Battery current with engine running?

[Koops]

With engine off, the ocean control panel indicates battery current as expected, increasing negative current as load is increased (mains power not connected).
With engine on and driving, I see the battery current varying frequently between say -9A and +9A (typically).
Any ideas why this is happening?
I wonder whether the T6 BMT is providing battery assist to the engine via the leisure batteries, using the alternator as a load for braking and a motor for energy recovery? Seems a bit advanced to me!

 

[Loz]

BMT allows the alternator to ‘idle’. In this mode power flows back across the split charge relay hence you see this.
That is why I changed the relay for a more efficient DC-DC charger.

See my post here; https://vwcaliforniaclub.com/thread...arger-to-replace-vw-split-charge-relay.23466/

 

[Koops]

Thanks, I’ll read your linked post. However 12V x 9A = approx 100 Watts which is a fraction of the power for braking or acceleration, so I think the VW bluemotion environmental benefit is rather inflated!

BMT allows the alternator to ‘idle’. In this mode power flows back across the split charge relay hence you see this.
That is why I changed the relay for a more efficient DC-DC charger.

See my post here; https://vwcaliforniaclub.com/thread...arger-to-replace-vw-split-charge-relay.23466/

 

[Skewif]

Thanks, I’ll read your linked post. However 12V x 9A = approx 100 Watts which is a fraction of the power for braking or acceleration, so I think the VW bluemotion environmental benefit is rather inflated!

There’s only so much the poor skinny fan belt can do to slow down a 3 ton van.... Every little helps though.

 

[El Greco]

I doubt if the benefit justifies the complexity, cost and subsequent maintenance

 

[motacyclist]

I’ve just noticed the same over the last couple of days. Before I left home I had the van on hook up to charge the batteries and cool the fridge. I was surprised to see >>20 amps in the display shortly after leaving home; and began to think the hook up must have got unplugged somehow and the fridge had run the batteries down.

When I got to site I went on hook up and it started charging at about 12 amps; gradually reducing.

It’s difficult to watch the panel display while you’re driving, particularly in bright sunlight, but it was a bit duller going home so I was able to watch it a bit more closely. It was generally swinging from plus 16 amps to minus 16 amps, sometimes more. I switched off the fridge and it was just the same. I thought maybe the a/c was running off the leisure batteries so I switched that off, just the same.

It seems to me that VW have made some changes to our previous 180 BMT model and I suspect they are using the leisure batteries in some way to reduce loading on the engine. A bit more investigation seemed to show that the amps were being shoved back in to the batteries when the engine was on overrun, and draining them when under load. Looking under the passenger seat, I was surprised to see how hefty the red cable was going to the battery so obviously the wiring is built to take those sort of loads, rather than just a few amps for the fridge and LED lights.

I don’t really know what would be pulling that sort of current though, around 200 Watts. Maybe I have a fault ...

 

[thehorse]

I A bit more investigation seemed to show that the amps were being shoved back in to the batteries when the engine was on overrun, and draining them when under load.

That is what you'd expect. The leisure batteries are being used to increase the energy storage of the engine battery. When the engine is on the overrun / you are slowing down then the alternator will dump energy into the batteries. When you are accelerating then it is better to avoid the alternator loading the engine using more fuel / sapping power, so the electrical power is delivered from the battery rather than the alternator.

 

[motacyclist]

That is what you'd expect. The leisure batteries are being used to increase the energy storage of the engine battery. When the engine is on the overrun / you are slowing down then the alternator will dump energy into the batteries. When you are accelerating then it is better to avoid the alternator loading the engine using more fuel / sapping power, so the electrical power is delivered from the battery rather than the alternator.

Yes I would expect it on the vehicle battery, but using the leisure batteries seems to be a bit of a dodge to bring the emissions/fuel consumption down. It might seem a laudable idea, but your fridge or parking heater could stop working early because the leisure batteries were already partly depleted since they were being used for other than leisure purposes to help you up that hill.

You should be ok after a long descent though!

 

[Loz]

Yes I would expect it on the vehicle battery, but using the leisure batteries seems to be a bit of a dodge to bring the emissions/fuel consumption down.

Not by design, just a by-product of not upgrading the old split charge relay setup.