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[omega2018]

If you use a voltage sensing relay to switch it in, it should just get disconnected when the car's battery is not being charged and it means it won't be discharged by the car's systems. That behaviour is probably OK unless you're relying on it to be charged heavily while on the move.

Need it fully charged to run the fridge whilst on the move, and again, to use the mover when I get home, as it's too tight a turn to reverse into it's parking slot.

The fridge should be run from the vehicle battery when on the move

Agreed, but my point is the leisure battery will only be maintained at the same voltage as the car battery, i.e. 80%.

Not enough to use the mover.

are you sure about the 80%?  the caravan battery is in parallel with the car battery so should end up in the same state of charge as the car batt.  are euro 6 cars in normal use really leaving their battery at only 80% charge?  if so it's good news for battery manufacturers.

Apparently so ... reason is, I believe, to allow "space" for the regenerative braking to put the elecktrickery it makes .. so the alternators "backoff" saving power (fuel) and the braking tops up the battery instead ..... 

yes i got that. it's simply the car telling the alternator when it's braking and the alternator then putting max load on the belt. no electicity actually comes directly from the brakes/wheels.  I can see the car would do that when  loads are on, i.e. supply the load more from the alternator while braking and the battery when not.  i don't see the need to not keep the battery 100% charged though.

[Kevin Wood]

I don't think that would solve the fridge problem when running on 12v, as I believe, the fridge only works when the alternator is charging.

The fridge might struggle if it is driven by the VSR, as it might not be "on" for a sufficient proportion of the time to maintain temperature. In that case, it would need to be controlled by an ignition switched feed, I think.

[omega2018]

i think there may be a misunderstanding here.  regenerative braking to my mind means an electric car getting amps by using the electric motors to slow the car.  in a fossil fuelled euro 6 car surely all it means is the alternator maxes its load on the fan belt when the car is braking and outputs high amps to the battery and any other electrical load needing it. fossil euro 6 cars have no other source of amps than the alternator and the battery, i.e. they have no electric motors driving/taking energy from the wheels.

i don't think euro 6 cars would deliberately run with the battery off charge (i.e. below 12.75V).  has anyone with such a car done a voltage reading while running?

[Doctor Gollum]

Is there any room to fit a second, conventional, alternator to power the trailer?

[YZ250]

Manufacturer specific tow electrics should disable the 'smart' part of the smart alternator when it senses that the caravan is plugged in. It also disables the stop start to maintain the battery power for the fridge and the Al-ko Trailer Control (ATC).
Assume yours is not manufacturer specific towbar and electrics so maybe a voltage booster within the van may work, but fitting one of those may come close to buying the specific electrics pack. 
Alternatively, always tow with your lights on. 

[Kevin Wood]

i think there may be a misunderstanding here.  regenerative braking to my mind means an electric car getting amps by using the electric motors to slow the car.  in a fossil fuelled euro 6 car surely all it means is the alternator maxes its load on the fan belt when the car is braking and outputs high amps to the battery and any other electrical load needing it. fossil euro 6 cars have no other source of amps than the alternator and the battery, i.e. they have no electric motors driving/taking energy from the wheels.

i don't think euro 6 cars would deliberately run with the battery off charge (i.e. below 12.75V).  has anyone with such a car done a voltage reading while running?

Yes, it's simply offsetting the load on the alternator until such time as it comes "for free", such as when engine braking is in use.  In order to achieve this you need to maintain the battery in a state of partial discharge so that it is able to take advantage of such opportunities as they arise, hence, in the normal scheme of things, it's maintained at less than 100% fully charged.

[omega2018]

i think there may be a misunderstanding here.  regenerative braking to my mind means an electric car getting amps by using the electric motors to slow the car.  in a fossil fuelled euro 6 car surely all it means is the alternator maxes its load on the fan belt when the car is braking and outputs high amps to the battery and any other electrical load needing it. fossil euro 6 cars have no other source of amps than the alternator and the battery, i.e. they have no electric motors driving/taking energy from the wheels.

i don't think euro 6 cars would deliberately run with the battery off charge (i.e. below 12.75V).  has anyone with such a car done a voltage reading while running?

