get to work!

[MonzaRacer]

One most people have issues with is depending on specific tools finding problems all the time. Yet the good tools and companies that made them are gone or worn out.
Most likely your issues was with either weak/worn brushes/brush springs. and possibly a weak diode allowing ac voltage to get to the battery.
Putting a volt meter on the battery and having the head lights on and flip the high beams on and off, new alternators SHOULD put out no more than 40 mV (thats millivolts ) I can live with 60mV and 60mV is the limit for the old alternator. if the alternator spikes over 100mV it can damage the battery, it will drop the life of a battery from 4 or 5 yrs to a year or less in some cases.
fords will have idle issues with 100mV to 150mV even , like when an ac clutch engages and releases, the load ups the alternator output then it spikes.
Some testers will miss these small spikes and cars still have issues.
And remember there are rebuilt, these get new bearings and brushes, and rest tested and cleaned up even the regulators get reused if testin g ok. reman get replacement parts either refurbished stators, rotors and pulleys and maybe a new regulator. then there are new-man all new but aftermarket, and this is where they MUST be from a good company as some have different regulators, like some fords. Some regulators ONLY work from computer control and some will simply turn on with key on through a resistor or idiot light and work fine.
Good thing is you seem to have it fixed.

[megaladon6]

Well the alt did not completely fix the issue, but close. Shes now good down to around 10, but by 5 she gets weak--she'll start but will drop enough voltage to reset the clock.
oh and its a bosch rebuilt direct from germany