Split charge relay question

[neomagik]

Hi all,

I am currently constructing a small caravan, kind of like a teardrop but with more of a traditional caravan shape.

The vehicle I currently have will not necessarily be the one that ends up towing the caravan but when I do come to start towing, does the split charge relay usually come as part of the towbar electrics or must I fit one in the caravan?

Just for information I have run a 13 core cable for the road lights and obviously have the integral permanent and ignition controlled supply available within the cable if needed.

Any help would be gratefully received for future proofing.

 

[ProfJohnL]

The split charge relay is normally fitted in the car, so the pins in teh plug only become active when the cars alternator has excess capacity.

Most modern caravans also have a Habitation relay, which splits the way the power from the car is used.

Rather than trying to rewrite a quite involved peice of text, you may find this quite useful

https://caravanchronicles.com/guides/understanding-the-leisure-battery-charging-circuit/
I cannot vouch for its accuracy.

 

[ericmark]

Years ago split charging was enough to keep on going if you moved every other day using a relay or diode, however the power we use today it may stretch the battery life a bit, but does not really put enough in the battery to be much good, having the fridge work, or the anti-snake yes, but if you really want to charge the battery off the car, your looking at some sort of inverter.

As to what is fitted as standard on the car BS 7671 shows an ignition and non ignition supply but does not actually say what should be fitted on the car, it could just be a simple fuse.

The problem is if the car saves energy by switching off the alternator then if the split charging relay uses voltage to activate it could switch off while travelling.

It is easy to theorise and say fit a split charging diode on car and inverter on caravan which should work with all cars, but really you need to consider what will work with your car and your caravan. Also of course do you really need it?

In the old days I found start the car and rev the engine you may get a 20 amp charge, but within 5 minutes that has dropped to an amp or two, so 3 hours towing only put 3 Ah into the battery, hardly worth the effort.

Even the inverters are likely less than 10 amp but same 3 hours towing will likely put 15 Ah into the caravan battery still not great but far better than 3 Ah.

 

[Guzzilazz]

The problem is if the car saves energy by switching off the alternator then if the split charging relay uses voltage to activate it could switch off while travelling.

It is easy to theorise and say fit a split charging diode on car and inverter on caravan which should work with all cars, but really you need to consider what will work with your car and your caravan. Also of course do you really need it?

Hmm, not sure that I agree with this... A proper VSR wired in correctly doesn't work in the way you indicate... I refer to my experience in the marine sector, where a VSR Is the best solution to the charging problem. The alternator, even if smart works by monitoring battery voltage. With my boat, when I start the diesel, I run the glowplugs for about 10 sec, drawing 40 Amps before cranking and drawing loads of current. As the engine runs, the VSR is directing all alt output to the Engine battery, but within less than TEN SECONDS, you can hear the engine come under load as the VSR clicks over, and parallels both banks. Therefore, the alternator is responding to a lower voltage than the engine battery alone, so a smart alternator will continue to charge at a higher rate until the paralleled banks reach the appropriate voltage to indicate that the alt can shut down or reduce the charge.

The only caveat that I would make is that the cabling to the caravan battery will be much smaller than would be ideal for a proper split charge regime (my boat banks are connected by battery cable), so the voltage detected by the alternator would be lower.

 

[neomagik]

Thanks for the responses. Most helpful.

I will also be connecting a solar panel at some stage to keep the battery topped up throughout the year.

Is there any provision I could make now in terms of wiring to make things easier to connect when I buy the panel(s)?

 

[ProfJohnL]

Thanks for the responses. Most helpful.

I will also be connecting a solar panel at some stage to keep the battery topped up throughout the year.

Is there any provision I could make now in terms of wiring to make things easier to connect when I buy the panel(s)?

It will depend on which panels you choose as the will determine where the panels connections will be, but including a channel or conduit to bring the solar panels wires down to your switch gear and battery area.

 

[Guzzilazz]

I would add that you need to identify a space for a solar controller. The cabling from the panels runs to the controller, and then you would need cabling from the controller to the battery. Good practise is that the cable run from the controller to the battery should be as short as possible

 

[neomagik]

Again thank you for the recommendations.

From the link provided by ProfJohnL, I have bought a Voltage Sensing Relay (VSR) and bench tested it to ensure it works as I need it to…… and it does.

The relay came with 6 meters of 4.5mm cable which is rated to carry a nominal 42 amps.
I’m thinking of using this cable as the main cable from the leisure battery terminal to the caravan fusebox which is less than a meter away. I would install a 30A fuse in this cable as close as possible to the battery terminal.

