Split charge system advice

[Padraic]

Hi all, I'm looking for some advice on the split charge system. Some background info first.
A few months ago I fitted a Cyrix dual voltage sensing split charge relay, in order to enable charging of leisure battery and engine battery when on hook-up. However, after researching leisure batteries, I learned that they require different charging profiles than lead acid batteries, so I then decided to remove the dual voltage sensing split charge relay and revert back to the standard system.
In the future I plan to fit a Ctek mains charger like Sidepod did for charging the engine battery when on hook-up.
So I also recently fitted a tow-bar and a 13 pin socket. In order to have it wired up correctly, I fitted a permanent + supply and a engine running + supply.
I've wired the engine running + supply via a relay, however I'm struggling to find a suitable engine running trigger for the relay.
Firstly I just taped in to the connections for the split charge system, however that started to give an alternator warning on the MFD.
Second attempt was to use the D+ connection (this connection gives alternator voltage when the engine is running) from the fuse box under the dash. same result, alternator warning on the MFD.

So after some more research, it seems that the standard split charge relay is already fed from the D+ from the alternator.
This D+ connection, is only able to supply a low current, and when the second relay is connected, it draws too much current and puts the alternator in to fault and affects the charging of the engine battery.
As this seems to be the only correct way to enable switching of the split charge relay and relay for the engine running + feed to the 13 pin tow-bar socket, I want to find a solution.

So my plan is to use the D+ connection to switch on a small standard type relay. This will work okay as the D+ is already capable of switching on the one split charge relay.
This relay will in turn switch on the split charge relay and relay for the engine running + feed to the 13 pin tow-bar socket. With this set up, there should be no faults from the alternator and both relays should switch on when the engine is running and the alternator is supplying the required voltage.
Should I source a relay with a resistor to limit the current flow through the relay, or a relay with a diode to dissipate the stored energy in the coil?

Any suggestions or opinions on this plan of mine would be greatly appreciated.
Regards Padraic.

 

[sidepod]

I’ll get my head around this tomorrow

 

[Padraic]

I’ll get my head around this tomorrow

Hopefully I’ve explained it so it makes sense.
In a nut shell, I’m gonna use a smaller relay to switch on two larger relays.

 

[sidepod]

First off, why power your engine running perm supply via a relay?

Secondly for the relay coil (trigger) just come off the leisure battery main feed on the split charge relay ie the big red cable that is live when engine is running.

 

[Padraic]

First off, why power your engine running perm supply via a relay?

Secondly for the relay coil (trigger) just come off the leisure battery main feed on the split charge relay ie the big red cable that is live when engine is running.

Firstly, This supply is to run a fridge in a caravan when it’s on tow, so it could draw up to 10 AMP, so this should only be on when the engine is running.
Secondly, wouldn’t each cable on each side of the split charge relay be at 12volts, even with the engine off, so this would keep the relay triggered all the time.

 

[sidepod]

At the split charge relay there are two large red cables on one connection and a single large red on the other.

The first pair are only live with engine running, the other live permanently as it connected to the leisure battery.

When the engine stops the relay opens and the two sets of connections become isolated therefore my initial argument stands.

 

[Padraic]

At the split charge relay there are two large red cables on one connection and a single large red on the other.

The first pair are only live with engine running, the other live permanently as it connected to the leisure battery.

When the engine stops the relay opens and the two sets of connections become isolated therefore my initial argument stands.

Okay, I thought the pair of red cables were connected to the engine battery, if what you say is right it’s just a direct feed from the alternator. I’ll have to check with a meter.

 

[Padraic]

In short, I need a trigger that is at at zero volts unless the engine is running and the alternator is working correctly.

This is already achieved for the factory fitted split charge relay. However this trigger feedbfrom the alternator is only capable of switching one relay.

So I’m proposing to use this already available engine running to trigger one small relay that in turn will trigger the existing split charge relay and the relay I’ve fitted to supply the feed to the 13 pin socket for the fridge connection.
I hope this makes sense.

 

[sidepod]

Ah yes. Sorry my bad. Doh.

 

[sidepod]

I’ll have a think.

 

[sidepod]

Ok I think this is what you want.

Replace the split charge relay with a similar double pole relay ie an extra set of contacts doing exactly the same as the first.
Tee off the engine battery feed into the second contact then off that to the caravan supply.
This away the caravan will have a perm eng running supply but not when you stop.
Any good?

Relay is simple to source.

 

[Padraic]

Ok I think this is what you want.

Replace the split charge relay with a similar double pole relay ie an extra set of contacts doing exactly the same as the first.
Tee off the engine battery feed into the second contact then off that to the caravan supply.
This away the caravan will have a perm eng running supply but not when you stop.
Any good?

