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

 

[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.

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[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.

 

[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.

 

[AdrianChen]

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

If it is a true D+ coming from the alternator, then as the alternator is not turning/producing there would be no D+ signal.

If it is a simulated d+ I guess it would need to be tested....

 

[AdrianChen]

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.

Yes from my declining memory with the early simple dash light when the ignition is turned on a current passes through the dash light to a terminal (L on many) on the alternator and this is used to provide the field current needed to start the alternator (or "excite" it). Once the alternator starts, a secondary diode trio provides a current that is fed back to the L circuit so reducing the current flow, and the dash light goes out. The diode trio is also connected to the D+ terminal providing that output, but it's main use is as the source for the ongoing field current the alternator needs via whatever regulator is installed.

I guess nowadays there is a computer or two involved, but the principles will be similar.....

 

[yorkiebar]

If it is a true D+ coming from the alternator, then as the alternator is not turning/producing there would be no D+ signal.

If it is a simulated d+ I guess it would need to be tested....

I would expect the same for a true D+, it was the "ignition on" signal I was questioning (and if it would be active when the stop-start kicks in).

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[AdrianChen]

I would expect the same for a true D+, it was the "ignition on" signal I was questioning (and if it would be active when the stop-start kicks in).

Sorry not sure, and my old brain addled so answered the wrong question!

 

[autorouter]

If it is a true D+ coming from the alternator, then as the alternator is not turning/producing there would be no D+ signal.

The alternator doesn't 'turn off' while the engine is running. The voltage level varies - sometimes it is high enough to charge the starter battery, sometimes it is down at 12.5V which will not put any charge into the starter battery. But it doesn't switch off.

The vehicle lights, wipers, fan etc. all work fine on 12.5V, powered by the alternator. Even a 3-way fridge would work fine from that voltage. However if you need to refill a leisure battery to 100% while driving, a smart alternator won't do it. Even worse, if the leisure battery is connected to the starter battery via the split charge relay, the leisure battery may be at a higher voltage than the starter battery, so it will drain into the starter battery to refill it.

A B2B (DC-DC charger) takes power from the alternator at whatever it supplies. If it only outputs 12.5V, it can use that. It converts the voltage to whatever the leisure battery needs, according the the charging profile selected and the state of charge. As a bonus, it prevents any back-flow of charge from the leisure battery to the starter battery.

The alternator D+ signal is on when the engine is running, even if the smart alternator is not at the higher charging voltage. If the B2B is triggered by the D+, it will work correctly, charging the leisure battery whenever the engine is running, and switching off when the engine stops.

Sorry not sure, and my old brain addled so answered the wrong question!

Edit: same here.

 

[AdrianChen]

The alternator doesn't 'turn off' while the engine is running. The voltage level varies - sometimes it is high enough to charge the starter battery, sometimes it is down at 12.5V which will not put any charge into the starter battery. But it doesn't switch off.

The vehicle lights, wipers, fan etc. all work fine on 12.5V, powered by the alternator. Even a 3-way fridge would work fine from that voltage. However if you need to refill a leisure battery to 100% while driving, a smart alternator won't do it. Even worse, if the leisure battery is connected to the starter battery via the split charge relay, the leisure battery may be at a higher voltage than the starter battery, so it will drain into the starter battery to refill it.

A B2B (DC-DC charger) takes power from the alternator at whatever it supplies. If it only outputs 12.5V, it can use that. It converts the voltage to whatever the leisure battery needs, according the the charging profile selected and the state of charge. As a bonus, it prevents any back-flow of charge from the leisure battery to the starter battery.

The alternator D+ signal is on when the engine is running, even if the smart alternator is not at the higher charging voltage. If the B2B is triggered by the D+, it will work correctly, charging the leisure battery whenever the engine is running, and switching off when the engine stops.

Edit: same here.

I was referring to pre-stop start alternators when the engine is stopped. As I said to yorkiebar I was mistakenly referring to D+ not "ignition on signals". I really must stop posting in the mornings before my brain boots up...... or in the afternoons when its tired.

But thanks for clarifying the record - I guess the D+ will mimic the voltage output of the alternator if it's generated by an old fashioned diode trio, but that should still be enough to trigger the stuff a van needs!

 

[fred_jb]

Just be aware that some people have experienced an issue with Renogy B2Bs taking power from the starter battery, at least for a while, when the D+ signal is not present. This seems to be due to these devices working both in response to a D+ signal, and will seemingly also switch if the starter battery voltage is at a level high enough to suggest it is being charged from the alternator when in fact it isn't. Possibly when you have a dedicated solar panel for the starter battery, or if it has only recently been fully charged by the alternator and has not yet subsided to a normal voltage level. Renogy's suggestion is put a heavy duty relay in series with the starter battery feed to the B2B, and arrange for this to only close its contacts in response to a D+ signal.

