I failed to jump start a weak/bad battery.

[sapote]

2006 SMG.
Yesterday the starter was clicking but couldn't start, and measured the batt only 12v. Last drive was on last Friday evening. Thinking I might have not closed the trunk completely and discharged the batt, so I connected the charger for 10 hours and disconnected it for the night and left the hood open . This morning measured 12.7v and car started right up with 14.1v engine idling (means good alternator), and drove for 20 miles, turned off engine and measured only 12.4v (might be due to acessories had not slept yet) but this was low after a long drive I thought. This evening came out to drive and only clicking. A friend helped connected the 5 ft long batt jumper cable from his running engine to my fender jumper posts, wait for 10 minutes, tried start but only cliking, and the zenon headlight was strobing (switch was at 0 off position), perhaps the voltage during cranking dropped too low to cause the light control system crazy. We let the cars connected for charging some more, but still no crank. I put in a new batt and it ran perfect, drove 20 miles home, turned off and measured the batt, again only 12.4v (can the wake up accessories load the batt down this much?)

The funny thing is that the "dead" batt now measured at 12.7v disconnected.
I had the engine hood opened so tomorrow I can measure the batt without waking up the accessories.

It seems the M3 needs more cranking current and the jumper cables couldn't provide it; the same cable was used to jump the 325i wagon with no problem.
Any ideas?

[Xmetal]

How old is the "dead" battery? Batteries tend to act funny when a cell is about to go bad, but hadn't gone completely bad. You can't really judge a battery's "health" just based on it's unloaded voltage. Get a battery tester that does a load test in order to get a proper gauge on it's "health"...it's not too expensive. My 2.5 year old battery tested good last year...at least the tester indicated an 80% "health"...whatever that means. All the sudden, I couldn't start the car last week. Like you, I thought the car had somehow drained the battery. So, I pulled out the battery and had it fully charged overnight. Voltage measure 12.8V unloaded after fully charged. Put a tester on it and it was bad - only had about 35% of the rated CCA, but it still gave decent unloaded voltage reading!

You can also buy one of those Bluetooth voltage meter/analyzer and hook it up to your battery and monitor it to see if the car is doing anything strange.

Jumper cables are a mystery too since it's difficult to judge it's quality. I also had a similar situation a few years ago where I couldn't jump a car with my jumper cable. Did everything you did - wait a few minutes for the dead battery to charge. Rev'ing the helper car while trying to start the dead car, etc. Nothing. Then my neighbor had what seems to be a better quality cable. Borrowed that and the dead car fires right up with that new cable.

[sapote]

How old is the "dead" battery? Batteries tend to act funny when a cell is about to go bad, but hadn't gone completely bad. You can't really judge a battery's "health" just based on it's unloaded voltage. Get a battery tester that does a load test in order to get a proper gauge on it's "health"...it's not too expensive. My 2.5 year old battery tested good last year...at least the tester indicated an 80% "health"...whatever that means. All the sudden, I couldn't start the car last week. Like you, I thought the car had somehow drained the battery. So, I pulled out the battery and had it fully charged overnight. Voltage measure 12.8V unloaded after fully charged. Put a tester on it and it was bad - only had about 35% of the rated CCA, but it still gave decent unloaded voltage reading!

You can also buy one of those Bluetooth voltage meter/analyzer and hook it up to your battery and monitor it to see if the car is doing anything strange.

Jumper cables are a mystery too since it's difficult to judge it's quality. I also had a similar situation a few years ago where I couldn't jump a car with my jumper cable. Did everything you did - wait a few minutes for the dead battery to charge. Rev'ing the helper car while trying to start the dead car, etc. Nothing. Then my neighbor had what seems to be a better quality cable. Borrowed that and the dead car fires right up with that new cable.

I should have known better: This morning checked the old Bosch batt but pristine condition externally, and it was manufactured on July 2015 -- 8 years old. What fooled me is that I charged it Monday, disconnected charger overnight, Tuesday cold morning 12.7v and started the car up perfect, drove 20 miles with 14.1v alternator charging, parked for 8 hrs, then just clicking when started again. Why in morning was fine but 8 hrs later it failed badly? Yes, I should have done the load test on Tuesday morning by turning on the headlight for 5 minutes, then try to crank the engine.

On the jumper cables, I think the alligator clamps are not good as the contact surfaces are too little on the points of the teeth. They should have a curve surface to maximize the contact from the clamps to the batt cable terminals. But I believe the M3 engine needs much higher cranking current than normal cars.

[sapote]

Others had similar issue of jump start these cars with low/dead battery using jumper cables?

With a weak batt, have anyone tried push start an SMG? I remember reading a guy tried push-start by rolling down the hill and popped the SMG into gear and it worked. Don't remember if he double click into 2nd or 1st gear. My batt was bad enough it could shift out of 1st into neutral for push start.

Be very careful when using jumper cables to start: don't remove the jumper cables off the two cars too soon, as this disrupting the high charging current to the dead/discharged batt can cause the very high voltage Load Dump, and this can fry the DME. Wait 3 minutes or more before removing the cables.

[eacmen]

Yea you can bump start an SMG car:

https://youtu.be/BsO_IOun2iI?si=XKMRebH8uIiNwgoJ

[sapote]

Yea you can bump start an SMG car:

https://youtu.be/BsO_IOun2iI?si=XKMRebH8uIiNwgoJ

Yes, in the video it jumped into 2nd gear. In my case I couldn't get it out of 1st for pushing for some reasons even though the batt was still good enough for the headlights. Yes, it's the same video that I mentioned. Thanks for posting it.