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Battery Charging


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20 hours ago, beetle1 said:

Hi folks, apologies if this has already been answered, just about to charge my Mk8 R with Ctek MXS 5.0 and there’s a AGM mode on it.  Any ideas if the battery is AGM?

My battery is a 5QS 915 089 C


I know on other forums people have emailed the manufacturer to confirm. 

I find it annoying these days that you have to charge a car battery even if it’s 2-3 old.


My experience is my wife’s Fiesta that has stop/start and it flagged up a low battery warning when the car was about 2 years old. The Ford dealer actually said to drive regularly with lights on, heated rear window etc. to load the battery to force the alternator to charge the battery. I don’t  think modern batteries charge that well in normal use, from our experience regular charging is needed, even if you are using the car a fair bit. 
Ridiculous really compared to a few years ago. 

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hmm..... not mentioned anywhere in this thread (I think) and accepting the important question regarding how to charge AGM/EFB batters in cars with Start Stop - perhaps the more critical issue is the  simple matter of the physical placement of the battery itself and the connection of the charger leads


Yes, it's in the owner's manual - but not everyone reads the tome that VW generously provides at the buyer's expense!!


There are 2 x physical aspects at play when charging batteries in cars that have Start Stop installed:


First, these vehicles also have regenerative braking - which means that under conditions of engine braking, the alternator voltage is increased so that the battery receives additional charge. This system captures the car's normally lost kinetic energy as electrical energy and it stores this in the battery.  Of course for regenerative braking to work, there must be sufficient spare capacity in the battery to receive this additional charge. VW guarantee that there is always sufficient spare battery capacity for regenerative braking by ensuring that the battery is NEVER fully charged. The headroom in battery capacity that is allocated for regenerative braking is about 20% of full charge. I very much doubt that even sophisticated external chargers understand (and cater-for) the designed headroom for regenerative breaking in car's with Start Stop. Happy to be advised otherwise!!


Second: cars with Start Stop have a special module bolted onto the negative battery terminal (it's called J367) that monitors ALL (emphasis on ALL) the electrical current flowing out-of and into the battery over the battery life.  So, over the life of the battery - J367 provides a complete history of the electrical demands placed on the battery.  Have a look on your car - J367 is as shown in the picture below. (ignore the yellow-ish gunk - it's an anti-frosting spray that dealers use in southern Australia)


The car's Energy Management System (EMS) uses the database from j367 to determine the best charging regime for optimal battery longevity. These decisions are different for EFB and AGM batteries and the EMS uses the battery type that is specified in the Gateway module to understand which battery is installed in the car. So, if the battery type alters when the battery is changed  - the Gateway module needs to be told about the changed battery manufacturing technology (else the EMS will make sub-optimal decisions).


Also key to the ability of the car's EMS to make correct charging decisions is the accuracy of the electrical current measurements into and out-of the battery (i.e. from J367).


If the battery is removed from the car and bench charged with an external charger - J367 will be blind to the re-charged battery capacity and the EMS's understanding of the historic  electrical current measurements for the battery will be wrong.


Also, if the battery remains in the car - but the negative lead from the charger is placed directly onto the negative battery terminal - J367 will be out-of-circuit for the external charging period. Again this will result in J367 being blind to the recharged battery capacity and once more, EMS decisions will be wrong because the historic data for the battery will be incorrect,


Moral of the story - there's only ONE way to charge EFB/AGM batteries in cars with Start Stop: Leave the battery terminals connected and ALWAYS  connect the negative charger lead to the special charging post that's bolted to the car body .


For emphasis: NEVER - ever connect the negative charger lead directly to the negative battery terminal!!






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7 hours ago, dv52 said:

For emphasis: NEVER - ever connect the negative charger lead directly to the negative battery terminal!!

You can’t be any clearer here Don - noted.   May I ask, for peace of mind and should the occasion ever arise….should a battery jump starter or booster be attached the same way when trying to start a car with a flat battery?

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@NRW: Excellent question - my bad for not including jump-start in my previous mental meanderings!


Short answer = YES!

Slightly longer answer = the same basic principle applies in cases of  jump-start  AND battery charging because both processes have ostensibly the same impact on the battery  -meaning that both add electrical energy to the battery, which is important for the EMS to understand (albeit, the primary purpose of jump-start is to crank the engine whilst the external battery is paralleled with the native car battery)



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