Alternator requirements - DC to DC charger

[Riverbug]

There is alot of information available about DC to DC chargers but everything I find talks about the charger, installation, batteries, etc. But the only information I can find about the alternator itself is that it has to be "appropriately sized" to handle the extra draw. How does one know if their alternator is sized to handle a DC to DC charger? I have a truck on order that will have a 240 amp alternator. How would I know whether that can handle a 30 or 40 amp DC charger? I assume I would need to know what the truck itself is drawing at the same time the DC charger is charging? But how is that determined?

[MoonShadow_1911]

There is alot of information available about DC to DC chargers but everything I find talks about the charger, installation, batteries, etc. But the only information I can find about the alternator itself is that it has to be "appropriately sized" to handle the extra draw. How does one know if their alternator is sized to handle a DC to DC charger? I have a truck on order that will have a 240 amp alternator. How would I know whether that can handle a 30 or 40 amp DC charger? I assume I would need to know what the truck itself is drawing at the same time the DC charger is charging? But how is that determined?

There is alot of information available about DC to DC chargers but everything I find talks about the charger, installation, batteries, etc. But the only information I can find about the alternator itself is that it has to be "appropriately sized" to handle the extra draw. How does one know if their alternator is sized to handle a DC to DC charger? I have a truck on order that will have a 240 amp alternator. How would I know whether that can handle a 30 or 40 amp DC charger? I assume I would need to know what the truck itself is drawing at the same time the DC charger is charging? But how is that determined?

It looks like you have a diesel GMC/Chevy truck on order, based on the 240 amp alternator. I'm also assuming you have dual batteries in this truck.

So, it looks like the standard alternator is a 220 amp one. I have been told (and haven't really validated this) that the HD trucks have a "robust" charging system and is generally sized 25% LARGER than what is absolutely necessary to run the truck. On a purely technical level, the 240 amp alternator should be able to handle a 30 amp DC-to-DC charger, provided you have the correct wire size going from the alternator to the DC-to-DC charger. A 40 amp charger is pushing it.

Now, base on experience with DC electronic circuits and such things, the truck should be able to handle the 30 amp DC-to-DC charger with no problem and be just fine with the 250 amp alternator. The only caveat to that statement is if the battery bank in the coach is "empty" or dead, then you might have an issue, but, as long as you have at least 50% of available battery bank power, you should be just fine.

You did not mention what type of battery bank you will be using in the coach. For a LiFePO4 bank, 50% would be considered around 11.3 VDC. For a traditional lead acid or AGM battery bank, 11.8 VDC is considered 50% (these numbers are guidelines only).

As the voltage drops in the bank, the "force" to charge them increases, so, a typical battery charger will increase the voltage and amperage (to set levels) to feed the battery bank at it's highest charging rate. Several other factors play in to this "set level", such as temperature.

Most RVs do not come with a temperature sensor in the battery bank that feeds back to the charge controller, so it will push "industry standard" safe levels.

A DC-to-DC charger usually will be able to provide high amperage and possibly voltage to the battery bank, so this needs to be kept in mind.

If you want the tow vehicle to maintain the charge in the batteries as you move from site to site, this shouldn't be a problem, or if you want to use the tow vehicle to maintain the batteries in a solar system due to driving at night or during weather (cloudy, rainy, snowy, anything that blocks direct sunlight from the panels), again, this should be just fine.

If you want the tow vehicle to act as a battery charger, it might not be such a good idea.

The wire size I would recommend is 8 GA, but you probably could get away with 10 GA. This is over a 25' run from the alternator to the DC-to-DC charger. Your installation might necessitate a different wire size.

Hope this helped and that I didn't "geek out" on you too badly.

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[Riverbug]

It looks like you have a diesel GMC/Chevy truck on order, based on the 240 amp alternator. I'm also assuming you have dual batteries in this truck.

So, it looks like the standard alternator is a 220 amp one. I have been told (and haven't really validated this) that the HD trucks have a "robust" charging system and is generally sized 25% LARGER than what is absolutely necessary to run the truck. On a purely technical level, the 240 amp alternator should be able to handle a 30 amp DC-to-DC charger, provided you have the correct wire size going from the alternator to the DC-to-DC charger. A 40 amp charger is pushing it.

