W204 - Battery Charging Question

[mpc]

I went to start my car this morning and the battery was flat. Its a C250 W204 2011 Estate.

I hooked up a battery charger this morning (9.30am) and have left it most of the day. I have kept checking on it every few hours to see progress (my battery charger has a % charged display) and it seems to be hanging at 80%. Should I keep it charging in hope it will gt to 100% or is it likely that 80% is the best i'm going to get?

 

[OneForTheRoad]

Maybe crank up the amps if you can and force it.

 

[m80]

Are you in a rush?
Trickle charge / discharge is better for the battery. A very rapid charge over a sustained period can see off a battery. That's more to do with heat generated and warping the plates.

The initial voltage can rise fast when charging, and likely that's what you're seeing. But that isn't a fair measure of the amount of energy in the battery.

If you're in a rush to use it then as soon as it starts, using it will be about the fastest way to charge it. The alternator can put out a high amount.
Otherwise just leave it to do its thing.

 

[markjay]

Stop/Start AGM batteries are typically only charged to 80% capacity, apparently this optimises energy efficiency. There's a battery management module on top of the positive terminal that manages that. It shouldn't stop the battery from charging fully if you connect the charger directly to the battery terminal, but it might stop the charging if the charger is connected to the charging point in the engine bay and not directly to the battery - I do not know. Either way, regardless of why the battery is 'stuck' on 80% charge, this should be enough to start the car and drive it away. The battery will be charged best while driving. Put the transmission in Sport mode to ensure higher engine revs.

 

[Dazsw]

It all depends on how old the battery is a well.

 

[MarcusRomer]

My W204 is charging at 15.2 volts, this doesn’t sound normal as sometimes when starting the car its labouring to start. A company has offered to diagnose the problem at £80. I feel that I should just replace the alternator as the car is a 61 plate and it’s never been changed.

Does anyone feel this could be the best move as an alternator at this age could fail again and spending £80 every time is not cost effective?

 

[Petrol Pete]

^^^ Marcus . How many miles on your car ? Is it on the original battery ? Replacing the alternator sounds a bit desperate . Is it the type of alternator where you can just replace the regulator ? looks a bit like this. Cheap and easy to do.

I would do a bit more fault finding before you start replacing things.

 

[MarcusRomer]

^^^ Marcus . How many miles on your car ? Is it on the original battery ? Replacing the alternator sounds a bit desperate . Is it the type of alternator where you can just replace the regulator ? looks a bit like this. Cheap and easy to do.

View attachment 124634 I would do a bit more fault finding before you start replacing things.

^^^ Marcus . How many miles on your car ? Is it on the original battery ? Replacing the alternator sounds a bit desperate . Is it the type of alternator where you can just replace the regulator ? looks a bit like this. Cheap and easy to do.

View attachment 124634 I would do a bit more fault finding before you start replacing things.

 

[MarcusRomer]

^^^ Marcus . How many miles on your car ? Is it on the original battery ? Replacing the alternator sounds a bit desperate . Is it the type of alternator where you can just replace the regulator ? looks a bit like this. Cheap and easy to do.

View attachment 124634 I would do a bit more fault finding before you start replacing things.

Hi Pete, my car has done 98,050 and I’m on my second battery. The original one was replaced 2 years ago.

 

[OneForTheRoad]

Hi Pete, my car has done 98,050 and I’m on my second battery. The original one was replaced 2 years ago.

15.2 v will fry a battery dry , a new regulator is all you need , as advised already.

 

[190]

In the old days you could say replace the voltage regulator and that will fix the problem.

Voltage regulators work by controlling the average excitation current to the rotor windings. when a voltage regulator fails the excitation current usually becomes unrestricted and that would cause the alternator to go to full voltage output which in my experience of driving down the M62 with a model 8 Avo perched on the dash can be anything up to 18 volts. So 15.2 volts as has already been pointed out, might not be a voltage regulator failure. That's one thing. The 2nd things is do we know that smart alternators even have a recognisable voltage regulator to replace. So called smart alternators aren't smart at all they simply have the rotor excitation current controlled by an ECU. It's the ECU that's smart. How is that done ? Does the ECU replace the voltage regulator or manage it in some way in which case either could be a fault.

I'm just making the point that it's more complicated than it used to be and it may no longer be possible to say replace the voltage regulator and that will fix the problem.

If someone here is experienced with "smart alternators" can they tell us the answer.

 

[Fastcar155]

Hi , Stirling Power Products in Droitwich are superb and worth a call.

Specialise in marine products and thier experience in 12 volt batteries is second to none.