car wont start without a jump

[Kershaw]

i bought this e30 maybe 5-6 months ago. owner said he put in a new battery. i believe him. it was pretty damn shiny.

i drove it around for 5 months, no problems. about a month ago i'd get hiccups while driving. sometimes it refused to rev past 1500rpm. (had to granny shift it and not double clutch it like i should.) so i parked it and drove my rx8 for the past month.

it has oil all over the cylinders #5 and #6 spark plugs. so i figured the hiccups/lack of power was a blown head gasket. maybe fried piston rings.

well i decided to drive it around yesterday. i had to jump it. battery was dead. it drove 90% of normal after i jumped it. and the engine starts REALLY easily. i mean, i've never seen an engine start this easily all my life. you barely touch the ignition and it comes to life. the battery had >4 volts, and with a jumper turning over the engine at about 50rpm it started. no joke.

so i'm confused. a lack of compression should make it harder to start yet? i need to do a compression test and a leak down.

is it possible that the alternator is not keeping the battery charged? if the alternator were toast though, wouldnt i not be able to drive it all?

[jlevie]

The dead battery after the car being parked for a month isn't unexpected. Normal parasitic draw (the current drawn from the battery when the engine is off) could easily deplete the battery to the point that the car won't start. And if the car suffers from excessive parasitic draw it would take even less time to drain the battery.

If you are going to park a car from an extended period of time an battery tender will prevent the battery from going flat.

[Kershaw]

but i jumped it and then drove around for at least half an hour. everything seemed normal. 11.9 volts after i shut the car down.

but then when i tried to start it a few hours later the battery was dead again.

[jlevie]

If the battery didn't fully charge in 30 minutes of driving, the alternator becomes the prime suspect.