BenzIsWin

Hello forum, got a couple battery questions.
I've been away from home for 4 months and my buddy who's been looking after my place informed me this morning that the battery in my 09 SL550 is completely dead. He says he has tried to start it a few times and has put his (more powerful) tender on it, but it appears to be completely dead at this point. So first question, is there a particular or Mercedes specific battery I need to get, or will any (my model specific) AC Delco, Interstate, Die Hard, etc do the trick? And since the car has sat with no power for at least 2 months (perhaps even longer), will the ECU have lost it's mind with that much down time? Lastly, will I likely have to replace the Accessories battery as well? I'll be back home in two weeks, and can have that battery tested. But I'm just trying to get an idea of how many delightful surprises I'm in store for when I get back to the vehicle.

As always, thanks in advance for any information anyone can impart.

g0rsq

Remember... Two batteries on the car! Starter and consummer.

Where was the tender connected? Should have been the consumer battery, in the boot.

More powerful battery tender...That wories me, as a tender is basically a trickle charger, with some clever bits (so very low power). Damage can be done if attempts made to "jump" start using other car, or starter unit. Cars have been destroyed by connecting jump start devices incorectly.

If no harm been done, you should not get any electrical issues, just clock, and syncronise windows.

If battery (either or both) have been completely flat, they are probably destroyed now, so likely two new replacements, though the starter battery generelly does not discharge on its own.

Rear battery can be replaced by any quality AGM battery. Stick to branded batteries.

I would be just as worried about mechanics (brakes, suspension etc) if left unused for this time!!!

Frederick NL

"If no harm been done, you should not get any electrical issues, just clock, and syncronise windows" - plus steering angle sensor. All routine, done in a few seconds. The bigger issue here, now, is that jump starting MUST be done on the front (starter) battery, never on the rear battery. It might still be worth your while to try a correct jump start. A jump over the rear battery would have upset the electrical system.
To simplify your quest I would charge the rear battery totally disconnected from the car and see if it holds power (probably not..).

cdk4219

With as many problems that come with bad batteries on these cars, and electrical issues from jump starting and high amp chargers, I would be inclined to replace both batteries. I don’t like to spend money if it’s not needed, but these cars aren’t fond of weak batteries.

Make sure you connect the starter battery first and then consumer battery last and disconnect in the reverse order. And I wouldn’t recommend jump starting or high amperage chargers ( which increase voltage)

BenzIsWin

GREAT!! Thanks for all the great information guys! My buddy has a "name brand" trickle charger that he used and connected it to the starter battery. But I didn't know that I should have had my cheaper trickle charger to the consumer battery the whole time. But it's cool, now I know. But it looks like I'll be replacing BOTH batteries. I'm not going to chance jump starting that thing, so I'll just bite the bullet and spend the money.

But thanks again for the great information!! I really appreciate it.

cdk4219

Lots of people have good luck with ctek smart chargers, I have one and it works well.

High Technology

The front is a V4 flooded (wet-cell) battery, which is available in the big-box auto parts stores (Advance, AutoZone, etc.) If yours doesn't stock it, it should be able to be ordered.

The rear is a H6 AGM (size 48) -- most H6 are AGM, but DO NOT put a flooded battery in there. Aside from the electronics not liking non-AGM batteries, it's technically inside the passenger compartment (albeit in the trunk), and I recall reading long ago that only AGM batteries are safe for that.

If you're doing your own replacements, check some of the threads here for steps -- order matters on these cars! REMEMBER the two batteries are connected through a SAM, so once you remove the rear cables, remember the front battery is still in there and has some voltage (even if "dead"). So make sure the red cable doesn't contact any metal else you'll potentially fry something.

My hack for changing batteries dual-battery cars -- get a set of rubber gloves (I borrow the long rubber gloves used for house work from our cleaning bucket) and I turn them inside-out, and put each disconnected REAR cable in a rubber glove, then wrap them with blue painter's tape covering the the edge of the glove and first inch of the insulated cable existing the globe(so they don't fall out when I come back to reinstall the battery). YES, OVERKILL, but well worth wasting a couple of 4-inch pieces of tape to avoid messing up some electronic device in the car. If you don't have those, any rubber glove should work fine.

