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Old 07-29-2021, 02:12 AM   #1
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Default DC to DC charger question

Hi folks,

When wiring up a DC to DC charger to charge the house batteries, is that charger meant to be supplemental to the existing alternator charging or is it meant to replace that source?

Thanks,
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Old 07-29-2021, 04:13 AM   #2
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Replace, assuming the charge source is the alternator.
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Old 07-29-2021, 10:09 AM   #3
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A DC to DC charger is not a DC power source by itself. It needs a a DC power source for it to work.

Example setup: Engine -> Alternator -> Engine Battery -> DC to DC charger -> House Battery

DC to DC chargers are typically set up to run only when the engine is running.
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Old 07-29-2021, 10:49 AM   #4
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Originally Posted by markopolo View Post
A DC to DC charger is not a DC power source by itself. It needs a a DC power source for it to work.

Example setup: Engine -> Alternator -> Engine Battery -> DC to DC charger -> House Battery

DC to DC chargers are typically set up to run only when the engine is running.
I was hoping to leave everything as it is and just run a positive and negative cable directly from the starter battery posts to the new charger and then run a positive and negative from there, back to the house batteries. (Through the shunt, of course). I'd also have to run a small wire to turn it on, through a switch. Sounds like it will be more complex than that.
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Old 07-29-2021, 11:21 AM   #5
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What you describe would work as long as you remember to do it. If you forget to turn it on then no charging. If you forget to turn it off then dead starter battery.

See this topic for alternate example: https://www.classbforum.com/forums/f...way-11572.html

Maybe the DC/DC unit you're considering can be triggered like that.
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Old 07-29-2021, 11:27 AM   #6
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I was hoping to leave everything as it is and just run a positive and negative cable directly from the starter battery posts to the new charger and then run a positive and negative from there, back to the house batteries. (Through the shunt, of course). I'd also have to run a small wire to turn it on, through a switch. Sounds like it will be more complex than that.

Others would know more about the actual connections on various models, but if you are doing a single alternator setup it isn't much more than you mention.


The units are usually setup to run only when the engine is running as Marko stated and done with an engine sense wire that you could put an interrupt switch in to be able shut it off when the batteries are full. It would be nice to have a shutoff for the power to the B to B so you can remove the voltage from that line for maintenance and possible parasitic loss and many just us a low power use automatic charge relay or a manual one. Fuses on both ends of the cables for safety. No need to run the negative back to the engine compartment as the chassis ground is plenty and the shunt for a monitor doesn't go in the line anyway. You still use a single cable from the house batteries to the body of the van with the4 shunt in it. The starting battery grounds to the chassis directly as you don't want you monitor to see any of that current, only the house batteries.
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Old 07-29-2021, 12:15 PM   #7
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I'm looking at the Renogy 40 amp charger. This would be for my C. I've got 4 GC2 batteries in there and it can take a lot of driving to bring them up to snuff from 50% using the single stock alternator. The coach is huge and I can pretty easily run large or small wires with fuses to where they need to go. A small trigger wire with a switch in the cab is easy.
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Old 07-29-2021, 12:41 PM   #8
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I'm looking at the Renogy 40 amp charger. This would be for my C. I've got 4 GC2 batteries in there and it can take a lot of driving to bring them up to snuff from 50% using the single stock alternator. The coach is huge and I can pretty easily run large or small wires with fuses to where they need to go. A small trigger wire with a switch in the cab is easy.

At 40 amps you have at least 5 hours IF you could get the batteries to absorb that much until full, which they won't do. Realistically, you will be looking at probably 10 hours of driving to full, or so. You will also be taking probably 60 amps from the alternator by the time you figure inefficiencies based on what we have heard about that. Putting on what the van uses, which could be 20-50 amps depending on what is running, plus whatever is running in the van like a frig on DC or charging devices, you will be putting a pretty good sized load on the single alternator.


What batteries and monitor will you be using? That is interesting to see where the low charge rate will fall in relation to the battery type and manufacturer specs for recommended minimum charge rate. Lifeline, for instance would like .2C amps for discharges of 50% or more.
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Old 07-29-2021, 01:12 PM   #9
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I'm using a Victron 712 to keep an eye on things.

I've only checked what was going on once. I dry camped for a few days and used an indicated 140Ah. I took to the road and it wasn't long before the monitor showed the batteries were only getting <10 amps, typically between 3 and 7. This was during the day, running no lights and a propane fridge. Strangely, every once in a while it would jump up to >16 amps but return back down within 10 minutes. Pretty dismal. Fortunately, my destination had a place to plug in.

I've since added 400 watts of solar, but I haven't had a chance to test it.

I did learn on interesting tidbit though. The setting for flooded batteries on the MPPT controller was 14.6v in boost mode. Evidently, that exceeds the threshold for the dashboard idiot light and illuminates it. Turning the boost mode down to 14.2, corrected that, or so it seems with my limited testing.
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Old 07-29-2021, 02:12 PM   #10
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I'm using a Victron 712 to keep an eye on things.