Thanks, appreciate the response. Helpful, but I was hoping to find a more concrete response as to how one would measure the actual draw requirements vs capacity. Maybe that's not realistic. I read that the risk of not having the right sized alternator is that you risk burning it out, but not one person explains how one determines that it is appropriately sized. I assumed there would be a specific way to measure it, but perhaps not. All I can think of is to measure the amperage being output from the alternator while turning on all the accessories I can and just assume that whatever is left over is the amount of headroom I have to work with. That just doesn't seem very reliable in my mind, but maybe it is? My guess is I'm overthinking it and most people just install the charger and don't think anymore about it as long as it doesn't seem to cause any problems.

[wudwork]

I also have a diesel with the 2nd large alternator and battery; and really want to use that to charge lithium batteries. What is stopping me is the trucks computer. You would have to run your charge wire direct to battery terminal; same place your truck battery(s) are wired. What voltage will the computer see ? Will it get upset with "out of range" voltages? Ideally we would have the 2nd alternator not attached to anything but the belt drive and then there would not be a computer in the decision process; other than the buck converter. Like you, I could not find any real life values, other than anecdotal and don't want to risk anything issues with onboard computer.

[MoonShadow_1911]

Thanks, appreciate the response. Helpful, but I was hoping to find a more concrete response as to how one would measure the actual draw requirements vs capacity. Maybe that's not realistic. I read that the risk of not having the right sized alternator is that you risk burning it out, but not one person explains how one determines that it is appropriately sized. I assumed there would be a specific way to measure it, but perhaps not. All I can think of is to measure the amperage being output from the alternator while turning on all the accessories I can and just assume that whatever is left over is the amount of headroom I have to work with. That just doesn't seem very reliable in my mind, but maybe it is? My guess is I'm overthinking it and most people just install the charger and don't think anymore about it as long as it doesn't seem to cause any problems.

You are looking for hen's teeth! So, the only way to do what you are looking to attempt to do, is the following (and it's probably not a complete list either!):

1. Get OBDLink and the MX dongle https://www.obdlink.com/.
2. Get the OBDLink app for Android or iOS.
3. Learn how to use OBDLink to get you the amperage usage of everything in the truck (I THINK it does this).
4. Hitch up and turn on EVERYTHING in the truck that you can possibly turn on, and go for a drive.

This will give you some kind of starting point, but, I'm not sure how accurate this will be.

Yes, it is true that you could burn out your alternator if you have one that doesn't produce enough amperage for the DC-to-DC charger and the stuff for the truck. I believe statements like this are more along the lines of people who are trying to use the truck as a battery bank charger with the DC-to-DC converter. In cases like that, it might just be better to hook jumper cables up, from the truck to the battery bank, and let it charge them a bit that way, at idle. I've done this a number of times, but only with FLA or AGM batteries.

A good, high waulity DC-to-DC charger should have protection circuitry engineered into it so that it won't burn out the alternator, or blow up the battery bank. I haven't done much research on them, but give me a few hours and I'll see what I can come up with.

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[Riverbug]

I also have a diesel with the 2nd large alternator and battery; and really want to use that to charge lithium batteries. What is stopping me is the trucks computer. You would have to run your charge wire direct to battery terminal; same place your truck battery(s) are wired. What voltage will the computer see ? Will it get upset with "out of range" voltages? Ideally we would have the 2nd alternator not attached to anything but the belt drive and then there would not be a computer in the decision process; other than the buck converter. Like you, I could not find any real life values, other than anecdotal and don't want to risk anything issues with onboard computer.

I haven't seen that it is an issue in the Superduty's computer or anything. I read where one guy mounted the charger in the truck and wired it to an outfitter switch rather than the parking lights or other source, so he could turn it on/off independently; I like that idea. I have to imagine the 240amp alternator is enough for the charger since I've seen others do it, but it bothers me that I don't know how to confirm actual capacity. If I will have the headroom for a 40 or 60 amp, that's what I want to run. I just want to "know" that I have the headroom. If I only have capacity for 30 amp, I'll take that if need be. (I have a 200AH lithium battery, with plans to add a second battery this summer.) We mainly stay in campgrounds with hookups but have trips to AK, CO, Nova Scotia, etc., planned where multiple nights will be spent boondocking while traveling from point to point and maybe more often if we have the available reserve power.