Relay is simple to source.

View attachment 35355

Yea, and the first relay is triggered by the original factory fitted split charge relay trigger wires.
Here is another schematic.

 

[sidepod]

Similar but yours uses thee relays, as opposed to one (replacement) ?

 

[Padraic]

Similar but yours uses thee relays, as opposed to one (replacement) ?

Yea, I could source a double pole double throw relay, which can handle the split charge load, although I’ve not seen one yet.
I’ve already got the two relays, so the smaller one shouldn’t cost much more, and no point wasting money by not using them.
Also the second relay could be used as a backup for the split charge relay should it ever fail.
Anyway, you think the idea is okay and should give me the results I’m after?

 

[sidepod]

I don’t see any issues.

 

[sidepod]

Thinking about this a bit more, it does require that all of your relays are N/O when de-energised (shelf state) as drawn. This will ensure there is no current drain when the vehicle is parked.

Also, I’m not sure what the KL30 is?
If this is a battery then the whole scheme will rely on charge state.

 

[Padraic]

Thinking about this a bit more, it does require that all of your relays are N/O when de-energised (shelf state) as drawn. This will ensure there is no current drain when the vehicle is parked.

Also, I’m not sure what the KL30 is?
If this is a battery then the whole scheme will rely on charge state.

Yea all three will be NO, to ensure do current draw when de-energised. They will also have diodes to ensure safe dissipation of the stored energy from the coil when switching off.
KL30, is the permanent live from the fuse board under the dash. So yea it will rely on charge state, but should be fine as it’s only designed to switch on with the engine running.

 

[sidepod]

Cool. Good man. Make sure you size the cables for voltdrop, it’s a long run from the fuse board to the tow ball.

 

[Padraic]

Cool. Good man. Make sure you size the cables for voltdrop, it’s a long run from the fuse board to the tow ball.

Yea, it’s a long run, but I sourced the proper VW cables second hand from Germany. They plugged straight in the the fuse board and had the proper fuse holder that fitted in to the spare slots under the dash.
They even had the correct connectors to plug in to the tow bar wiring loom.

 

[Rascal1]

That is a lot of wiring to achieve a feed to a caravan when the engine is running.
It should be easy to find some thing that is only live when the ignition is on.
And taking Sidepods point about cable sizing,I doubt the original loom was designed to run a fridge.
As regards the VSR using the wrong charging profile for the engine battery,the alternator is charging the leisure battery with a profile that is designed for a normal lead acid battery.
You could refit the VSR and use the original relay to feed the caravan..The engine battery will not be upset
by the difference in the profile.

 

[Padraic]

That is a lot of wiring to achieve a feed to a caravan when the engine is running.
It should be easy to find some thing that is only live when the ignition is on.
And taking Sidepods point about cable sizing,I doubt the original loom was designed to run a fridge.
As regards the VSR using the wrong charging profile for the engine battery,the alternator is charging the leisure battery with a profile that is designed for a normal lead acid battery.
You could refit the VSR and use the original relay to feed the caravan..The engine battery will not be upset
by the difference in the profile.

It’s supposes to only be wired with an engine running feed, if it was running with an ignition on feed, there is a chance the ignition could be left on and flatten the engine battery.
I had though about putting a switch on it, but I’m gonna go with the above plan, I just want it done correctly, then it’s done one and for all.

 

[Padraic]

Similar but yours uses thee relays, as opposed to one (replacement) ?

Just thinking about this a bit more. If I used one double pole double throw relay, it may have a very large coil to activate the two sides of the relay.
This may cause too much current draw and give me the same fault on the alternator.
I might be safer to stick with the three relay set up, even though it adds more parts that could fail in the future.

 

[sidepod]

I don’t think an extra set of contacts will increase the current draw of the coil by very much.

When searching for relays the current rating of the coil is usually quoted.

Keep in mind, the main reason the alternator signal is used to control a split charge relay system is to protect the leisure battery in the event of alternator failure ie the leisure battery doesn’t end up being drained by ignition/lights etc.

For your fridge supply switching it’s less important so any “engine off” signal will suffice
If you don’t want to swap out the existing split charge relay for a double pole unit then I’d just fire the relay off the ignition switch. Nice and simple and it keeps it all separate from the split charge system.

 

[Padraic]

Hi all, so I finally finished this job today.
Decided to use the same relays as the split charge relay, due to them having diode protection on the + & - of the coil. Also one could be used as a spare of another ever failed.
So as discussed earlier, I’ve set it up so that the top relay is triggered by the factory circuit that triggers the split charge relay. This relay in turn triggers the split charge and towbar fridge circuit relays.
Tested and working fine.