Not holding charge

[fumstoo]

Just finished my m42 turbo and it wont hold a charge. Brand new alternator, tested and passed. New battery, getting anywhere from 10.5-11.5 volts when the car isnt running. the alt seems to be grounding fine but im going to run an independent ground to the chassis. When i start the car, have to jump it, the volt reading i was getting at the battery dropped steady and quickly from 11.5 to 7.2 where it stayed in that area till the car died. Any ideas?

EDIT: Can someone get me a picture of how number 8 in this diagram is hooked up? If i have something wrong here could that be doing this? im also thinking the wire between the starter and alternator

[Geoff]

10.5 to 11.5 is not a good reading for a battery, even at rest.    it should be more like 12.5 to 12.8 at rest, then 13+  when car is running and  charging the battery.   if you have 1 bad cell in the battery, it will read low and not take a charge.
                                                                            Geoff

[bmwman91]

Yeah, does the battery voltage fall measurably over the span of 12-24 hours without running the car at all?

[DesktopDave]

If you hooked up any of those wires on that diagram to the wrong place you'd have a really serious short.  That's all unfused as well...not a good place to make mistakes.  That's all positive except for the grounding strap in the trunk.

Disconnect the battery until you get that wired up right, otherwise you'll kill it.  PB batteries don't like to be quickly drained.

If I get a chance I'll snap a pic.

[fumstoo]

Battery is brand new. although i dont think that is the problem. At lunch i went and got some different reading with my meter. brought one from work to make sure mine wasnt reading wrong. the alternator ground isnt ohm'ing. Was told to add an independent ground to the alternator so im going to give that a try. And i would really appreciate that picture, dave. Thanks for the replies

[fumstoo]

Added ground wire from alternator to chassis. Problem solved