Given all of the electronics in the cars, I also do this with the positive cable under the hood so I'm positive (pun intended) that cable never touches a grounded piece of metal since it's connected to some devices that might carry a residual voltage.

Obviously this is all covered by a disclaimer to do any work on your car at your own risk -- and I'll close with "some of my favorite people are O.C.D. like me" ...

brucewane

Battery disconnection order, as stated in the owners manual -
Disconnect consumer battery negative terminal
Disconnect starter battery negative terminal
Disconnect consumer battery positive terminal
Disconnect starter battery positive terminal

Install in reverse order.

Wet/flooded type batteries produce hydrogen gas, both during charge and discharge. We all know hydrogen is highly flammable, but it's also corrosive. This is why you want an AGM battery for the consumer battery in the trunk. Do be aware - AGM batteries don't produce hydrogen gas in normal use, but they still can if they are overcharged. So it's not a bad idea to go ahead and connect a vent tube to your AGM consumer battery, your trunk floor has a hole with a rubber grommet for this purpose right next to the battery tray.

When you go shopping for a starter battery, you're probably going to find all suppliers have pretty much the exact same battery at nearly the same price. The R230 starter battery is an odd size (group V4), so there's little demand and no incentive for retailers to carry the same kind of good/better/best battery offerings you typically find for most cars. So don't think you're going crazy when you keep finding the only battery offered has 520CCA and a 35Ah reserve capacity. And it'll be right around $189

The consumer battery is a common size (group H6), so you can actually shop around a bit for that one. I've had an Everstart AGM from Walmart for about two years, no issues whatsoever.

I keep a battery tender on my consumer battery since my car is not my daily driver. I do not keep a tender on my starter battery; if the car's been sitting more than a month, I'll swap the battery tender over to the starter battery for 24 hours. I've never had any issues with "consumers offline" messages or difficulty starting.

slkfla

I have had good luck on batteries at Advance Auto, good prices and warranty. Replaced both Starter and Consumer batteries on my SL550. They have replaced their private label with Die Hard across the board and have exact fit for both types.

BenzIsWin

This forum is AWESOM!! You guys are GREAT and always provide outstanding information, which has me THOUSANDS and all kinds of headaches!! I really appreciate all of your excellent responses and information, and I certainly hope to return the favor one day!!
THANKS AGAIN!!

Tommyboy928

if put the key in the ignition, car should have started , front batteri live around 6 years , and the never charge out by themselfs , rear batteri, is either have trickle charger, or disconnect if go away for more than 12 days.

MBCO

I replaced both batteries this past year and purchased from the Stealership parts department who gave me a discount. Turned out $5 more and they had both in stock saving me a trip.
Parts dude said MB uses Interstate to make and mark batteries. Not sure if other Interstate outlets carry batteries for SLs, but might be worth a call.

BenzIsWin

Thanks again everyone for all your replies. I went and picked up a CTEK trickle charger for the accessories battery, and it seems to be doing the trick. But I'm about to go out of town again for about 2 months, and a thought came to me. Should I add a trickle charger to the starter battery as well? That battery went completely flat while I was away last time, so I've replaced it with a new one. So would putting a trickle charger on that one be overkill and/or unnecessary?

Thanks in advance for any help!

g0rsq

Only necessary on consumer battery. However if leaving it for that time, maybe better to disconnect both batteries and leave them both on battery tender.

Some cars have had bad BCM modules that can set on fire!

See what others say....

BenzIsWin

Ok cool. I was thinking that as well. I've heard how those can start fires, so I'll probably just play it safe and go with your suggestion.

THANKS!

4Rock

Pardon my ignorance, but why charge only the consumer battery? I put BatteryTender leads on the starter battery and connected a battery tender junior (probably too small).

Tom Manning

The front battery has an easy and lazy life - all it does is crank the car. There's also no drain on it when the car sits for months, so it shouldn't go flat.

Rear battery is a different story. That one you need to keep working, charge up or disconnect when you leave the car unused.