Low Battery is better than a dead battery..
The new battery,
Is it store bought? Or is it a used/reconditioned battery new to you?
How many CCA's is it? I use a minimum 700 CCA's..
Since I live in Michigan, Where temps can change drastically.

Do you possibly have some thing wired direct? (Doesn't shut off with a key stroke) Like an after market Amp/Stereo?

Somewhere theres an open circuit that is causing a draw on the battery... IMO. Thus causing the "Low Batt" issue.

 

only thing was radar detector...I had garage check the draw and found nothing...positive battery to neg side of car with batt disconnected.

 

Go down to O'Reillys ask to have battery and alt. checked can do it with battery in car. Its free

 

I do know that the older non computerized GM car starters use to draw on a battery overnight when they had a bad starter solenoid..
I'm curious if that's still possible in these new computerized cars?

If so...
That could also be the cause of the battery drawing dead overnight?
Of course my knowledge base is mostly old school...
Maybe one of our electronics guys could answer this?

I also prefer when having an alternator tested ..To have it bench tested VS still in the vehicle... But that's just me..
That way you can test it multiple times.... and get a little heat built into it enough to possibly show a bad bearing or diode.
Which can also cause a battery draw and or low charge rate. Thus causing a battery failure.

 

will it show on a draw test? connecting amp meter across neg terminal when disconnected?

 

well i left it for 4 days without the radar plugged in and it started up in cold weather..gonna have to plug it into a fuse that shuts off with the key.

 

even though the battery may be new, ive seen more than 1 have a weak or bad cell and give the same symptoms. like you mentioned cold *** temps or extreme hot kills a weak batt every time. If were me id take it back and swap for another battery. that radar detector shouldnt kill the battery.

 

yea I had it plugged into my Bonneville but the batt was under seat and it was huggggge

 

I have a power inverter I sometimes leave plugged in. If it draws power to charge something like a laptop, it can drain the batter quickly. But it sounds like you are draining prematurely. In addition to the other advice offered, take a close look at the cables running between the battery, starter and alternator. My quickly dying battery (and a replaced alternator) didn't have a prayer with the corroded cables I was using. Once they were replaced, everything "lit up" and stayed fully lit with full power.

 

car started after sitting for 18 days,been single digits a bunch of days...to cold,have not driven the car since feb 2