Electrical Issue

[dobbo]

Went to start my 1980 928 auto today which hasn't been driven for a couple of weeks and battery is flat. Dash lights very dim and speedo needle sitting on 100kmh not even a click from solenoid. Battery is less than two years old and has never let me down before. Now on charge will see what happens. 

[Buchanan Automotive]

Once you charge up you're battery you have to check a few basic things }

A )  Voltage test
    With a digital voltmeter ( hand held multimeter ) with the engine turned off ( battery installed ) the battery should read 12.6V or a bit more
      Engine running in park with the handbrake on , the alternator output at the battery ( multimeter still connected ) should be 13.5v - 14.7v 
      Engine running but now headlights on & interior fan on , stereo on , foot on brake pedal ( brake lights illuminated ) and in Drive ( alternator running at its lowest speed ) be do not want to see less than 12.9v

B )  Amperage test , more important than voltage test
      Engine running but now with headlights on & interior fan , stereo on & foot on brake pedal ( brake lights illuminated )  & in D ( drive ) , simulating traffic conditions and in D the RPM is low & the alternator output will be at its lowest , ideally in this mode we would hope for positive amperage ( not negative ) , meaning the battery is still being charged , but if it is in the neg range , the longer the car is in this mode ( sitting at traffic lights etc ) you're battery is discharging = not good , but once the RPM go up , meaning no longer stuck at traffic lights the Alternator amperage output will go up 

The above amperage output test is far more clear cut than a voltage test , but you have to know what you are doing 

C )  Amperage draw test 

Engine off & like the amperage draw test with a multi meter , but this time we are simulating engine off and car locked up , we disconnect the neg lead from the battery & connect the multi meter so all the draining amperage is running through the multi meter & rear hatch closed & doors & bonnet closed 

In the perfect world we do not want to see more than 0.02 of one amp draw ,  by contrast we often see 0.04 of one amp draw , thats twice as much amperage draw than 0.02 , then we often see Porsche's with 0.12 of one amp  or more  = even worse , the higher the amperage draw the quicker the battery goes flat when the car is sitting 

We often recommend customers to disconnect their neg lead from their battery if their Porsche is not going to be used for more than one week  & particularly 2 weeks or more , because even in the perfect world an amperage draw of 0.02  or 0.03 ( excellent ) is still drawing from the battery 24 hours a daw for two weeks or more , this eventually kills batteries 

D)  Battery quality 

The cheap made in Indonesia or Philippines budget lead acid cheapie are just that = Cheap & these generally have a poor shelf life 

The better quality Calcium car batteries made in South Korea ( example ) are excellent & have a great shelf life , meaning disconnected when not in use 

[brian in buddina]

Bruce

Is there any down side to leaving the car on a trickle charger full time, if you only use your P car on weekends? Sometime it stretches out to 2 weeks between uses.

Brian

[ANF]

Very odd after 2 weeks @dobbo as I could leave my 83 928 for 1 to 2 months and it would still start easily. Either your battery is on the way out or something is drawing current still.....

[3legs]

Dobbo, do what I do. I disconnect the earth strap where it bolts to the body behind the tool tray and reconnect it when you want to go for a drive. It only takes a minute.

Mine used to do the same but now I don't have that problem anymore.

It won't upset anything especially if you have an aftermarket radio. 

[tazzieman]

I have an earth disconnect switch at the rear behind the tool tray panel.

Very useful when you are doing electrical work anywhere on these electron hungry selachimorphs. 
A security switch also...
 

[bumble]

Mine can easily go 3-4 weeks without running and not have battery issues - but that wasn't always the case.  Bruce spent quite a while isolating various electron munchers and also installed a new generation door unlocker/engine immobiliser.

Brian, I now (for the last two years) leave mine on a trickle whenever I know it won't be run for some time.  No issues so far, although unless you want a long expletive laden rant, Bruce might not be the one to ask ?

[dobbo]

  Thanks Bruce will do the tests you suggested battery still charging. The voltmeter on dash always indicated some charge going in although I know its not that accurate. Yes Bumble and ANF my car can sit for some weeks without issue but usually driven weekly it has never been a problem before. Thanks for all the replies Dobbo.

[StevepGT3]

these are a great way to keep an eye on whats going on, if you don`t have an accurate volt meter on your dash

 

https://www.ebay.com.au/i/182363060946?chn=ps

[Buchanan Automotive]

Hi Brian , I generally tell customers  do not leave a car battery on constant trickle charge month after month , I am not sure why ( technically ), but for several decades now we have noticed customers ( heaps of them ) who had their battery in their Porsche on a constant trickle charge deteriorate over just a couple of years ( or less ) , but the Porsche's that when not in use & had the negative lead of the battery disconnected , either via manual disconnection ( like I do with mine )  or via a good quality battery isolating switch , the battery ( quality Calcium ) will last for years & years & very little or no battery trickle charging needed ever

Advantages of the battery negative lead disconnected in a 928 when vehicle parked for weeks etc }

A )  Battery in the car ( quality , not junk ) battery will last & last & last

B )  Car can NOT catch on fire by its self and take you're house with it in the middle of the night while you are asleep , we lost a customers 911SC cabrio a few years ago , and it took part of his house with it , the trickle charger ( 240volt plugged into  ) did a wobbly in the middle of the night & the car caught fire & with the 911Sc fuel tank next to the battery , with the front carpets alight the fuel filler hose burnt through & whammo up she went & so did the house , car fires are more common than you think , particularly in old cars

