Driving while that light is on is an issue as the car is driving solely on battery power and is not being replenished by the alternator.

Aux battery primarily for operating the stop/go function and would not cause that light.

Reasons: loose (probably) or corroded (unlikely as the battery was just changed), and a failing alternator is less likely (though possible--warranty issue if it is at fault) after so few miles though IMO.

Check the batt connections--loosen completely, clean the posts, tighten and then put some white lithium grease on the posts/clamps to protect from the elements.

Will need to do the window reset again after reconnection

How do you re-synchronize Mercedes windows?

1. Close all the doors of your car. This is very important. Otherwise, re-syncing will not work.
2. Put the key to position 1 or 2. Do not start the car.
3. Pull the window button to the first position (the first click on the button) until the window completely closes. Hold for 2-5 seconds until you hear a click.
4. Check if the one-touch feature and the other problems are fixed. If not, repeat step 3 until they are.
5. Do the same for each window.

Alternative steps if re-synch procedure doesn’t work.

1. Close all the doors
2. With the key in position 1 or 2, push the window button until it is completely open. Hold the button for 5 seconds.
3. Pull it up to close and hold again for another 5 seconds.
4. Check to see if the problem is fixed. Repeat steps 2 and 3 as needed.

 

thanks will give it a go

 

Just checked the connections and the battery (thanks to the forum for showing me its hiding place) and they are super tight with no movement and the battery is indeed very new. It is however a Yuasa 80 amp no mention of AMG.

 

Think You mean "AGM" Battery..!! Does the Yuasa battery look new??

May well be that the secondary battery behind the passenger seat has failed due to relatively low mileage of Your SLK, but these normally just causes the Stop/Start system, to stop working..

If You have a Volt meter (DVM), would worth checking the main car battery voltage:

When the battery is fully charged, will be as high as 12.7 volts with the ignition turned off and car not used for 12 hours.
But more likely to be 12.4 to 12.6 volts depending on driving style or being topped up by a CTEK Charger:

Unlike the R171, the Face Lift R172 battery mainly charges during braking - to help the CO2 rating I guess....
I can say that after a 500 mile drive across France at a steady 130km/h, My R172 battery was now where near fully charged, but a spirited afternoon drive around the Cotswolds see's the 12.75 volts, the next day...!!

During braking, the voltage should rise to 14.8 volts:

Either way, if ones SLK is not being used much in the winter, worth investing in a CTEK Charger

 

That Yuasa is the aux battery for the stop/start function, not the main charging battery Got nothing to do with running the car or the warning light. Main battery is in engine bay, by firewall under drain pan.

Find a pic of location in post 7 here

Slk 250 battery location

I have 2012 slk 250 and haven't changed the original battery. Left it for 2 1/2 months due to a business out of the country. Came home yesterday and could not start the engine. When I put the key in, all the lights turn on at the console. When I start it, there is no cranking sound and the...

www.slkworld.com

OR here (dont know why they are different but it will be one of those two locations

 

John, the main starter battery under the bonnet should be of the AGM [absorbent glass mat] type. If that is what has been changed I would, as Len has said, check the connections closely. To access the starter battery the tray that covers it needs to be removed.

There are three turn rivets, two up under the scuttle and one to the front side of the tray.
While you are in there, check for water in the bottom of the battery compartment and also clean out the drain hole
Don't get confused with AMG of which yours is an 'AMG styled' SLK. It does not have an AMG engine.

 

Thanks everyone, just checked the main battery again and it is a brand new Yuasa YBX9115 AGM (sorry typo above)

 

Is there a date code on the battery? Should be. Shouldn't matter as AGM's are (said to be) good for 5 yr off the shelf but......

That little gizmo in post 7 ("14.8") is handy to have and is only a few dollars but if the battery is done, you need a load tester on it to be sure.

How to Test Your Auto Battery | Battery Testing & Maintenance | Autobatteries.com

Learn more about the importance of battery testing, when to replace your battery and where you can get a free battery test at AutoBatteries.com

www.autobatteries.com

.

It would be handy (and only a few bucks) to see if the alt is doing it's job when the engine is running. Won't tell you if the battery is done though.

