To properly preventing dc-dc charger from draining starter battery.

[TheMobileWanderer]

OK, so I am going over my electrical plan again, to make sure I haven't overlooked something or used a wrong-spec item.
So, I'm on the subject of battery protect.

If I were to use a D+ wire to ensure the dc-dc charger only operates when the ignition is on, couldn't I fall into the scenario that the ignition is on but not the engine and so, the starter battery could/would drain.

So then I saw a thing about fitting an oil pressure sensor, which, because it only has pressure if the engine is actually running, the dc-dc charger could be set to function only when there was oil pressure i.e., when the engine is actually running.

And then I remembered that I often over think things or generally complicate them.

So then I thought about connecting to the wire from the alternator, which should (I think) only be live if the alternator is charging the starter battery.

Is it OK to tap into the live from the alternator, to flip a relay thus enabling the dc-dc charger to function only when the engine is on, not when just the ignition is on?

 

[Driventemple]

If I were to use a D+ wire to ensure the dc-dc charger only operates when the ignition is on, couldn't I fall into the scenario that the ignition is on but not the engine and so, the starter battery could/would drain.

A proper D+ supply is only available when the engine is running, ie alternator charging.
If you are converting a Ducato there is a method of obtaining a true D+:

Converter's socket - MisterG

The Peugeot Boxer, Citroen Relay and Fiat Ducato can be equipped with an option for a “Conversion Interface” or Converter’s socket. Fiat: “Electrical Interface Supply Socket (For Conversions)” option 081 Peugeot: “Electrical connections interface box / Includes reinforced alternator” option JI01...

misterg.org.uk

 

[TheMobileWanderer]

Loving your signature, Driventemple
Thanks for the link, though I'm not converting any of those.

 

[bigtwin]

The problem that you’re concerned about simply isn’t an issue with a genuine D+ signal.

Some signals aren’t true D1+ but you can readily check to see if the signal is there (it shouldn’t be) when you turn on the ignition.

Ian

 

[Jabstatt]

Why would you have the ignition on without the engine running except for the odd few seconds?

It’s not good practice. All sorts of other stuff run as well!

Tony

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[TheMobileWanderer]

Why would you have the ignition on without the engine running except for the odd few seconds?

It’s not good practice. All sorts of other stuff run as well!

Tony

I wouldn't have ignition running on its own often, if at all. Other people might accidentally do it so, the important bit, I think, is to make sure the battery won't be drained by the dc-dc charger.

I'm also wondering why the habitation battery should be connected to the starter battery rather than directly to the alternator. I guess it's to prevent overload on alternator but I don't know.

 

[yorkiebar]

Why would you have the ignition on without the engine running except for the odd few seconds?

It’s not good practice. All sorts of other stuff run as well!

Tony

Is the "ignition on" signal active with stop-start systems when they stop the engine when stationary ?

 

[mikash]

Before fitting any extra sensors, you should check what signals your vehicle offers by default. D+ is a bit confusing term, because it can mean either ignition signal or engine running-signal. The latter would do what you want for the dc-dc. Many vehicles have both options available.
The main problem with ignition switching on the DC-DC is in my opinion not that it might eventually draw the starter battery empty if you forget ignition on, but that the extra draw from DC-DC comes up right when you need the most power from your starter battery for cranking the (possibly almost frozen cold) engine.

 

[autorouter]

I wouldn't have ignition running on its own often, if at all. Other people might accidentally do it so, the important bit, I think, is to make sure the battery won't be drained by the dc-dc charger.

There's one good reason why a D+ signal is used rather than the ignition key signal. The D+ only turns on when the engine has finished the starting up process and is running. With a split charge relay this avoids any complications where the leisure battery is connected to the starter battery while it is powering the starter motor. It can take hundreds of amps to turn a starter motor. A leisure battery is generally not built to provide short bursts of hundreds of amps, like a starter battery does. Also even if it could, the wiring from the leisure battery through the fuses and split charge relay would not be designed for hundreds of amps.

Fortunately if there's a B2B instead of a split charge relay, there will be electronics such as a diode to only allow flow one way, from the alternator/starter battery to the leisure battery, so that's less of a problem.

I'm also wondering why the habitation battery should be connected to the starter battery rather than directly to the alternator. I guess it's to prevent overload on alternator but I don't know.

In any vehicle, the alternator is connected permanently to the starter battery. This stabilises the alternator voltage output. Bad things happen if the link from the alternator to starter battery is not good. If you're lucky you'll get a flat starter battery. If you're not, you can get prolonged spikes of up to 60V for several seconds, repeated if the connection is intermittent.

Vehicle electrics are designed to withstand such events, but other stuff like your habitation devices probably won't be. The upshot of this is, don't mess with the alternator/starter battery connection. Any connection to the alternator should be made at the starter battery terminal.

 

[TheMobileWanderer]

Marvellous, autorouter. Very clear explanation.

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[Driventemple]

Before fitting any extra sensors, you should check what signals your vehicle offers by default. D+ is a bit confusing term, because it can mean either ignition signal or engine running-signal.

Sorry, no confusion. The D+ is derived from Dynamo Output - It can only happen when the engine is running. People throw the phrase D+ around and include the idea that this can be the same as 'Ignition On'

 

[mikash]

Sorry, no confusion. The D+ is derived from Dynamo Output - It can only happen when the engine is running. People throw the phrase D+ around and include the idea that this can be the same as 'Ignition On'

Yes, that's what I mean by confusing . It can mean different things depending on who's talking/writing.
F.ex.

https://nohma.com/van-conversion/how-to-find-your-vehicles-d-plus/

Is clearly talking about ignition signal, not engine run signal, as D+. Whicn in my opinion is wrong, but still quite common.
Also I've understood that for some vehicles it's not equal to engine run signal either, since the (smart) alternator can drop D+ when it's not providing power, even though engine is running.
So that's why it's confusing - always best to clarify what exactly is meant with D+.

 

[autorouter]

When you turn the key, the dash lights turn on. That's the 'Ignition' signal. When you start the engine, the alternator light goes out. That's the 'D+' signal. Lots of people get them confused, even some of the people that write the manuals.