Does anyone see a problem with this? The only issue I can see would be if there were a fault on the short piece of cable directly from the terminal, prior to the fuse but this is very unlikely. It’s like having short pieces of small size cable connecting from a busbar chamber prior to a protective device, which is permitted in commercial 400v 3 phase systems on the premise that the likelihood of a fault is very low.

 

[ProfJohnL]

Again thank you for the recommendations.

From the link provided by ProfJohnL, I have bought a Voltage Sensing Relay (VSR) and bench tested it to ensure it works as I need it to…… and it does.

The relay came with 6 meters of 4.5mm cable which is rated to carry a nominal 42 amps.
I’m thinking of using this cable as the main cable from the leisure battery terminal to the caravan fusebox which is less than a meter away. I would install a 30A fuse in this cable as close as possible to the battery terminal.

Does anyone see a problem with this? The only issue I can see would be if there were a fault on the short piece of cable directly from the terminal, prior to the fuse but this is very unlikely. It’s like having short pieces of small size cable connecting from a busbar chamber prior to a protective device, which is permitted in commercial 400v 3 phase systems on the premise that the likelihood of a fault is very low.

From what I understand by your reply, I can't see a problem. The likelyhood of a fault on a piece of conductor is usually related to teh current passing, and in this case I'd guess a 42A capacity is well above any likely currents that will arise, So if anything it may be a sledge hammer to to crack a small nut situation.

 

[neomagik]

Due to the helpfulness received so far in this thread I'm wondering if to start a new thread dedicated to the build as I'm sure I'll have further questions as it progresses. I'll post some progress photos from time to time too if that's permitted?

 

[ericmark]

The problem is if the car saves energy by switching off the alternator then if the split charging relay uses voltage to activate it could switch off while travelling.

It is easy to theorise and say fit a split charging diode on car and inverter on caravan which should work with all cars, but really you need to consider what will work with your car and your caravan. Also of course do you really need it?

Hmm, not sure that I agree with this... A proper VSR wired in correctly doesn't work in the way you indicate... I refer to my experience in the marine sector, where a VSR Is the best solution to the charging problem. The alternator, even if smart works by monitoring battery voltage. With my boat, when I start the diesel, I run the glowplugs for about 10 sec, drawing 40 Amps before cranking and drawing loads of current. As the engine runs, the VSR is directing all alt output to the Engine battery, but within less than TEN SECONDS, you can hear the engine come under load as the VSR clicks over, and parallels both banks. Therefore, the alternator is responding to a lower voltage than the engine battery alone, so a smart alternator will continue to charge at a higher rate until the paralleled banks reach the appropriate voltage to indicate that the alt can shut down or reduce the charge.

The only caveat that I would make is that the cabling to the caravan battery will be much smaller than would be ideal for a proper split charge regime (my boat banks are connected by battery cable), so the voltage detected by the alternator would be lower.

I love riding my bike on the canal tow path, and my son did at one time live in a narrow boat, even when waiting for the lock to fill or drain, I have never heard the narrow boat engine stop as with a car when at traffic lights, I will listen more carefully now, But I would be nervous letting the engine stop unless tied up, and if tied up I would expect it uses a human machine interface (HMI) called an ignition switch, as far as I am aware the engine of a narrow boat does not stop charging when under load and only charge when on over run as canals tend to be level there are no up and down hill sections.

I don't really like the idea of stopping the car engine at traffic lights, has not let me down yet, but whole idea of stopping engine for such a short time seems flawed, and it seems so many people stop at traffic lights, then decide to go closer to the car in front, and as soon as you release the brake to also move up, engine starts and will not re-stop, so most of the time it ends up running anyway.

Narrow boats do tend to have either twin alternators and stage charging with domestic one, or special inverters, and there is no reason why inverters can't be used with a caravan to charge it's battery from the car, I am sure it would work well.

However a split charging relay is rather useless today, one with VRLA on the car the voltage is reduced, and two the engine management turns off the alternator to save fuel, even before that the split charger relay was not very good, unless you boost the volts, battery charging takes time, a 50% discharged battery with a float charge will take around a day to fully charge up again, we simply don't tow for that long.

Lift the volts to 14.8 from 50% to 85% charged then drop to 13.8 volts for last 15% and then yes in 5 hours you can put a reasonable amount of charge in the battery. But at just 13.8 volt if you can get 13.8 volts by time it reaches caravan and first 5 minutes you may get 10 amp charge, but after 15 minutes that has dropped to around 2 amp if your lucky.

For a 85 Ah battery 50% charged to recharge in 5 hours it needs to maintain 10 amp charge for most of the time, that will not happen with a relay.