Previous car I had would start at 11.7V (using that gizmo) and never a problem. Knowledge says 12.6V so your mileage may vary.

Only alternative I see otherwise is the alternator. And that is warranty work as you've indicated.

At this point, I do think it's the alt.

 

That little gizmo in post 7 ("14.8") is handy to have and is only a few dollars but if the battery is done, you need a load tester on it to be sure.

Available on Amazon for £3.50 LED Digital Car Volt Voltage Meter Battery Vehicle Car 12v 24v Battery Voltage Meter Monitor Tester Checker: Amazon.co.uk: Car & Motorbike
(Or from FleeBay..)

I also have a different version with USB charging points, but be careful as one of these that I purchased from FleeBay knocked out the Radio signal when plugged in..!!

Previous car I had would start at 11.7V (using that gizmo) and never a problem. Knowledge says 12.6V so your mileage may vary..

Think My old R171/350 would start at or just start below 12.0 volts..... (Before I got power in the garage to run a CTEK)
But have to remember that when the car is un-locked and the ignition is turned on, the voltage displayed on the Cigar lighter/12 volt power socket will drop..?

 

I have just been looking at the CTEK range, what model is the best then?

 

The MXS 5 is more than adequate.

CTEK MXS 5.0 12V Charger and Conditioner MULTI XS 5.0 (56-975) MXS5

CTEK MXS 5.0 12V Charger and Conditioner MULTI XS 5.0 (56-975) The MXS 5.0 is a charger that offers state of the art technology that was previously only found in CTEK's professional product range. The charger can solve a broad range of battery-related problems and is the perfect charger for...

www.tayna.co.uk

Shop around though as prices vary considerably

 

The MXS 5 is more than adequate.

CTEK MXS 5.0 12V Charger and Conditioner MULTI XS 5.0 (56-975) MXS5

CTEK MXS 5.0 12V Charger and Conditioner MULTI XS 5.0 (56-975) The MXS 5.0 is a charger that offers state of the art technology that was previously only found in CTEK's professional product range. The charger can solve a broad range of battery-related problems and is the perfect charger for...

www.tayna.co.uk

Shop around though as prices vary considerably

I have just seen they do one especially for AGM batteries, it is only 3.8 amp though.
CTEK CT5 Start/Stop 40-106 Smart Battery Charger

 

Remember, it's NOT meant for charging your battery. It's meant to maintain your battery during periods of storage (ie >3 wk at a time).

I believe though that that "start/stop" is just advertising. My BT gets/maintains both batteries up to snuff and it's not advertised as "start/stop".

 

John, excellent feedback so far. Just as a guide, my 2013 SLK has 79K miles on the original (stock) battery, so it is hard to imagine that your car with only 16K miles is already on the second AGM battery, and you are already having a low battery issue with the new one.

If it is me, I'll find a way to get the car to the seller since it is still under warranty so they can do a thorough inspection, evaluation/diagnosis and repair. Who knows what issue it could develop in the future if not properly diagnosed and repaired.

Good luck.

 

Wow, what a great forum, so much friendly and constructive advice, thanks everyone.

The neighbour has just checked (socially distanced) for any error codes using a state of the art very expensive Snap On code reader and this was clear of any faults. Have ordered the battery charger you have recommended and will take it from there.

Again, many thanks all.

 

I am dating myself but, way back in the days of dinosaurs, I had a problem like this once before it was a loose alternator belt. It looks tight, but you could move the alternator pulley with the engine off which caused the charging light to come on. Just brainstorming here.

 

Latest update received the MXS 5 charger this morning and found that it has an AGM mode on it. Plugged it in and it showed the battery was below "the ready to use" i.e. 3 lights lit suggesting to me that it was not charged sufficiently. will let you know the outcome in due course.

Many thanks for the advice

 

Finally resolved this and it may be helpful to others. It ended up being a an issue with the pre-fuse box. Replaced it with a genuine MB part (total cost £343 including fitting and test) and it has caused no problems since.

 

Thanks for updating this

As usual, marked as Resolved BUT if the problem resurfaces, please return to THIS thread (do NOT create a new one) and we will update accordingly 😉