C )  If you are away & the keys for the 928 are in you're house , probably in the kitchen & you're hose gets broken into ( not  uncommon in Sydney & Melb ) & the young thief's ( probably plural ) find some car keys , it doesn't take Einstein  to figure out that that they will find the car to match the key & if you're battery is connected & charged up , Yippee she is a goner 

If however the battery negative lead is disconnected ( car completely dead ) & the boot carpet back in place & the rear hatch closed , the thief will probably think the battery has to be up front , like in a Subaru or Commodore  & if he or she finds the front lid release & then discovers where the hell is the battery , they will most likely just think this is way too hard & just throw the key , the exact thing happened to a friend of mine about 5 years ago & his 928S4 was within seconds of being stolen , all except the battery lead was off & the boot carpet was in place & the thief just gave up

Remember , with the key you use  to start you Porsche , even the most expensive remote control aftermarket system will have a remote control on the key ring , it was only that the battery was disconnected saved my friends 928S4 from driving up the road and into oblivion if the battery was connected & even worse with a trickle charger connected because the thief will think how nice it was for the owner to supply this excellent service 

Regards
Bruce Buchanan 

[tazzieman]

In the old days the dizzy rotor in your pocket whilst you were at the restaurant was all you'd need. 
My neg isolator switch is a marine one.

[Redracn]

Whenever my P car is not in use I connect it to a CTEK charger which is specifically designed to maintain a battery. Note that this is NOT trickle charging. The oldest is 7 years and still going strong as is the 6 year old one. Unfortunately the word trickle is overused and miss understood when it comes to batteries so you need to look closely at technical specifications. 

Only use a charger designed for battery maintenance of the particular battery chemistry you are using. 

While not applicable to older cars the more modern ones have a lot of learnt data relating to stuff like fuel mixtures and shift operation that will be lost and need to be relearnt if you disconnect the battery for too long. 

[Buchanan Automotive]

Funny you should say the dist rotor , I did exactly that on my dads 1970 Datsun 1200 coupe back when I was 17 or 18  years old ( 44 years ago ) and borrowed his 1200 coupe from time to time , with the Datsun you could flick the dist cap off with you're fingers  , takes one minute or less

A lot harder with a 928 or a 928S , and really hard on a quadcam 928S4 / GT / GTS 

Regards
Bruce B

[tazzieman]

35 minutes ago, Buchanan Automotive said:

A lot harder with a 928 or a 928S , and really hard on a quadcam 928S4 / GT / GTS 

hence the 21st century battery disconnect security upgrade!

btw the repco guy says they call the old style chargers "toasters" , with good reason!

[ANF]

Obvious solution, drive your cars more.....

[tazzieman]

5 minutes ago, ANF said:

Obvious solution, drive your cars more.....

Fine but if stuck in city traffic with the AC on and subwoofer thumping , eventually the electron leakage wins.
It seems these cars only work as intended when the ancient wiring and ancillaries are renewed . Funny that!

[firstone]

Mmm, leaking/older. ?

[StevepGT3]

Have all my vehicles on battery tenders/ maintainers, my 993 battery was still going strong at 11 years of age when I sold the car,and my wife's Chrysler still has its original battery from 2005.Both cars have been and were hooked up 100 % of the time when not in use.

[tazzieman]

"At colder temperatures, the battery’s ability to provide sufficient power to start and run a vehicle is diminished. Automotive batteries are rated in CCA (Cold-Cranking Amperage). This is the amount of current a battery can deliver for 30 seconds at -18 C without dropping to a specified cut-off voltage. A fully charged lead-acid battery can survive up to –50 C, but a battery with a low state of charge can freeze at –1 C. When the water in a battery freezes it expands and can cause irreparable damage to the cells."

 

[dobbo]

  Tested alt output 13.2v at idle with lights on and 14.5v at fast idle. Draw was .03a I think wasn't sure about this as I borrowed multi meter and wasn't 100% sure I had it on the right setting. 

       Drove it to work on the weekend in the dark with no problems will see if it starts next weekend.  Thanks all for advice Dobbo.

[Kevvie]

Does anyone have a recommendation for a "smart" trickle charger or alternatively just a lithium emergency jump starter?

My 986 Boxster is only going to be driven once every 2 weeks and I'd like it to start when asked. I have looked at both the solar chargers (my preference) and the emergency jump starters, but there are so many available and not a lot get good reviews.

[Peter M]

I would buy one of these from this supplier and just leave it plugged in between drives

https://www.ebay.com.au/itm/CTEK-MXS-5-0-12V-5AMP-SMART-BATTERY-CHARGER-4WD-CAR-CARAVAN-BOAT-MARINE-BIKE-AGM/292098236763?epid=2173987512&hash=item440269615b:g:UJEAAOSwrORbi4DH:rk:13:pf:0

Probably bought 5 or 6 Cteks from him in 5 and 0.8amp sizes over that last 6 years with no complaints. May have to drill out the eyelets to fit your battery clamps but that's no biggy. 

[P-Kay]

^ +1 Ctek