Quality charger is key. Cheap charger will not have the sensing and control logic of better chargers. Also the good charger will have more filtering, resulting in a smoother waveform. Cheap charger will have an ac component riding on the dc which is bad for your battery (and the car electronics).

The advice to disconnect the charger before cranking is solid. If you crank the car with the charger connected, you are effectively throwing a dead short across the charger output.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

How will the battery get charged if you unhook the charger?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Even the cheapest chargers have short circuit protection, so it's no big deal to crank the engine with the charger connected. I short the leads of my chargers all the time before use to see if it sparks, if it sparks I know it's on, and working.

The thing that you should never do is connect the charger wrong by reversing polarity of the leads. That could potentially fry the charger (if it does not have reverse polarity protection), or fry the alternator (depending on the voltage and current setting of the charger).

Edit: Why is an AC component on the DC waveform bad for the battery?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Not so much bad for the battery as it is for the electrnics. But a battery is a dc cell, and ac current can overheat the battery.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Ha ha... oops

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Where did you get this info from? Personal experience or pulled out of thin air? The old battery chargers with a transformer and selenium rectifiers have a little bit of AC ripple on top of the DC component but that does not overheat the battery. The new electronic chargers without a transformer have less AC ripple but does not charge the battery any better at the same charge rate.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

A battery will be CHARGED if you apply a higher outside voltage to the battery. This is like jumping a car with another battery. If you hook it up long enough then the battery voltage will go up, the jump battery voltage will go down. There will always be some AC ripples on the DC if the charger is plugged into AC unless you have a BIG FAT capacitor there to smooth it out. BIG FAT capacitor is expensive so draws your own conclusion. AC ripple does not hurt the battery so very few manufacturers put a BIG FAT cap in there.

As for AC component superimposed on DC. Actually the new and expensive charger with de-sulphate circuit uses the exact principle. High frequency AC superimposed on the DC to break up the sulphate, if any.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.