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[Auto Addict]

"Current MB's use the alternator differently to older models.

 Most have variable output alternators to help reduce fuel consumption.

 In most cases once a battery is fully charged they hold battery voltage between 12.6v-13v to reduce fuel consumption. If you apply the brakes then the voltage may increase to as much as 15v to recoup the "free" energy, if accelerating then it may drop as low as 12v. The engine ECU determines what voltage it wants to maintain at the battery based on load/temps/electrical consumers etc.

 All Bluefficiency models have this, and most other current models."

So, the leisure battery on my caravan may not always be at full charge, if I read the above correctly, as it will be balanced to the car battery.

If this is true, is there a way around the problem.

Car electric experts your thoughts please.

[Andy B]

you know there is 

https://caravanchronicles.com/2017/07/21/is-a-euro-6-engine-killing-your-leisure-battery/ 

[Kevin Wood]

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.

[Auto Addict]

you know there is 

https://caravanchronicles.com/2017/07/21/is-a-euro-6-engine-killing-your-leisure-battery/ 

Well yes, apart from it's out of stock, and no-one knows the price of it.

[Auto Addict]

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.

[henryd]

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

[Auto Addict]

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.

[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.

[Entwood]

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 ..... 

[Kevin Wood]

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.

Not if you use a voltage sensing relay. This will connect the leisure battery only when the alternator is kicking out a decent voltage, so the battery will be charged when the car decides to pump charge into the vehicle battery, and left standing when it isn't.

You would need to arrange for the fridge to get power from the car whenever the engine is running, however.

[Andy B]

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.

Not if you use a voltage sensing relay. This will connect the leisure battery only when the alternator is kicking out a decent voltage, so the battery will be charged when the car decides to pump charge into the vehicle battery, and left standing when it isn't.

You would need to arrange for the fridge to get power from the car whenever the engine is running, however.

If you read some of the article I linked to, the gist of it is that the smart alternator sees the leisure battery's voltage & tries to balance it out with the car's battery. I think a VSR is too simplistic ......

[Kevin Wood]

If you read some of the article I linked to, the gist of it is that the smart alternator sees the leisure battery's voltage & tries to balance it out with the car's battery. I think a VSR is too simplistic ......

Yes, but I don't think he's 100% correct. The two batteries' states of charge will track each other if they are simply in parallel all the time, of course. With a VSR, it won't pull in while the car's alternator is allowing the vehicle battery to discharge, because the terminal voltage will sink too low, so the leisure battery won't discharge.

During periods of regenerative braking or when the car decides the battery has got a bit low, the voltage will rise, the relay will pull in and the leisure battery will receive charge. The car will probably monitor charging current as an additional indicator of the state of the battery, so it may well see the extra current drain into the leisure battery and decide extra charging is required.

Once it's happy with the state of charge of the battery, or the opportunity for regenerative charging has passed, the voltage will drop once again, the leisure battery will be disconnected and it will retain its' new charge level. I think it would work.

As an alternative, however, a charge controller that can boost the car's output voltage and inherently blocks any discharge path back from the leisure battery would also do the job.

[Bigron]

Back in the days before this alternator complexity took hold, people used two fat diodes in a "bean tin" to split the charge between the two batteries; the diode voltage drop was hardly significant and the laisure battery could not discharge to the car.....happy, simple days! 

Ron.

[Kevin Wood]

Back in the days before this alternator complexity took hold, people used two fat diodes in a "bean tin" to split the charge between the two batteries; the diode voltage drop was hardly significant and the laisure battery could not discharge to the car.....happy, simple days! 

Ron.

Yep, before they decided that it's preferable to shorten the life of the battery, starter, alternator, etc. with all the environmental consequences of their manufacture and disposal, to achieve an extra femtomile-per-gallon.

A bit like the recent U-turn on speed humps, really, give it a few years and we'll have revolutionary new vehicles that save the planet by not stalling at every set of traffic lights. 

[Auto Addict]

If you read some of the article I linked to, the gist of it is that the smart alternator sees the leisure battery's voltage & tries to balance it out with the car's battery. I think a VSR is too simplistic ......

Yes, but I don't think he's 100% correct. The two batteries' states of charge will track each other if they are simply in parallel all the time, of course. With a VSR, it won't pull in while the car's alternator is allowing the vehicle battery to discharge, because the terminal voltage will sink too low, so the leisure battery won't discharge.

During periods of regenerative braking or when the car decides the battery has got a bit low, the voltage will rise, the relay will pull in and the leisure battery will receive charge. The car will probably monitor charging current as an additional indicator of the state of the battery, so it may well see the extra current drain into the leisure battery and decide extra charging is required.

Once it's happy with the state of charge of the battery, or the opportunity for regenerative charging has passed, the voltage will drop once again, the leisure battery will be disconnected and it will retain its' new charge level. I think it would work.

As an alternative, however, a charge controller that can boost the car's output voltage and inherently blocks any discharge path back from the leisure battery would also do the job.

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.