 

[dogster]

Just be aware that some people have experienced an issue with Renogy B2Bs taking power from the starter battery, at least for a while, when the D+ signal is not present. This seems to be due to these devices working both in response to a D+ signal, and will seemingly also switch if the starter battery voltage is at a level high enough to suggest it is being charged from the alternator when in fact it isn't. Possibly when you have a dedicated solar panel for the starter battery, or if it has only recently been fully charged by the alternator and has not yet subsided to a normal voltage level. Renogy's suggestion is put a heavy duty relay in series with the starter battery feed to the B2B, and arrange for this to only close its contacts in response to a D+ signal.

Hi fred_jb - I was reading this thread with interest as I had exactly this at the weekend.
I've a 50A renogy DC charger which seems to work well (no solar as yet). However this weekend I fitted an isolator switch to the leisure battery and discovered some weird behaviour when I shut the battery off. It appears the DC/DC output was still live with engine off - possibly as the van had a run home and the starter battery would be full. The feed to the electroblock is after the +ve shutoff but also connected to the +out from the DC/DC so the habitation systems were powering up - albeit unreliable.

It's back in storage now, as I ran out of time and energy to troubleshoot further. When I next get the van, time will tell if the starter battery is flat!

 

[fred_jb]

Hi fred_jb - I was reading this thread with interest as I had exactly this at the weekend.
I've a 50A renogy DC charger which seems to work well (no solar as yet). However this weekend I fitted an isolator switch to the leisure battery and discovered some weird behaviour when I shut the battery off. It appears the DC/DC output was still live with engine off - possibly as the van had a run home and the starter battery would be full. The feed to the electroblock is after the +ve shutoff but also connected to the +out from the DC/DC so the habitation systems were powering up - albeit unreliable.

It's back in storage now, as I ran out of time and energy to troubleshoot further. When I next get the van, time will tell if the starter battery is flat!

Interesting!

I'm a bit surprised that your EBL is connected twice, both before and after your switch. How have you done that?

My B2B output only goes directly to the leisure battery, and from there to the EBL via a 50A fuse, so bypasses the split charge relay, which is preferable as I believe it is only rated at 30A.

It doesn't sound like you have done this, but if you retain a starter battery connection into the EBL, this must be taken from the output of the B2B. If you also have the starter battery connected directly to the EBL as well as via the B2B, then you will be effectively shorting the output of the B2B to its input with unpredictable results!

In your case it seems that the B2B is seeing a high enough voltage on the starter battery to switch on and try to supply current to the leisure battery. As I mentioned, one way to stop this is to put a high current relay, controlled by the D+ signal, into the starter battery feed to the B2B.

 

[berni109]

Not read all of this thread.
But my DC to DC charger only starts to try to charge when d+ present and the vehicle battery is above a threshold I have set.
Fail safe situation.
Should have added I don't have a smart alternator but can't see it would make a difference.

 

[autorouter]

Not read all of this thread.
But my DC to DC charger only starts to try to charge when d+ present and the vehicle battery is above a threshold I have set.
Fail safe situation.
Should have added I don't have a smart alternator but can't see it would make a difference.

If you had a smart alternator, the vehicle battery would mostly be below the threshold that you set. So the DC-DC charger would not turn on for the majority of the time the engine was running. I suspect that is not what you want to happen.

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[fred_jb]

Agreed. A B2B is intended to subvert the energy saving characteristics of ECU controlled smart alternators which try to reduce charging unless the vehicle is braking or decelerating. It does this by boosting the low voltage put out by smart alternators when in energy saving mode, so that the leisure battery can still take large currents from the alternator even in situations where the vehicle is trying to minimise battery charging.

 

[dogster]

Interesting!

I'm a bit surprised that your EBL is connected twice, both before and after your switch. How have you done that?

My B2B output only goes directly to the leisure battery, and from there to the EBL via a 50A fuse, so bypasses the split charge relay, which is preferable as I believe it is only rated at 30A.

It doesn't sound like you have done this, but if you retain a starter battery connection into the EBL, this must be taken from the output of the B2B. If you also have the starter battery connected directly to the EBL as well as via the B2B, then you will be effectively shorting the output of the B2B to its input with unpredictable results!

In your case it seems that the B2B is seeing a high enough voltage on the starter battery to switch on and try to supply current to the leisure battery. As I mentioned, one way to stop this is to put a high current relay, controlled by the D+ signal, into the starter battery feed to the B2B.

Hi Fred_jb
Thanks for the follow up comments.
To clarify my fumbled description, there is only one connection to the EBL, and this is after the isolator - so effectively the B2B output and the feed to the EBL are on the switched side of the isolator. I also removed the original split charge relay fuse (30A from memory) from the EBL to avoid short/loop problems when I fitted the B2B a few months ago.
While I observed these strange behaviours, it all seemed to work according to plan (before I added the isolator) - I'll do some more investigation next time I've got the van, and might have to consider your relay idea if I get stuck.
Cheers!

 

[fred_jb]

Thanks for the clarification. I was pretty sure that your issue was just due to how the Renegy kit responds to high starter battery voltage.

I didn't initially have a problem with mine, but have seen it recently since I fitted a small solar panel and controller connected solely to the starter battery.

I guess I will be refitting the relay which I had originally put on as a precaution, but removed when it didn't seem to be needed!