Is it OK to disconnect the negative cable before charging a car battery?
 

That's what I've been doing ever since the time I overcharged a battery on a W123 (no computers) and blew out the alternator diodes - I hate to think what damage could be done on a modern car by leaving it all hooked up.

It's generally recommended that the negative cable be disconnected before charging a battery. This may require resetting radio theft codes, clocks, etc.

KarTek quite recently had a W210 battery related problem that promoted a discussion about not charging the battery in the car - it was all to do with the anti-theft system.

http://www.peachparts.com/shopforum/diesel-discussion/364549-1998-e300-how-do-you-reset-electrical-system.html

(seems to me disconnecting the -tve lead is a good idea - despite the radio code nightmares)

I haven't ever disconnected the battery while I had it on a charger but as was already mentioned it is recommended.

I remove the negative cable whenever I charge up the battery.

I pull both ends off and clean them as a matter of maintenance. Better to do it in summer than winter!

I've done it all 4 ways (everything connected, + disconnected, - disconnected, and everything disconnected) and never had an issue. I'm also careful not to charge at too high a rate. Not sure what's recommended these days (my knowledge is OLD) but so far so good.

Dan

For those who store their cars there are a couple of possible options.

For long term or a very low battery, I disconnect battery negative or positive and charge battery. After long termstorage, hook up and car starts right up. Just have to set clock and on W210, the radio code (easy to do)

However, with the current smart trickle charges, such as C-tek (even MB sells one as an accessory), you leave battery connected and charger maintains it as required. I now leave mine connected all winter while away.

On cars like W210, there is quite a current draw even when parked, so something has to be done. On some cars, battery is hard to get at, so the small maintainers are handy.

It's ok but in general you don't need to. What are your concerns? Best to read the owners manual and follow the manufacturers recommendation.

What do you call long term? Batteries will self discharge even while disconnected. I think it is recommended to charge a disconnected battery every 6 months to keep the charge up.

Jumping off with the negative on frame (strut mount bolt) is a good way to prevent bad things (tm)

FWIW, in my shop we do all kinds of things that require a battery charger to be connected, with the key on. Many software updates can't be done with the engine running and the last thing you want is to have a low voltage condition before the update is complete. Same goes for some SDS quick-tests.

Do what ever you like but what ever you do, DISCONNECT the power cord from the wall before you disconnect the charger from the battery. Also, let the battery sit for a few minuets before you reconnect it, if you disconnected it. When a battery is being charged, it generates hydrogen gas which can cause an explosion if there is a spark while unhooking or hooking things up. It is not a pretty site for the person or the car.

PaulM

ok... best advice I can give for use of a charger with a car battery...

use a good quality charger... connect it to the battery before you plug it into the wall, and before you set the amp and voltage settings...

I've never been concerned with unhooking the charger from the car before charging...

but AVOID starting the car with the charger connected and charging. it's best to charge the battery, then unhook the charger, then start the car.