e46 alternator question

[Josh (PA)]

Yesterday the solid red battery light came on in my E46 wagon. Quick research strongly points to alternator (or regulator on back of alternator). It doesn't look like a job I have an interest in doing, so I want to get it to my mechanic.

Here's where I need some feedback:

The shop is 21 miles from my house. Research shows best case battery only range is about 20 miles. I have a marine deep cycle battery that I use for my trolling motor. My thought is I can fully charge the e46's battery and the 2nd battery with a charger before heading out. If I don't make it, I can hook up the marine battery for the last few miles.

Questions:
1. Is this plan stupid? Should I just have the car towed?
2. If the plan is not complete nonsense, is it better to hook the two batteries in parallel so they discharge together with jumper cables or similar or run one battery down then connect the 2nd.
3. Is there risk to the electricals to not using a "properly spec'ed BMW" battery for a period of time?
4. Does the car use cycle effect the battery discharge rate? IE: I will shut off lights, clmate control and radio, but does speed/ acceleration rate, etc impact discharge. My assumption is the faster I go the better as the computer modules will all be pulling current/time. What about the sparks, etc?

Thanks for letting me bounce this plan off you guys.

[John V]

There is no issue with using a non-BMW battery, but the car may start to behave very strangely if you run it with a dying battery. You may also destroy the battery if you run it completely dead.

Running the two in parallel with jumper cables is certainly a reasonable plan electrically and should get you there. Just be sure your connections have no chance of coming loose and shorting power to the chassis and that the second battery can't slide around in the trunk. I would rather do that than have to fiddle with hooking up a second battery on the side of the road.

[clyde]

FWIW, the last time I was in a car that ran down a brand new battery wih a failed alternator, we made it 26 miles in LA Wednesday-before-Thanksgiving rush hour traffic. About 9 miles on surface streets and the rest on a very clear nested freeway. It wasn't until the very last little bit there appeared to be a problem. We were in the left hand lane (no shoulder) in mostly stopped traffic and immediately tried making it to to the right shoulder. We got there just in time. My guess is we were in the car for about an hour total. This was in a 1991 (or 89? Hard to remember now) Buick LeSabre.

[Nick M3]

I've run a race with a dead alternator. 20 minutes at maximum throttle. Just turn everything off. I like the parallel batteries solution for safety.

You might as well pull the existing battery and get it tested, though. It is technically possible for a fucked battery to throw the light. (Lupinsea here was, I believe, that rare special snowflake.)

[lip277]

I'd just tow it.
Too many electrical 'things' to worry about with this. IMO...
If this was an old carb'd car that only had the ignition system going when it was running (once started) - maybe I'd do it. But - BMW's are just too finicky.... electrically speaking for me to think this would be a good idea. Again - IMO.

[Josh (PA)]

Thanks John and Clyde. I'll give the plan a try this week and see how it all goes.

[rumatt]

It is technically possible for a fucked battery to throw the light. (Lupinsea here was, I believe, that rare special snowflake.)

Me too. (http://forums.carmudgeons.com/showthread.php?t=102487)

My battery light came on and a new battery fixed it. It's possible your alternator is fine.

[John V]

Yeah, checking the charging voltage is a good idea before replacing the alternator.

Bad batteries often kill alternators, though.

[Josh (PA)]

Thanks for the suggestions. I'll measure the charging voltage, and maybe swap the battery before making the final call.