[Riverbug]

You are looking for hen's teeth! So, the only way to do what you are looking to attempt to do, is the following (and it's probably not a complete list either!):

1. Get OBDLink and the MX dongle https://www.obdlink.com/.
2. Get the OBDLink app for Android or iOS.
3. Learn how to use OBDLink to get you the amperage usage of everything in the truck (I THINK it does this).
4. Hitch up and turn on EVERYTHING in the truck that you can possibly turn on, and go for a drive.

This will give you some kind of starting point, but, I'm not sure how accurate this will be.

Yes, it is true that you could burn out your alternator if you have one that doesn't produce enough amperage for the DC-to-DC charger and the stuff for the truck. I believe statements like this are more along the lines of people who are trying to use the truck as a battery bank charger with the DC-to-DC converter. In cases like that, it might just be better to hook jumper cables up, from the truck to the battery bank, and let it charge them a bit that way, at idle. I've done this a number of times, but only with FLA or AGM batteries.

A good, high waulity DC-to-DC charger should have protection circuitry engineered into it so that it won't burn out the alternator, or blow up the battery bank. I haven't done much research on them, but give me a few hours and I'll see what I can come up with.

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Hmm. Wonder if Forscan would provide that type of info? I'm thinking about getting that anyway for some of the other things it can do. Will have to research. I have time to research while I wait for the truck to arrive...

[MoonShadow_1911]

@Riverbug and @wudwork,

Here is the Renology DC-to-DC charger manual. It's all seems pretty straight forward and does offer some protection to the tow vehicle charging system.

I did mis-type when I said in needed to be connected to the alternator, it needs to be connected to the tow vehicle battery(ies) with an "appropriately sized alternator".

From the research I have done, if you can order a truck with a "heavy duty alternator", that should be enough for even the 60 amp DC-to-DC charger from Renology.

Several people have run 4 or 6 GA wire from the tow vehicle battery to an Anderson Power connector at the back of the truck. Yes, this means a second connector you will have to connect and disconnect, but you will be assured the wire run to the DC-to-DC charge can handle the amperage without potentially melting your 7-way plug.

You will need an appropriately sized fuse as close to the battery as possible.

To answer @wudwork's concern about the computer "seeing" a different voltage than what the truck actually is at, the DC-to-DC Renology charger is supposed to "block" voltage going back upstream, so the computer shouldn't "see" anything other that the tow vehicle battery voltage.

I've attached the manual for the Renology DC-to-DC charger.

[Riverbug]

@Riverbug and @wudwork,

Here is the Renology DC-to-DC charger manual. It's all seems pretty straight forward and does offer some protection to the tow vehicle charging system.

I did mis-type when I said in needed to be connected to the alternator, it needs to be connected to the tow vehicle battery(ies) with an "appropriately sized alternator".

From the research I have done, if you can order a truck with a "heavy duty alternator", that should be enough for even the 60 amp DC-to-DC charger from Renology.

Several people have run 4 or 6 GA wire from the tow vehicle battery to an Anderson Power connector at the back of the truck. Yes, this means a second connector you will have to connect and disconnect, but you will be assured the wire run to the DC-to-DC charge can handle the amperage without potentially melting your 7-way plug.

You will need an appropriately sized fuse as close to the battery as possible.

To answer @wudwork's concern about the computer "seeing" a different voltage than what the truck actually is at, the DC-to-DC Renology charger is supposed to "block" voltage going back upstream, so the computer shouldn't "see" anything other that the tow vehicle battery voltage.

I've attached the manual for the Renology DC-to-DC charger.

Thanks again. That sounds hopeful. I was planning to run 6 gauge to Anderson or similar type connectors at the back of the truck. I wonder if a 240 amp alternator on a diesel with two batteries is considered "heavy duty"? Seems like it should be.

[MoonShadow_1911]

Thanks again. That sounds hopeful. I was planning to run 6 gauge to Anderson or similar type connectors at the back of the truck. I wonder if a 240 amp alternator on a diesel with two batteries is considered "heavy duty"? Seems like it should be.

If you ordered the heavy duty alternator option, then you've got it, if not, you might be looking to upgrade. I'm a Ford guy, so GM stuff, I'm not so good with.

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