Yes, it's simply offsetting the load on the alternator until such time as it comes "for free", such as when engine braking is in use.  In order to achieve this you need to maintain the battery in a state of partial discharge so that it is able to take advantage of such opportunities as they arise, hence, in the normal scheme of things, it's maintained at less than 100% fully charged.

yes but you could still keep the battery at full charge and use the surplus amps to power everything else .  if euro 6 cars don't do that then the battery will inevitably be left at less than full charge, resulting in early replacement. 

i suspect euro 6 cars do leave their batteries fully charged and claims they don't may be marketing hype to sell  unnecessary devices. OP where is your opening quote from? i can't see any mention on http://techcenter.mercedes-benz.com/en/blueefficiency/detail.html

[Kevin Wood]

yes but you could still keep the battery at full charge and use the surplus amps to power everything else .

That's exactly what a conventional alternator does. Floating the battery at 13.8-14.4 V = keeping it fully charged and powering everything else from the alternator - all of the time the engine is running.

What a "smart" alternator does is to allow the engine ECU to reduce this voltage to perhaps 12.5V during cruising and acceleration conditions. Not low enough to seriously deplete the battery, but low enough that the battery will take no charge whatsoever unless it's pretty flat and, if it's fully charged, it will supply the car electrical loads. When braking, the voltage can be raised higher than the normal alternator output, to perhaps 15 volts or so, rapidly charging the battery and taking over all electrical load.

Don't forget that such a vehicle will probably be cursed with "stop-start" technology, so the loads on the battery are going to be quite high during normal driving as the battery repeatedly starts the engine and runs the accessories while the engine is stopped. By only replenishing such loads when the car is braking or when the battery does get low enough to cause concern, you save a little fuel.

Granted, the battery is going to keep getting cycled between partially discharged and fully charged, which will shorten its life, but since when have environmentalists been able to see the big picture?

[TheBoy]

And the silly Jag system periodically purposely discharges the battery a fair way to test it.  Because that is going to be contusive to battery longevity, isn't it.

Bloody do-gooders. The cull needs to start, as silliness is taking over.

[omega2018]

yes but you could still keep the battery at full charge and use the surplus amps to power everything else .

That's exactly what a conventional alternator does. Floating the battery at 13.8-14.4 V = keeping it fully charged and powering everything else from the alternator - all of the time the engine is running.

no i meant at full charge i.e. 12.75V not on charge (13.8-14.4V) which agreed is what a conventional set up does.

[TD]

Manufacturer specific tow electrics should disable the 'smart' part of the smart alternator when it senses that the caravan is plugged in. It also disables the stop start to maintain the battery power for the fridge and the Al-ko Trailer Control (ATC).
Assume yours is not manufacturer specific towbar and electrics so maybe a voltage booster within the van may work, but fitting one of those may come close to buying the specific electrics pack. 
Alternatively, always tow with your lights on. 

That makes sense for such cars.....id imagine it would make the ecu to get the alternator to produce a high enough output all the time 

[Auto Addict]

And the silly Jag system periodically purposely discharges the battery a fair way to test it.  Because that is going to be contusive to battery longevity, isn't it.

Bloody do-gooders. The cull needs to start, as silliness is taking over.

I knew  you're  tempted, but don't  tow a caravan with Jag

[Kevin Wood]

yes but you could still keep the battery at full charge and use the surplus amps to power everything else .

That's exactly what a conventional alternator does. Floating the battery at 13.8-14.4 V = keeping it fully charged and powering everything else from the alternator - all of the time the engine is running.

no i meant at full charge i.e. 12.75V not on charge (13.8-14.4V) which agreed is what a conventional set up does.

But unless you allow the battery to discharge to some extent, there is no benefit, because you're not shifting any of the load to a later period of braking. If the alternator is chucking out 12.75, it'll be powering all of the car's systems, the only difference is that it won't be charging the battery much if it's depleted.

[TheBoy]

And the silly Jag system periodically purposely discharges the battery a fair way to test it.  Because that is going to be contusive to battery longevity, isn't it.

Bloody do-gooders. The cull needs to start, as silliness is taking over.

I knew  you're  tempted, but don't  tow a caravan with Jag

Oh, I'm devastated, I can't tow a burger wagon. Oh well

[Kevin Wood]

Oh, I'm devastated, I can't tow a burger wagon. Oh well

It's not that you can't, just that your burgers will defrost in the 'fridge.