s320 2018 Battery Misery - HELP!

[kingsize]

s320 2018 32k miles

If anyone can help/advise I would be so grateful - am at the end of my tether.

For the last no of months I have experienced repeated flat battery if car not driven for a couple of days. Flat as in won't even turn over and on one occasion completely wiped the SRS system producing a constant error message requiring a reset.

I am driving MUCH less than I used to for various reasons - sometimes will not drive for a couple of days and then just for 30 mins or so.

So, at last service 3 months ago, I had the battery replaced (as the servicer said it was the 'big battery' and it was £250 - that's fine).

Within a week, the problem was back, with the car barely starting - turning it over and over, electrics going haywire and 'catching' if you will, at the last. Then it drives fine until the next time.

Left the car with an auto electrician (highly regarded) - his advice was to 'drive it more'...

I left the car undriven for 7 days recently - it started first time and then wouldn't the next morning.

I have invested in a Ring Ultraboost 1500 Jump starter which works superbly and has started the car 4 times now when needed.

The battery was flat after leaving it (for 24h after repair) with an MB dealer to fix the aircon.

I sat in the car for 50 mins using the Comand with the engine off - got a warning to turn the engine on and of course it wouldn't start.

I get messages from the Mercedes Me app - "the charge level of your vehicles started battery is critical' & '12v battery very low'.

So...

1. Is there a SECOND battery that needs replacing? My internet searches have proven inconclusive.

2. Can anyone offer any thoughts at all - I'm genuinely at my wit's end - I love my car but am losing faith in it, which grates somewhat.


Any advice/suggestions glady received.

 

[DSM10000]

When you turn the car off a number of systems remain active for some time. This is normal but occasionally systems do not "go to sleep" but and cause a parasitic current drain.

Did your "highly regarded" auto electrician" check for current draw with the car shutdown?

 

[Delderek]

Maybe originally fitted with a Tracker, that has a small rechargeble battery, that is recharged from the vehicle battery. This small battery could be faulty, happened to a friend of mine. Took him 4 months beore someone diagnosed it.

 

[kingsize]

Than you both. Dont know if any drain was sought or indeed detected I’m afraid.

Can’t go to MB dealers as they are unbelievable chancers who I simply couldn’t believe under any circumstances- they could tell me anything (and indeed have done so in the past).

Does anyone know if there is a second battery?

 

[DSM10000]

Than you both. Dont know if any drain was sought or indeed detected I’m afraid.

Can’t go to MB dealers as they are unbelievable chancers who I simply couldn’t believe under any circumstances- they could tell me anything (and indeed have done so in the past).

Does anyone know if there is a second battery?

I am not sure if there is a second battery (unless it is a hybrid?) but regardless you need to have an auto electrician or MB independent check for any parasitic drain and try to isolate any of the module or circuit that could be causing the problem.

If you let us know which town or city you are in /near to then members will hopefully be able to suggest someone

 

[l5foye]

Than you both. Dont know if any drain was sought or indeed detected I’m afraid.

Can’t go to MB dealers as they are unbelievable chancers who I simply couldn’t believe under any circumstances- they could tell me anything (and indeed have done so in the past).

Does anyone know if there is a second battery?

Yes your car will have an auxiliary battery. If it is flat, it can cause all sorts of problems.

 

[keith of kent]

On the passenger side B post there will be a QR code sticker. Scan this with a QR code reader on your phone and it will show the location of batteries within the vehicle.
I recently got a replacement auxilary battery for my E350 from Tayna batteies for £50.99 incl postage(arrived the next day)
Easy to do assuming you can use a small spanner, you don't need to pay a garage.
I also got in 2022 a replacent main battery from them F21 Varta Start-Stop Plus AGM Car Battery 12V 80Ah £144.96 delivered

 

[DSM10000]

On the passenger side B post there will be a QR code sticker. Scan this with a QR code reader on your phone and it will show the location of batteries within the vehicle.
I recently got a replacement auxilary battery for my E350 from Tayna batteies for £50.99 incl postage(arrived the next day)
Easy to do assuming you can use a small spanner, you don't need to pay a garage.
I also got in 2022 a replacent main battery from them F21 Varta Start-Stop Plus AGM Car Battery 12V 80Ah £144.96 delivered

The OP has had the main battery changed 3 months . It is still discharging so he needs to find why.

 

[ChrisPDuck]

The rate of drain seems high enough for it to be a faulty alternator diode pack or alternator regulator. I had a similar rate of drain and expected to find faulty diodes. After a bit of a fight to get the alternator out I took it to be refurbished, and they told me the diodes tested fine. I asked them to change the regulator while they had it, and to my surprise that fixed the drain problem.

I'd say get a vehicle electrician to test your alternator. If you can get to the alternator easily enough, and you have a digital multimeter you can test it yourself:

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(I promise you, you'll never get this guy's accent out of your head!)

 

[kingsize]

Thanks - I'll see if I can find out about this.