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Old 08-19-2016, 02:28 PM   #1
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Dead Battery Question

A recent ACL reconstruction has had me down for several weeks, and the battery in the Boxster, which I suspect was nearing end-of-life to begin with, is nearly dead. There was enough juice to lower the window when I opened the car door manually, but I'm going to buy a new battery this weekend.

I know it's advisable to keep power to the car when the battery is removed, either through the fuse panel or with a cigarette lighter adapter/charger, but would it hurt anything to simply keep my trickle charger connected to the battery leads when I remove the battery? Would that provide sufficient juice to keep the DCM and radio from going whacky? Any concerns with doing this?

Electrical is my weak point...

Thanks!

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Old 08-19-2016, 02:49 PM   #2
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I know it's advisable to keep power to the car when the battery is removed
Who told you that?
That's wacky

I don't even keep the battery connected to the car when I charge it. There are too many sensitive electronics in the car to have a device that is plugged into AC connected to the power system. Stray EMF, surges, charger failures...the chances are slim, but still not worth the risk to me.

Last edited by particlewave; 08-19-2016 at 02:52 PM.
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Old 08-19-2016, 03:12 PM   #3
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Having a trickle charger connected to the car is not all that unusual. Porsche actually markets one with their name on it. I would opt for a Battery Tender. IMHO, better product and price.

Porsche Charge 95504490054 - Genuine Porsche - 955-044-900-54 | Pelican Parts

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Old 08-19-2016, 03:19 PM   #4
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There's no valid reason to power the car with an AC charger with the battery out.
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Old 08-19-2016, 04:34 PM   #5
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Thanks guys...The internet of course can be a fear mongering entity...Reading about folks starting engines after a battery replacement with cam timing off, and other odd stuff brings some caution...

Whew, at least it's just a simple battery replacement, no different than my prior '78 SC...

By the way, does disconnecting the battery wipe the DME?
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Old 08-19-2016, 04:37 PM   #6
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By the way, does disconnecting the battery wipe the DME?
The car will revert to its factory DME settings and have to relearn its fuel maps and such.

No biggie. Put in the new battery and drive!
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Old 08-19-2016, 06:54 PM   #7
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By the way, does disconnecting the battery wipe the DME?
I do the calibration every time I disconnect the battery.
Just turn the ignition on after the battery has been connected so the dash lights illuminate (no engine start) for about 60 seconds. You may hear the DME hum and click. Then switch ignition off and remove key for 10 seconds - that's it. The DME calibration is now complete and there's no wait time to re-learn anything.
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Old 08-20-2016, 10:03 AM   #8
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Thanks gang, really appreciate it!
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Old 08-20-2016, 11:25 AM   #9
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No issues with disconnecting the battery. I would even suggest it is a good thing to let everything reset every so often. Just make sure you have your radio code. If you don't a few people on this forum have a way to decode it from the radio serial number.

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I do the calibration every time I disconnect the battery.
Just turn the ignition on after the battery has been connected so the dash lights illuminate (no engine start) for about 60 seconds. You may hear the DME hum and click. Then switch ignition off and remove key for 10 seconds - that's it. The DME calibration is now complete and there's no wait time to re-learn anything.
I have never heard about this before, I'll have to try it next time I disconnect the battery!
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Old 08-20-2016, 01:06 PM   #10
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Hi gang,

Just so I'm clear on the DME calibration. If I turn the key on for 60 seconds, and then off for 10, does that restore the old settings? I think I'd prefer to have it learn my personal driving habits if that's possible.
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Old 08-20-2016, 04:05 PM   #11
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When the battery is disconnected the DME resets, the only way to keep the settings is to attach a battery maintainer. When you go to reconnect the battery (assuming you let it reset), you can either drive the car and it will calibrate the DME over time, or as Steve pointed out you can make the DME calibrate itself all at once (versus over time) with the key.
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Old 08-20-2016, 09:58 PM   #12
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As Rick says. Calibrating the DME circumnavigates the learning curve.
Its easy to check - just disconnect the battery for a couple of minutes, reconnect and start the car. You will feel the tickover slightly lumpy and (if you drive it) the performance is not quite as crisp as the engine management system adjusts to fuel octane rating / ambient air temps/ throttle body positioning etc. Leaving the ignition on for the +/- 60 seconds allows the DME to revert back to what it was.
At least that's how I understand it.....
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Old 08-21-2016, 06:25 AM   #13
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From what I understand it allows the DME to read and store the cold to warm, value for the MAF sensor heat. throttle body sensor and pedal sensors (0). Reset the learned map to factory value. If the battery goes down slowly to dead or there is a spike these values can be corrupted because they are in volatile memory. Not doing this can mess with the performance for about 30 miles.

I shorted a wire in my relay box and it screwed up my throttle positions to the point that I was going into limp mode and throwing throttle codes. This corrected the problem.

I was told to have all other electrical systems off when doing this. Fan, Temp unit, Radio, lights, doors closed no alarms dinging, etc.
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Old 08-22-2016, 12:33 AM   #14
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I replaced my battery about a month ago. The only thing that bothered me is that I didn't find the PIN code for my PCM 1 until a week later. :P
I also took the opportunity to fix the battery tray, as it looked awful. Check yours when you have the battery out anyway. It's an easy job.

Simple DIY: Battery tray refurbish

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