[inspectorman]

A week sitting at Gatwick was enough to completely flatten the battery - 60Ah or not. It was at least two years old, so I had no problem replacing it (apart from paying, of course) but there was a curious symptom driving back once I got it jump started.

It didn't want to idle at all - with minimal electrical loads on the system - even after three hours of driving. Is it a; the adaptive engine management not adapting; b; the large load caused by a flat and probably knackered battery; c; something more sinister?

Also curious was the open circuit voltage on the knackered battery when I took it out - 12.8V - theoretically about 90% charge. But at that state it only just started the beast to get it around to my garage for the battery swap.

Finally - I know the handbook says any volt display below 10V on starting means the battery is not too healthy - but even with the new shiny trickle charged Varta in, the instantaneous volt display on starting varies between 11V and 8V. Running volts are nice and normal at 13.9 - 14.4 V

 

[billj]

Hi, inspectorman. Welcome to SaabScene.

Yes, 12.8V open-circuit voltage would indicate a fairly good charge if the battery were healthy. Even with this open-circuit voltage, though, a duff battery could have trouble supplying a decent starting current. A common cause of battery failure in modern batteries is due to internal shorts caused by buckled plates touching, or, quite often, a conductive sludge that builds up on the bottom of the cells, eventually reaching the bottom of the plates and shorting them, reducing the effective capacity of the battery. This sludge moves around too, and can cause intermittent problems before the battery finally dies. This can lead to the situation I found myself in, where after jump-starting the car at work, I drove straight to a battery shop where the battery refused to misbehave and they just about flatly refused to replace it! Then all the electrics cut out briefly as I braked for a roundabout on the way home. A new battery the next day fixed it and it's still running well two years later.

I can't explain the reluctance to idle. What did the voltmeter say as you were doing this? Bear in mind that an internal short might have cleared once you removed the battery.

Finally, the internal voltmeter might not be a great guide to battery condition. My CSE had been turning over slowly since I bought it so I bought a new battery for it when I wanted to take the Aero off the road for a few days. The voltmeter had been showing around 8V (sometimes slightly less) after starting. With the new battery (big Bosch 74Ah that fills the battery tray), the engine now turns more confidently, but the starting voltage on a cold start is still around 9V. It is possible, of course, that there is a slightly high resistance somewhere that is dropping the voltage during starting. The engine still doesn't turn quite as quickly as I would have expected with a new battery.

 

[mark e]

Hi and welcome.

There's another possibility given the high terminal voltage of your battery- that there's corrosion crept in to the crimping of the main power lead to the battery. I know this becuase I've finally traced an intermittent problem on mine to the short lead that goes between the battery and the positive terminal distribution block.

Having replaced it and claned all the terminals, I don't drop below about 10.4V on starting at the moment.

As for poor idle, I'm not sure- has it done it since? You could try disconnecting the new battery for a couple of hours and then taking it out for an adpation run.

As for something more sinister- I hate to ask but did you follow the correct procedure for jump starting? ie Jump start car has engine running, then you connect dead +ve to jump +ve, then jump -ve (battery) to dead -ve chassis (normally engine block, NOT battery) then you leave it that way for 5 mins or so, then turn off the jump car engine BEFORE you try to start your car. This offers both vehicles maximum protection from potentially damaging voltage surges and transients.

Good luck!

Mark

 

[inspectorman]

Thanks for both of your replies. I wasn't looking too hard at the voltmeter when it started to think about stalling - getting it into neutral, left foot braking, and holding the revs up on the throttle without hitting anything in front in wet Friday night traffic was quite enough! As it began to stall the volts were plummeting through 13.6V and going south, I recall.

I guess the best way to spot the internal corrosion on the battery lead is with a thermal imaging camera. Not having one is a handicap so I'll resort to feeling it after starting.

Idling is now fine, and life is very normal. If I had to bet on it, I'd say voltmeter readings are now up a little - either because the battery is fully carged, or its internal resistance is lower.

Jumping was done with a battery on a trolley. The very nice man from the parking company said he'd done a lot that week - perhaps a lot of alarms were active during the weather of Sun / Mon 10 days ago?

But a nice shiny battery with a brand name on it does a lot for my peace of mind.

 

[sgould]

Make sure the earth lead connection to the body and engine are corrosion free as well. As a brief check, if they are corroded they often get quite hot under prolonged starting.

 

[johncc]

If you have Trionic then the rough idle will be due to ECU relearning after battery disconnected, I think. That is why it has now sorted itself.

 

[inspectorman]

The answer to my first question lies in a Haynes handbook - from a friend who originally asserted that I must have something deeply tragic wrong with my wonderful Saab

"According to Haynes manual on Engine Management and Fuel Injection Systems (which I had
forgotten I had)

"Adaptive systems

The ECM is adaptive to changing engine operating characteristics, and constantly monitors the data
from the various sensors.....

As the engine or its components wear, the ECM reacts to new circumstances by adopting the
changed values as a correction to the basic map...

Most adaptive systems will lose their settings if the battery is disconnected. Once the battery is
reconnected and the engine is restarted , the system will need to go through a re-learning curve.
This usually occurs fairly quickly, although idle may be poor until the adaptive process is
completed."

So that's the question answered