Caravan Fridge/Battery Charging from car.

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I know it's not strictly cars but I think there are a few on here who know their stuff.

I'm fitting a 20amp Sterling B2B charger in the boot of my car, to charge leisure batteries in caravan while towing and recharge same leisure batteries in car while out and about on holiday. All cables are massive and connection between car and caravan will be Anderson connectors.

I was going to install seperate heavy duty cable from car boot (via VSR) to caravan fridge (I don't have 7S wiring in car but understand it doesn't really work anyway), but I'm starting to wonder whether this is necessary - I'm thinking I could just run cable from caravan battery to VSR just before fridge, this would power fridge, I guess from a combination of caravan battery and supply from B2B. The VSR cuts off at 12.9v so should cut power when engine switched off, so fridge doesn't try to run off battery.

Questions -

the Sterling B2B is a 3 stage smart charger so the voltage varies, but is always above the 12.9v VSR cut off, I'm pretty sure the fridge will accept whatever voltage it gets, but will the battery charging be affected by the current drawn from the fridge, or is it the voltage that determines how the charger works?

surface charge - obviously I want the fridge to shut off as soon as I switch the engine off, but if the battery has just had 20-30 amps pushed into it and the surface charge is giving a voltage in excess of 12.9v the relay will carry on supplying current to the fridge - from my freshly charged battery! Hopefully I'm worrying unnecessarily and either the voltage won't be above 12.9, or if it is it will drop very quickly.

Any advice appreciated, but please bear in mind that I'm a labourer, not a physicist!
 
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Not sure if it is different for a caravan but our camper set up is completely different from what you are proposing. The leisure batteries are charged from the alternator via a VSR with no need for "massive cables " 2.5mm is more than adequate.
Also not sure what type of fridge you have, our battery/mains/gas one only uses the battery to maintain a low temp in the fridge it will not cool it down independently from the 12V dc leisure batteries as it is a very low wattage element. It has to be chilled by either mains or gas before you begin a journey. If your caravan does not have a built in charger then your smart charger should be part of the caravan & used on site with a mains hook up.
 
Thanks. That's the theory but it's well documented that the voltage drop on 10m of 2.5mm cable is quite considerable, hence the market for alternative setups (very common in Australia and the States, not so much here where people just plug in). We tour/camp off grid for weeks at a time so need an effective charging system, not least because when we get home we have a fairly torturous uphill route to park the caravan which means the motor mover pretty much drains a fully charged 110amp battery.
We use gas for the fridge when on site, but I believe the 12v is actually quite effective when supplied with sufficient current (10amp) from the alternator.

Screenshot_20210703-085155.png
 
When we were kids my old man would hook up the car via the towbar plug and run the engine.

Long periods of engine idling can also severely reduce the life of the engine and the oil.

I'd look at getting a seperate generator petrol/diesel mini generator,
 
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Not sure if it is different for a caravan but our camper set up is completely different from what you are proposing. The leisure batteries are charged from the alternator via a VSR with no need for "massive cables " 2.5mm is more than adequate.

Sorry, but you are wrong. 2.5mm is absolutely fine for the 20amp of current, but as this is only 12v volts drop is considerable and often ignored by manufacturers and installers. I installed 2x 4mm end to end of my towcar, to overcome the volts drop and make sure there was enough volts at the caravan to run the fridge and charge the battery.

One possible difference is that my 2x 4mm also supply my caravan's road lighting, via my can-bus system.

I was going to install seperate heavy duty cable from car boot (via VSR) to caravan fridge (I don't have 7S wiring in car but understand it doesn't really work anyway), but I'm starting to wonder whether this is necessary - I'm thinking I could just run cable from caravan battery to VSR just before fridge, this would power fridge, I guess from a combination of caravan battery and supply from B2B. The VSR cuts off at 12.9v so should cut power when engine switched off, so fridge doesn't try to run off battery.

I really have no idea why you are involving a B2B - the 12S works fine via a VSR, providing the VSR is correctly set and you use an adequate size of cables rated for volts drop, rather than just the current. I've been an avid caravanner for the past 50 years, towing thousands of miles around the UK. My fridge always arrives at destination frozen and the leisure battery fully charged, even when that destination might be 300+ miles away. It has to recharge on the go, because I use a 'mover', to get my caravan in and out of my drive.

A simple test is with it all coupled up and engine running at a fast tick-over, put a meter across the leisure battery - it should be higher than 14.1v to enable it to properly charge.
 
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As I said a caravan set up may be different but in our camper the fridge is connected to the leisure bettery & draws it's supply from there not the car so just charging the leisure battery from the alternator draws little current.I presume that caravans have compressor fridges unlike our 3 way fridge in the camper & therefore needs connecting differently although I cannot understand why you would need to run it via the tow car & not the leisure battery.
 
