Charging profile for a 2018 Passat 132 TSI

[guyburns]

Anyone know the details of the charging profile for a 2018 Passat 132 TSI?

During a wild storm on Saturday night here in Devonport, a group of people were supposed to come over from Sheffield (40 minutes away) for a movie night in our home theatre. I thought: what if we lose power? So, I brought out the cheapie 400W inverter (the projector has a 300 W bulb, so I thought 400W would be enough), connected it to the Passat's battery, set the car on idle – and it didn't work. The inverter didn't have enough umph.

So I started to look at the battery voltage – 12.1 volts and seemed to be falling very slowly. Turned off and started again. Same thing. Fired up my partner's 2007 Yaris – 14.1 volts independant of revs. But that level of voltage triggered the over-voltage turn-off for the inverter.

Good Lord. This is getting complicated. Forget the inverter, how is my car charging the battery?

Back to the Passat. This time, I took it for a drive first, with a voltmeter plugged into the 12 V outlet. 13.4 volts while driving, and the same when idling in the garage. What's going on? How come the charging voltage is all over the place.

Took the car on a longer drive and it seemed to settle down at 13.4 volts. However, when my partner touched the brake, the voltage jumped up to 14.8 volts for a while, then dropped back to 13.4.

So, anyone know how the charging works? Twice, at idle, without putting into gear, charging was not occurring, and the battery voltage was 12.1 volts. Driving, the battery was either at 13.4 or 14.8 volts, depending on…

[MurrayKhodd]

My guess is that what your seeing is the result of VW's repenerative braking. Depending on your level of charge it may unload your alternator for better fuel economy then charge extra strongly when you brake. I suspect such fast charging puts a fair bit of strain on the battery, possibly more than the stop-start system everyone turns off.

MK

[Hillbilly]

My guess is that what your seeing is the result of VW's repenerative braking. Depending on your level of charge it may unload your alternator for better fuel economy then charge extra strongly when you brake. I suspect such fast charging puts a fair bit of strain on the battery, possibly more than the stop-start system everyone turns off.

MK

Regenerative braking and the car holds the battery level down to about 90% to allow for topping up when you brake. My 2010 Passat had that as well so is not anything new

[guyburns]

Thanks for the suggestions.

BATTERY CHARGING
According to Battery University (maybe the best site for battery info), there are three stages when properly charging a lead acid battery (including AGM types):

• Current-limit phase: current limited to a constant value while charging at a constant voltage, typically 14.1 - 14.7 volts.

• Topping phase: current naturally falls while charging at the same constant voltage as before. Charging is terminated when current drops below a certain low level.

• Float charging, typically at 13.5 volts.


MY TESTING
My limited testing reveals there are three charge regions for my vehicle: ~12.1 volts, 13.4 volts, and 14.8 volts. This is what appears to be happening:

1. Under some conditions, such as idling or highway driving with low-electrical load, the battery is allowed to discharge well below the nominal 12.6 V – to improve fuel economy. At some point (12.0V?, 11.9V?) charging must resume.

2. Under certain other conditions, the battery is float charged at 13.4 volts, slightly lower than typical for a lead-acid battery, probably so that battery can accept a decent amount of regenerative charging (as suggested by the replies).

3. Under braking, regenerative charging comes into play at 14.8 Volts. This won't harm the battery because the current would be limited.


FURTHER TESTS
This all started when I wanted to use my Passat to power a 240V inverter, which are best fed at the highest possible voltage to reduce the current from the battery. Inverters are typically rated at 15.0 V max, 10.5V min. For example, 500W at 15 V requires ~33A; but at 10.5 V, ~48 A.

Someone local is selling a pure sine wave inverter, and they've offered to bring it around for testing. Is my Passat, at idle, going to allow its battery to slowly discharge until the inverter drops out? At what voltage will charging kick back in? Can I induce charging by touching the brake pedal at the start?

I'll find out. Maybe my Passat is not suitable for this application. The Yaris certainly is: a nice steady 14.1 volts under all conditions.