Hey guys, I read some threads on using a disconnect switch, I use my car mostly on weekends, sometimes the car would stand for 2 weeks. I have a 1988 S4 Auto, this is bad for the battery, running down every time, shortening lifespan.

I know some of you uses a disconnect switch connected to negative side of battery to cut of the battery completely.


Are you happy with them ? I don't have a problem with the digital clock being out or to be reset? or would you recommend trickle charger instead.


cheers !
Johan

 

The previous owner of mine installed one in the centre storage between the rear seats and I use it constantly as an anti theft device and battery saver when I leave the car for a couple of weeks. I was considering using a small rechargeable battery and some diodes to constantly power the radio and clock but haven't got round to it. Don't make the ground cable any longer than you have to as there's a lot of cranking amps going through it.

 

There are a few variations on how to connect a battery "cut-off switch," but it is easy to do and it works fine for extended storage where a power source is not available for a battery maintainer. It is always a good idea to track down the source(s) of any significant battery drain.

Here's mine:

 

HI all i use a battery brain with remote fob works great.

 

It is generally better to use a good quality battery maintainer than to turn the car off routinely. However I still think a battery disconnect is handy - Especially on a 928 where the battery is all the way in the back - even just unbolting the ground strap each time is a PITA.

So I'd say add in a battery ground switch (similar to as shown) for maintenance or emergency purposes and ALSO add a convenient permanent connection point for a battery maintainer (or even permanently install the maintainer itself) for routine use.

I have a battery isolation switch and permanently installed Schumacher maintainer, and also a higher current connection to the battery - even when isolated - for a higher current charger option.

I do find all three are quite useful options.

Alan

 

thanks guys, the disconnect switch looks neat !


I was thinking of bringing out a wire from the positive side of the battery, properly insulated in the boot to maintain the battery with maintainer inter connected towards the center earth point,



sound good Alan !

 

Some pictures for reference:

Permanently installed battery maintainer (in front passenger fender - with front grill 110V plug in).

Rear power outlet - always on, connected to battery side of switched ground. Also used as a medium current (10A) fused charger port - with clip on charger plate connector

Battery switch @ ground strap point.

Alan

 

A must in any 928, clean install

 

Alan, thanks for the pics, looks great !

 

Nothing wrong with the add ons but you should understand the amount of current being drained and correct it if excessive. The amount being drained should be no more than about 10mA if my memory serves me correctly and a 70 amp hour battery should last 7000 hours or 290 days in a perfect world [never the case though].

In a practical world if you cannot leave the car for a month and get it to start then it needs sorting out. You might take an aspirin for a headache but you would not take one everyday for the rest of your life would you?

 

Yes and no Fred (don't agree with you), actually many cars today with all the electronics, tracking devices, cpu memory, internal clocks, design flaws in car radio's etc, etc can drain a new battery within two to three weeks if not in use, ask BMW owners.


I've read sometime back some car manufacturers even warned new car owners about this.


And then most batteries drains itself over a longer period and it most cases influenced by temperature variations.


Small 'basic ' cars like for example my Hyundai Getz can be left for a month and start-up easy, although I don't like that, I believe any engine should turn-around once in atleast two weeks or less for the oil to move up and about, covering top engine parts like valves/seals etc (not to 'dry' out )


and off course its bad for any car to be left for long periods for many reasons, not going into that now.


I 'm not having headaches at all, my 928's battery is 4 years old now, I've never kept in on charge or anything like that, but it does now run down within a week and the battery fails the load test so its due for replacement, like I mentioned earlier, this is more of a weekend car, not a daily driver.


like many classic car owners keeps their batteries on trickle chargers or use disconnect switches, I just need to know how it works for fellow 928 owners...

 

Your memory is faulty Fred. At best you will be 30mA, most cars will be higher if they have add ons - like modern head units, alarm/KE and phone adapters. Less than 50mA can be considered OK and relatively normal.

You have to allow that most batteries are well into their working lives and don't maintain full RC, especially since many aren't even that well charged in the heat of summer.

You math is suspect further - you need to have lots of energy left to start the car (>50%). I would bet most 928's don't fare so well after 3-4 weeks...

Alan

 

No disrespect Alan, but mine does fine.

Routinely left for a week at a time when I'm at work. Fires up fine. Every so often, it doesn't get driven during my week home, so it can go 3 weeks sometimes.

Did that a couple weeks ago. Car sat from July 18th to Aug 7th . Started up just fine. The factory alarm is not armed (parked in my locked garage) and my Alpine head unit doesn't take much power to retain the clock and memory settings.

Battery is fairly old (came with car, so at least late 2012, maybe older).

 

Try 4 weeks?

That you can go 3 weeks doesn't really change my opinion overall - I think a relatively small percentage of 928's will make it past 4 weeks - you are lucky though!

BTW testing this theory will come at the cost of some lost battery capacity - running the battery that low is not good for it - especially if a) its already old, & b) if it stays that way for an extended period.

So overall I'm more interested in any history of this, rather than suggesting anyone deliberately do this as an experiment.

Alan