Battery immediately drains upon Negative cable connection

Question

Experienced problems starting engine.

Removed battery, checked fluid level, hooked it up to trickle charge, and it readily accepted the charge.

Put it back in and reconnected both cables, and engine started.

Allowed to sit overnight, and checked battery voltage at 10.3VDC and engine would not crank.

Jumped off and allowed to run >10 minutes, battery voltage reading 14.8V at that point.

Removed Negative cable, and battery voltage slowly drops, while engine continues to run.

Shut off engine, measured amperage draw with Negative cable disconnected (parasitic draw): 32.5mA.

Left Negative cable off and

Waited 30 minutes: battery voltage down to only 12.7VDC

Waited another 30 minutes: battery voltage down to 12.63VDC

Reconnect the Negative cable to battery negative, heard electrical 'zzt' noise, battery voltage immediately drops to 10.4VDC, and engine will not start.

Bad ground, bad alternator, etc.?

Thanks for your help and advice.

Answer 1

What I suspect is, your battery can only charge with what is considered a "surface charge". This usually means one or more of the cells are bad in the battery. When in this condition, the battery will somewhat accept a charge, then will sometimes work. Once the surface charge is gone from it, which can happen from any parasitic draw and be depleted very rapidly, the battery voltage will immediately drop and will no longer have enough power to start the engine. It might work long enough after being charged to get the car started once, but that's about it until it charges back up. Extended cranking is going to be out of the picture.

Considering the voltage at the battery when running is at 14.8vdc, this is a good sign your alternator is working as designed. It may be a tad high, but considering if it's trying to recharge a battery which basically isn't taking a charge, it will be running a tad high (should be in the 13.8-14.2vdc arena normally). Also, your 32.5mA draw is really insignificant.

Battery life is usually in the 3-5 year arena. If your battery is anywhere in there, take it into an auto parts store and have them load test it. They'll be able to tell you if the battery is okay or if it needs replaced.

Answer 2

The zap you hear when you reconnect the battery and the immediate voltage drop suggest you have a short somewhere which is trying to consume a lot more current than your battery can supply. Have you recently done any work on the car? any bulbs blown? any fuses burnt out? If you have any cable harnesses visible inspect them for damage.

As a last resort debug by hooking up each electrical system one by one and measuring current consumption, this way you can narrow down to a specific subsystem.