I've only checked what was going on once. I dry camped for a few days and used an indicated 140Ah. I took to the road and it wasn't long before the monitor showed the batteries were only getting <10 amps, typically between 3 and 7. This was during the day, running no lights and a propane fridge. Strangely, every once in a while it would jump up to >16 amps but return back down within 10 minutes. Pretty dismal. Fortunately, my destination had a place to plug in.

I've since added 400 watts of solar, but I haven't had a chance to test it.

I did learn on interesting tidbit though. The setting for flooded batteries on the MPPT controller was 14.6v in boost mode. Evidently, that exceeds the threshold for the dashboard idiot light and illuminates it. Turning the boost mode down to 14.2, corrected that, or so it seems with my limited testing.

Have you been following the discussions on the Victron defaults being way to conservative and indicating full batteries way early?


Depending on whose batteries you have they will probably want .5-1.0%C amps to be totally full if they are AGM. That is 2-4 amps on 400ah of battery. So you got at least close. The jump up in amps shouldn't really happen and if it does it should only be for an instant. Usually when we see that it is because there is something connected to the battery ground side that is on the wrong side of the shunt.


What solar controller and battery brand do you have? 400 watts of solar should easily do the finish charging to full if the controller is capable of controlling it well. Unfortunately most don't, so getting them to best compromise settings is needed.
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Old 07-29-2021, 05:12 PM   #11
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Have you been following the discussions on the Victron defaults being way to conservative and indicating full batteries way early?

Yeah, I'm the guy that started that thread.


Depending on whose batteries you have they will probably want .5-1.0%C amps to be totally full if they are AGM. That is 2-4 amps on 400ah of battery. So you got at least close. The jump up in amps shouldn't really happen and if it does it should only be for an instant. Usually when we see that it is because there is something connected to the battery ground side that is on the wrong side of the shunt.

I only saw it do it once on a 300 mile trip. I don't know why or if it ever did it again. I've taken other long trips but the batteries were full when I left so not many amps were going in and I never saw a jump in current.


What solar controller and battery brand do you have? 400 watts of solar should easily do the finish charging to full if the controller is capable of controlling it well. Unfortunately most don't, so getting them to best compromise settings is needed.

The controller is a Renogy Elite 40amp MPPT. Not all that terrific but for this project, the physical size was the important factor. So was price.
I think I need to see how the solar works before I consider a DC to DC charger. I just haven't had the opportunity to run the batteries down and take a trip. It works great in the driveway. I just don't know how they'll work in conjunction with the engine alternator.
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Old 07-29-2021, 05:51 PM   #12
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I think I need to see how the solar works before I consider a DC to DC charger. I just haven't had the opportunity to run the batteries down and take a trip. It works great in the driveway. I just don't know how they'll work in conjunction with the engine alternator.

The panels have plenty of capacity and compatibility of the controller won't be and issue as far as anything horrible happening. What you likely will run in to is the controller going into float based on the voltage it sees from the alternator. The controller, IIRC, is an algorithm one with a max timer likely. When the algorithm looks at the system and sees full battery voltage or more it will probably calculate a very short absorption time. Not a issue while driving except maybe losing the capacity it could add. But when you stop and need it to finish off the charging to actual full over a longer time, it will be in float and not do that charging at a high enough voltage. You might be able to put a load on to drop the voltage it sees once you stop, but with 400ah it would need to be a big load.


As you say, you won't know until you see what it is doing in the real world. This is a big flaw in the solar setups that need to finish charge after driving and with very few ways around it.
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Old 07-29-2021, 08:04 PM   #13
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The panels have plenty of capacity and compatibility of the controller won't be and issue as far as anything horrible happening. What you likely will run in to is the controller going into float based on the voltage it sees from the alternator. The controller, IIRC, is an algorithm one with a max timer likely. When the algorithm looks at the system and sees full battery voltage or more it will probably calculate a very short absorption time. Not a issue while driving except maybe losing the capacity it could add. But when you stop and need it to finish off the charging to actual full over a longer time, it will be in float and not do that charging at a high enough voltage. You might be able to put a load on to drop the voltage it sees once you stop, but with 400ah it would need to be a big load.


As you say, you won't know until you see what it is doing in the real world. This is a big flaw in the solar setups that need to finish charge after driving and with very few ways around it.
Yeah, I was kinda anticipating problems when the alternator light on the dash lit and the solution was to turn off the panels. I can try a different profile like GEL which only goes to 14.2v on boost instead of 14.6v or even the Li profile which is 14.4 volts and no float. Either one of those should keep the voltage low enough to prevent the light from illuminating. It would be nice to be able to force the controller into different modes like the PD converters.

