Alternator output dropping - need some electrical experts

[hari-bhari]

I upgraded from my stock 80A to a 130A alternator in December. Initially I would get a solid 14.0-14.1V with the car idling and while driving around (the carputer software so I can see it live). Anyways, recently I noticed that when I first turn on the car the alternator output is 14.0V, however after driving for about 10-15 minutes it slowly makes it way down to 13.5V and stays there. Also when I turn off the car my battery voltage drops to 12V and by the next morning its around 11.7V. It's an Optima red top so I know its a good battery, but looks like it's got a chronic drain on it.

It's possible the drain may be due to my alarm system or keeping power to the 500mA USB hub (rigged to stay on) while the car is off. I didn't think that this would be enough to drain the battery much, but I guess it could be compounded by the fact that I only take two 10-minute trips to school and back each day (meaning maybe I'm not recharging the battery much).

I've ran an extra ground cable from the battery to chassis, as well as an extra power cable from the alternator to the positive terminal of the battery, so I don't think it's an issue of power flow. I get no alternator whine while driving so I'm guessing my main ground is fine.

For the time being, I've pulled the fuse from the alarm as well as disconnected the USB power so I should not have any drain while the car is off. I'm still waking up to 11.6V-11.7V on the battery. It's is only 5 months old so I'm taking it back to be recharged/replaced. Luckily the car cranks fine with the exception of last weekend when I didn't drive for 3 days and it was dead in the morning (I didn't check the battery voltage since I had to have it jumped).

So, basically I have three questions:
1) Why is my battery still getting drained?
2) What is the significance of the alternator output decreasing after 10 minutes of driving.
3) Do I need to possibly rig a second battery in the trunk?

Thanks.

[corlon]

go you have a different battery lying around you could try. maybe you higher oupput has cooked the optima and now the signs are showing. if you put a differnet battery in and the problem goos away you might not need that much extra amperage.

[hari-bhari]

the battery does not know what car it's in. and the alternator amperage should only match the draw of the car+battery, not exceed it. i mean people roll around with 200A alternators.

[Mole]

you may have a parasitic draw comming from somewhere...they're a pain to figure out where it's drawing...

[Aotoyota]

my guess is the carputer. 500mA is a lot for a car. thats half an amp per hr??

try disconnnecting the carputer first then we can further investigate.

and.. what was the switched source to turn on your amps?

[hari-bhari]

500mA is the MAX draw of the usb devices, i dont think its drawing that much. but the thing i use converts from the battery's 12V so i'm not sure of its efficiency. the rest of the carputer is OFF when the car is off, and the amp is trigger to turn on through the power supply 5s after ignition (to avoid speaker thump)

[solara90]

use a test light or DMM and pull one fuse at a time and when the light goes off or it goes to zero you found your draw