My 2012 SLK250 has had continued battery problems. (under charged, boosting with de-sulfination charger always recovers the battery)
I have read that the LIN control of the alternator was intended to only keep the battery charged to 85%, so that the Engine Control Module could use the remaining 15% of battery capacity to regenerate/charge the battery during coasting to a stop.
Typically I was only seeing proper charging during coasting and heavy loads. Also charging was inhibited during cold starts for a period of time. Typical static battery voltage after driving was 12.3
Today I removed the LIN control wire from the alternator. I observed a jump in charge voltage by 0.7 volts, as the alternator regulator has a backup mode of operation. no "Check engine lights". The charge voltage is very constant now, with a brief delay after starting... The static battery voltage is now 12.7 after this change.
I hope that this will give me a 100% charged battery when I park the car from a drive, instead of 85%...
The alternator behavior is optimized for performance and fuel consumption not only battery charging.
It sounds like your problem is actually the battery control module.
It is putting out a steady 14.1 - 14.3 volts, regardless of the load with the "lin" wire disconnected.
Battery never recovered above 12.3, a couple of shops tested the battery with the smart electronic battery testers with the small 18 ga wires and said the battery was fine. Then Batteries Plus put one of the old school battery tester with the jumper cable leads on the battery and it failed badly. (Down to 8 volts, with a 200 amp load)
Managed to fit a group 94R, 80AH battery in it's place. (In place of group 48, 70AH) Static voltage seems to be settling at 12.7
Working on mounting an ammeter in the car, with a discharge indicating red light. I was surprised that the 40 amps I saw on the ammeter at idle was an actual discharge rate, because the alternator had not reached minimum output speed yet. When the car is cold, the fast idle allows the alternator to come online right after starting.
Many times I have started the car and ran it at idle, thinking it was recovering the battery... Not again.
Found a battery de-sulfinator, which works in the car, no trickle charger needed, the "Pulsetech PowerPulse Battery Maintenance System".
The car should charge at idle, albeit slowly, if this is not the case there is indeed a problem. But it can also be you have an electrical issue placing too high a load.
Once the alternator reaches the speed to where it starts outputting (Turn On Speed), then it remains outputting at idle. Cold starts take it to 1400 rpm and then later 600-800 rpm has alternator output. Starting hot, no output at 600 rpm, until I rev the engine to 1300, then idle output is fine.
The article I read about Lin controlled alternators described the fact that the regulators have a "Turn-on Speed". I can not test, because I do not have access to any other "Lin" controlled alternator cars. You really can not tell anyway, unless to hook up an ammeter at idle.
OK, I finally found the source of my intermittent parasitic battery drain.....
The Alternator
44K miles
120 Amp valeo 014-154-14-02
The amount of drain was temperature and time related, larger drain with higher temp, and larger drain the longer the alternator was connected to battery.
So cold with the battery recently connected, very hard to find...
I installed an aftermarket alternator. This alternator charges from idle, no more having to bring the engine speed up to 1400 rpm to get the alternator on line.
Also no more parasitic battery drain, this has been a five year problem...
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