This is why I posed the original question. I was just wondering if a DC to DC charger is as easy to hook up as the simple diagrams I see and if I needed to disable the factory battery charging system.
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Old 07-29-2021, 08:43 PM   #14
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This is why I posed the original question. I was just wondering if a DC to DC charger is as easy to hook up as the simple diagrams I see and if I needed to disable the factory battery charging system.
Presumably your coach has some sort of device between the chassis battery/alternator and the house batteries that connects the batteries together when the batteries are being charged and disconnects them when they are being discharged. That device would have to be removed from the circuit and the B2B put in its place.

Whether you need ignition sense wire or not depends on the B2B. My Victron senses voltage, so doesn't need ignition sense.

You'd want to make sure the wiring is properly sized and fused.

The B2B will only charge in one direction, so your shore power or solar will no longer keep the chassis battery charged, and because your chassis battery will no longer be tied to the house battery (and solar panels) your alternator light problem will go away.
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Old 07-29-2021, 09:35 PM   #15
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Presumably your coach has some sort of device between the chassis battery/alternator and the house batteries that connects the batteries together when the batteries are being charged and disconnects them when they are being discharged. That device would have to be removed from the circuit and the B2B put in its place.

Whether you need ignition sense wire or not depends on the B2B. My Victron senses voltage, so doesn't need ignition sense.

You'd want to make sure the wiring is properly sized and fused.

The B2B will only charge in one direction, so your shore power or solar will no longer keep the chassis battery charged, and because your chassis battery will no longer be tied to the house battery (and solar panels) your alternator light problem will go away.
Thanks.

I do have a system that charges both the house and chassis batteries at the same time in my coach. I'm guessing (and it's truly a guess because I haven't tried to test my theory) that the reason for the alternator light is that the voltage supplied by the solar charge controller and seen at the chassis battery is just too high. Renogy thinks 14.6v is OK for my house batteries but Ford thinks it's too high for the starter battery. If I can lower that voltage by changing the charge profile, it should be problem solved and everything can remain as is with both battery banks being charged. The voltage from the alternator and from the controller is not cumulative so they can coexist as long as one of them is not too high, I think.
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Old 08-06-2021, 08:45 PM   #16
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What you describe would work as long as you remember to do it. If you forget to turn it on then no charging. If you forget to turn it off then dead starter battery.

See this topic for alternate example: https://www.classbforum.com/forums/f...way-11572.html

Maybe the DC/DC unit you're considering can be triggered like that.
I just posted an update on the topic that marcopolo mentioned above. The controllers do a good job of automatically managing the DC-DC charger so I don't have to think about it. I am very happy with this as a replacement factory setup that pushed unregulated current through a battery separator. I would measure current as high as 150 amps with that configuration.
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Old 08-08-2021, 02:04 AM   #17
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Eric, listen to what @Michael said ==> you must disconnect any connection between your chassis battery and house batteries when you insert a B2B charger. And, of course, your solar controller will then be connected solely to your house batteries, thus, you should get no warning lights on the vehicle panel as the B2B charger will essentially block the higher voltage from the solar panels from reaching your chassis battery and associated circuitry.
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Old 08-08-2021, 02:58 AM   #18
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Eric, listen to what @Michael said ==> you must disconnect any connection between your chassis battery and house batteries when you insert a B2B charger. And, of course, your solar controller will then be connected solely to your house batteries, thus, you should get no warning lights on the vehicle panel as the B2B charger will essentially block the higher voltage from the solar panels from reaching your chassis battery and associated circuitry.

Wouldn't you be able to jump around the B to B in the coach to engine direction with a one way automatic charge relay or a manual so it would be not in the circuit unless the engine is off? Perhaps I am missing something, though. You would need a B to B that wouldn't trigger off the voltage once the charging was going past the B to B input as it would turn it on.



Having the option of the reverse charging can come in handy, especially if he wiring is big enough to also use it for emergency start if chassis battery dies.
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Old 08-08-2021, 11:17 AM   #19
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Eric, listen to what @Michael said ==> you must disconnect any connection between your chassis battery and house batteries when you insert a B2B charger. And, of course, your solar controller will then be connected solely to your house batteries, thus, you should get no warning lights on the vehicle panel as the B2B charger will essentially block the higher voltage from the solar panels from reaching your chassis battery and associated circuitry.
I see that now. I would need to remove the isolation solenoid.

It all looks rather simple. If I was to purchase the non-isolated version of Victron's B2B, I would only need to run one wire from the chassis to the ground post on the unit. I might be able to install a Trik-L-Start the same way you would install one if you had an isolation solenoid.
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Old 08-08-2021, 01:26 PM   #20
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I see that now. I would need to remove the isolation solenoid.

It all looks rather simple. If I was to purchase the non-isolated version of Victron's B2B, I would only need to run one wire from the chassis to the ground post on the unit. I might be able to install a Trik-L-Start the same way you would install one if you had an isolation solenoid.
That's what I did. A Victron B2B to charge the house battery from the chassis, and and a trik-l/amp-l start to charge the chassis battery from the house. They are wired back-to-back.
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