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E46 (1999 - 2006)
The fourth generation 3 Series (E46 chassis) was introduced in 1999 and set the standard for engineering and performance during it's years of production including being named to Car & Driver's 10 best list every one of those years! ! -- View the E46 Wiki

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Old 11-09-2012, 06:55 PM
steveh330i steveh330i is offline
Location: Orange County, Ca
Join Date: Oct 2012
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Mein Auto: 2003 330i
Disconnect battery - part 2

2003 330i:

This is follow up to my original post on the same subject. Because of where the car is parked - a carport with no power outlets - I don't have the option of connecting a battery tender or trickle charger to my 330i when I leave it untended for weeks or longer. I plan on using a quick disconnect tool on the negative post of the battery - you unscrew the knob on the tool to interrupt the battery circuit, thereby cutting power. I'm not worried about reprogramming the clock or seat/mirror settings when I reconnect. But, after reading the Wiki and some posts about losing memory in the onboard computer(s) (ECU or DDE/DME, which I don't understand), I'm wondering what the effects will be on those units, or any other electrics on the car.

Any help to this BMW newbie will be greatly appreciated.
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Old 11-10-2012, 05:33 AM
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jonathan2263 jonathan2263 is offline
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There should be no affects. The E46 is electronically simpler than its successors and shouldn't need reprogramming of anything except the clock and maybe radio station pre-sets after long periods of battery disconnection.

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Old 11-10-2012, 07:26 AM
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jvr826 jvr826 is offline
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Location: In the woods. Santa Cruz County, CA
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Some of these disconnect kits (Griots?) come with a fused jumper cable that will keep a small bit of current flowing so you don't lose your clock and presets. Honestly, a reset every now and then is not a bad thing. The ECU learns your driving habits and if you do a lot of slow city cruising, your car will learn to be a blue-hair type driver. :-) I reset mine before track weekends so I can retrain it for that activity quicker.

I use one of those quick disconnects on mine so I can remove my sound equipment quicker. One time at a track weekend, pre disconnect, I broke the neg cable securing bolt and had to drive hours home with it just friction fit.

Nothing bad will happen by leaving it disconnected, but batteries do discharge over time anyway. I'd suggest removing the battery and keeping it on a charger if we're talking months. If you don't want to do that, you'll definitely want to charge it up before driving the car. A drained battery will stress the alternator, possibly to a point of failure. They are not designed to recharge a flat batter... just replace the charge lost from starts.
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Old 11-10-2012, 07:37 AM
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cwsqbm cwsqbm is offline
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The E46 doesn't care if the battery is disconnected. I've done the same in the past many times (I just remove the negative cable completely from the battery - only takes a few seconds with a 10mm wrench). I don't think I even lose the radio presets ('04 mark iv nav system, so YMMV).

My E30, on the other hand, has a non-functioning radio right now because one button I need to press to put in the code doesn't work.

If you're going to be gone for months, just pull the battery all the way out and bring it inside and hook up a battery tender.
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Last edited by cwsqbm; 11-10-2012 at 07:38 AM.
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Old 11-10-2012, 08:09 AM
steveh330i steveh330i is offline
Location: Orange County, Ca
Join Date: Oct 2012
Posts: 15
Mein Auto: 2003 330i
Thanks for all the info. There may be times when I'm gone for months, and in those cases I will remove the battery and put it on a battery tender or float charger.
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