As I said a caravan set up may be different but in our camper the fridge is connected to the leisure bettery & draws it's supply from there not the car so just charging the leisure battery from the alternator draws little current.I presume that caravans have compressor fridges unlike our 3 way fridge in the camper & therefore needs connecting differently although I cannot understand why you would need to run it via the tow car & not the leisure battery.

Sorry, but you still have it wrong....

Caravans use the same system of evaporation fridge on both 12v and 240v (via EHU), or gas. On 240v they have a thermostat, on 12v they don't need one because the 12v is not capable of cooling the fridge down, only of keeping it cool. On 12v they draw around 120w, or 10amps, which would quickly kill a leisure battery, so they are only powered when the engine is actually running via the voltage operated relay. The power is not from the leisure battery, but from the car and it's alternator, which also charges the leisure battery at the same time - the two supplies are (should be) completely independent outputs of the relay.

I really don't know where you might have got the idea that a fridge could be run from a leisure battery? Possibly, you misunderstood the reason for permanent 12v supply going to the fridge - that is for the ignition system for the gas, when you run it on gas.
 
I have a B2B because I've always wanted one and I understand they will charge a heavily depleted leisure battery better than an alternator and we seem to be very "power hungry" off grid caravaners. I used to have an inverter and a mains smart charger in the boot but it was a bit cumbersome.

Going back to the original question, my fridge will effectively be connected to the leisure battery if I used the single cable approach, but only while it is receiving a 20amp charge from the B2B, so will this be a problem?
jj4091 suggests that his fridge in his campervan is connected like this (I wonder if it has a relay to cut the supply when the engine is switched off?)

I certainly don't have the knowledge to argue with anyone on this and I don't know how long your cables are but my calculator tells me there will be a fair drop which may result in a less than optimum charging voltage. I don't have a clue what range of voltage the fridge element tolerates.
Screenshot_20210703-131011.png

My cables are probably oversized but they're just recycled jump leads so cost me practically nothing:

IMG_20210703_131659928.jpg
 
Just been browsing a motorhome forum and apparently some 3 ways are connected directly to the leisure battery (possibly something to do with the AES which chooses the best power source for the fridge) and their B2B's are fine with it so must be ok. No-one seem too sure why this was and a few had been caught out with flat leisure batteries - I can only assume it's to maintain the fridge during short stops - for longer you'd turn on the gas or plug in.
 
I have a B2B because I've always wanted one and I understand they will charge a heavily depleted leisure battery better than an alternator and we seem to be very "power hungry" off grid caravaners. I used to have an inverter and a mains smart charger in the boot but it was a bit cumbersome.

Going back to the original question, my fridge will effectively be connected to the leisure battery if I used the single cable approach, but only while it is receiving a 20amp charge from the B2B, so will this be a problem?

I still fail to see what you are trying to do or what purpose the B2B serves. The B2B is designed to charge one battery, from another, by boosting the voltage of the donor battery. You will have your vehicles starter battery as the donor charging the larger (?) amp/hour leisure battery. The B2B will be around 90% efficient and will rapidly become depleted and unable to start your vehicle - that is why you have two batteries, one for starting, one for your domestic needs.

I hope you understand you cannot normally rush the recharging of a lead acid battery? They will take a large initial charge current, but that rapidly falls away as the batteries voltage rises - that normally takes around 16 hours empty to full. You can reduce the time only by increasing the charge voltage, which then damages the battery.

I accept your point that there might be systems around to run the fridge for a short stop, from the leisure battery, but I have not come across them. You can achieve that, by bypassing the VOR with a switch. On a short stop I just rely on the fridges insulation to keep it cool, for a longer stop away from EHU I switch to gas.

When we used to use the caravan off-grid, long ago - I would have two leisure batteries, one in use, one in the boot connected to the VOR, so it would charge when we went off touring the area. Now I always have EHU - a 20amp PSU charges and keeps my leisure battery charged, but I still would not dream of running the fridge via the 12v. As already said, the 12v system in fridges is a bit of a compromise for keeping the contents cool, whilst on the move.

A much more sensible option to the B2B would be a decent sized solar panel and/or a wind powered generator, but there is no way you can run your fridge long term from the leisure battery. 120w at 12v is 10amps. A 120 amp/hour battery would perhaps keep that going, with no other loads, for a maximum of around 10 hours, but as the battery voltage falls, the ability of the fridge to remain cool will fall off.
 
I certainly don't have the knowledge to argue with anyone on this and I don't know how long your cables are but my calculator tells me there will be a fair drop which may result in a less than optimum charging voltage. I don't have a clue what range of voltage the fridge element tolerates.

The calculator confirms that even a single 4mm is not adequate, if you assume 14.2v at the car battery, less the 0.754 drop, that means 13.44v available at the leisure battery - not enough volts to charge it effectively.
 
Does your tow vehicle have start-stop technology or 'intelligent' charging? If it does then you ought to be asking a dealer or the manufacturer for their advise on charging a leisure battery from your car. (Have a thought that VW's - for instance - have a software 'switch' that has to set in the car computer for charging a leisure battery.) Many modern vehicles only charge the vehicle battery on overrun or constant speed cruising.

I'd be less worried about cable size than I am about single insulated cables. I agree that cable resistance will cause voltage drop but will also limit current flow. It will just take longer to charge the distant battery (the voltage will rise as the current reduces).

Old employers test vehicles had vehicle and equipment batteries - split charging with high output alternators - Vehicle 1 had the ability to power a 1kw5 sine wave inverter for several hours at fast tick over as well as 15A @ 12V, equipment battery feed on 6MM cable just in case equipment battery had to start the car; 2nd vehicle could cope with 25A @ 12V all day at fast tick over or 4 hours plus just on equipment battery alone - that battery was connected on 3mm double insulated cables via split charge relay. Both vehicles 1990's technology though. (I gather replacement vehicle has a second alternator to feed the equipment battery.)
 
I'd be less worried about cable size than I am about single insulated cables. I agree that cable resistance will cause voltage drop but will also limit current flow. It will just take longer to charge the distant battery (the voltage will rise as the current reduces).

Keep in mind that there is already a 10amp load, causing the volts to drop all of the time. I agree that were it not for that fixed load, the battery would eventually charge.
 
I still fail to see what you are trying to do or what purpose the B2B serves. The B2B is designed to charge one battery, from another, by boosting the voltage of the donor battery. You will have your vehicles starter battery as the donor charging the larger (?) amp/hour leisure battery. The B2B will be around 90% efficient and will rapidly become depleted and unable to start your vehicle - that is why you have two batteries, one for starting, one for your domestic needs.

I hope you understand you cannot normally rush the recharging of a lead acid battery? They will take a large initial charge current, but that rapidly falls away as the batteries voltage rises - that normally takes around 16 hours empty to full. You can reduce the time only by increasing the charge voltage, which then damages the battery.

I accept your point that there might be systems around to run the fridge for a short stop, from the leisure battery, but I have not come across them. You can achieve that, by bypassing the VOR with a switch. On a short stop I just rely on the fridges insulation to keep it cool, for a longer stop away from EHU I switch to gas.

When we used to use the caravan off-grid, long ago - I would have two leisure batteries, one in use, one in the boot connected to the VOR, so it would charge when we went off touring the area. Now I always have EHU - a 20amp PSU charges and keeps my leisure battery charged, but I still would not dream of running the fridge via the 12v. As already said, the 12v system in fridges is a bit of a compromise for keeping the contents cool, whilst on the move.

A much more sensible option to the B2B would be a decent sized solar panel and/or a wind powered generator, but there is no way you can run your fridge long term from the leisure battery. 120w at 12v is 10amps. A 120 amp/hour battery would perhaps keep that going, with no other loads, for a maximum of around 10 hours, but as the battery voltage falls, the ability of the fridge to remain cool will fall off.

I have two leisure batteries in the caravan, the idea is to charge the flattest one when touring. When sited for a longer time I will do the same as you, put the flattest in the car boot to top up while driving around the area.

The B2B only works when the engine is running so can't flatten the starter battery.

I thought that the B2B, being a 3 stage smart charger, would charge my batteries more effectively from a deeper stage of discharge than my alternator (which I assume operates at a fixed voltage to top up the tiny bit of drain from starting the engine).

I did assume it would charge quicker than the alternator, everything else being equal - sounds like I'm wrong, but if it'll put 10 or 20 amps in while driving that's ok.

I don't want to run the fridge on battery (it runs on gas on site), I do want to power it when driving and was going to just fit larger "S" cables (again as you have) but have now realised that I can just piggyback off the battery in the caravan that is being charged, as long as I have a relay to disconnect the fridge when the charger is not supplying current - or it will be running off battery.

I have just bought a 100w solar panel as well! But more for summer than winter and night driving.

I don't know whether it's fortunate or not, but the car is old and there is nothing much smart about it, certainly not the alternator.
 
I have two leisure batteries in the caravan, the idea is to charge the flattest one when touring. When sited for a longer time I will do the same as you, put the flattest in the car boot to top up while driving around the area.

The B2B only works when the engine is running so can't flatten the starter battery.

Your alternator will perfectly adequately charge the leisure battery, if the leisure battery is charged via the VOR and decent sized cables. The B2B will not do a much better job.

I thought that the B2B, being a 3 stage smart charger, would charge my batteries more effectively from a deeper stage of discharge than my alternator (which I assume operates at a fixed voltage to top up the tiny bit of drain from starting the engine).

I did assume it would charge quicker than the alternator, everything else being equal - sounds like I'm wrong, but if it'll put 10 or 20 amps in while driving that's ok.

That is not how batteries charge in practice - they take an heavy initial current but that soon drops back to a trickle as the batteries own internal voltage rises - it then drops right back to a trickle.
 
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