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  1. #1
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    DC to DC Charging

    Ok, this isn't specifically a solar question but seems the right place to post this question...

    I picked up my new truck yesterday. I ordered the alternator upgrade to 397 amps; not sure if that is a single or dual alternator setup yet. I did the upgrade so that I could install a DC to DC charger. Two questions:

    1) Using a clamp meter on the negative wires for each of the truck batteries (with all the accessories running) should give a good idea of what the truck's current draw is vs the remaining headroom available for driving a DC charger for the lithium batteries in the camper, correct?

    2) If I want the DC charger to push about 40 amps to each of two lithium batteries in the camper (2 - 206AH SOK batteries), I would need an 80 amp DC charger, correct? (The batteries are rated for up to 50 amps charge input.)

    Something tells me it won't be that simple but the first two questions on my list I am trying to answer is how much headroom I have with the truck alternator and how to correctly size the DC charger. Thanks.
    Chad
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  2. #2
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    Quote Originally Posted by Riverbug View Post
    Ok, this isn't specifically a solar question but seems the right place to post this question...

    I picked up my new truck yesterday. I ordered the alternator upgrade to 397 amps; not sure if that is a single or dual alternator setup yet. I did the upgrade so that I could install a DC to DC charger. Two questions:

    1) Using a clamp meter on the negative wires for each of the truck batteries (with all the accessories running) should give a good idea of what the truck's current draw is vs the remaining headroom available for driving a DC charger for the lithium batteries in the camper, correct?

    2) If I want the DC charger to push about 40 amps to each of two lithium batteries in the camper (2 - 206AH SOK batteries), I would need an 80 amp DC charger, correct? (The batteries are rated for up to 50 amps charge input.)

    Something tells me it won't be that simple but the first two questions on my list I am trying to answer is how much headroom I have with the truck alternator and how to correctly size the DC charger. Thanks.
    First question - measuring: sure check what you are pulling for current. At least you will have a baseline (assuming you get the right accessories running all at the same time & the alternator doesn't some how back feed current and mess up a reading). So real answer - I'm not sure. But with a 397 alternator I'd say just go ahead and connect to the batteries and forget all this measuring.

    Second question - amperage. It all depends on how much amperage you want to give the DC to DC charge to charge the batteries. Since they will probably take all you can give them, pump away. But know this - you will need at least a 4 gauge wire (negative and positive) directly from the truck battery to the charge just to get 40 amps. So 80 amps? hmmmmm -- gonna need some serious cable.

    There are a lot of threads on this forum about DC to DC charges. It might be worth some time to explore a lot. They might give you a lot of good ideas and answer a lot of questions you have.

    Here is an important question to ask yourself - are you really going to deplete those batteries between tow trips? And will you have shore power once you arrive at your site? If you aren't traveling for days on end without charging, and you will have shore power each time you stop then my opinion is that the only good reason for a DC to DC charger is to keep the charge voltage at a set point.

    I put in a 100ah battery and a 40 amp DC to DC charger (you can read about it on this site) and don't think I needed to do it at all. Even after traveling for up to 6 hours without using the charge I still have at least an 80 SOC. And if I run the DC to DC charger it's like the battery never even gets used - and I don't think that is healthy; I'd rather use the battery and let it charge up again when I stop. So most of my traveling is done with the charger off, controlled via an up fitter switch.
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  3. #3
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    Quote Originally Posted by traveldawg View Post
    First question - measuring: sure check what you are pulling for current. At least you will have a baseline (assuming you get the right accessories running all at the same time & the alternator doesn't some how back feed current and mess up a reading). So real answer - I'm not sure. But with a 397 alternator I'd say just go ahead and connect to the batteries and forget all this measuring.

    Second question - amperage. It all depends on how much amperage you want to give the DC to DC charge to charge the batteries. Since they will probably take all you can give them, pump away. But know this - you will need at least a 4 gauge wire (negative and positive) directly from the truck battery to the charge just to get 40 amps. So 80 amps? hmmmmm -- gonna need some serious cable.

    There are a lot of threads on this forum about DC to DC charges. It might be worth some time to explore a lot. They might give you a lot of good ideas and answer a lot of questions you have.

    Here is an important question to ask yourself - are you really going to deplete those batteries between tow trips? And will you have shore power once you arrive at your site? If you aren't traveling for days on end without charging, and you will have shore power each time you stop then my opinion is that the only good reason for a DC to DC charger is to keep the charge voltage at a set point.

    I put in a 100ah battery and a 40 amp DC to DC charger (you can read about it on this site) and don't think I needed to do it at all. Even after traveling for up to 6 hours without using the charge I still have at least an 80 SOC. And if I run the DC to DC charger it's like the battery never even gets used - and I don't think that is healthy; I'd rather use the battery and let it charge up again when I stop. So most of my traveling is done with the charger off, controlled via an up fitter switch.
    Thanks Dawg. You make some good points about measuring the truck current and the wiring requirement. I'll have to research the wire requirement and determine if it makes sense to go with a lower amperage charger. Regarding the need for it at all, we will be boondocking for a week at a time on a 7 week trip to CO next year, then lots of off grid for a few months to AK in 2024. So, it would be convenient to have it to charge as needed while we overnight consecutive nights without hookups. When parked for a week, we can charge with the generator but I'd like to not depend on it if charging in transit can offset some of that need. Bottom line, I think it would work well for us. At the moment, I feel like I would prefer this route over solar panels but I suppose I will never know until I try one method or the other. Point taken on searching the forum for more information...will do.
    Chad
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    Surprisingly few threads specific to the topic of DC to DC chargers here, unless I just don't know how to use the search function. In any case, a couple of more specific questions:

    1) When installing the wiring at the vehicle, most people appear to connect the wire to the battery rather than the alternator. I saw only one YouTube video where the person referenced that Victron prefers it be connected direct to the alternator rather than the battery. But that's the only time I heard that and can find no other information that references this method. Any thoughts on whether that would be better or worse than connecting at the battery?

    2) When installing the ground wire at the vehicle I see most people run the ground wire (in tandem with the hot wire) from the battery to the back where they place a quick connect plug to go back to the camper. Is there any reason that one shouldn't just connect a short ground wire from the chassis at the rear of the vehicle to the quick connect plug rather than run from battery to rear?
    Chad
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    Site Sponsor Scocciante's Avatar
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    Works great Click image for larger version. 

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  6. #6
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    Quote Originally Posted by Riverbug View Post
    Surprisingly few threads specific to the topic of DC to DC chargers here, unless I just don't know how to use the search function. In any case, a couple of more specific questions:

    1) When installing the wiring at the vehicle, most people appear to connect the wire to the battery rather than the alternator. I saw only one YouTube video where the person referenced that Victron prefers it be connected direct to the alternator rather than the battery. But that's the only time I heard that and can find no other information that references this method. Any thoughts on whether that would be better or worse than connecting at the battery?

    2) When installing the ground wire at the vehicle I see most people run the ground wire (in tandem with the hot wire) from the battery to the back where they place a quick connect plug to go back to the camper. Is there any reason that one shouldn't just connect a short ground wire from the chassis at the rear of the vehicle to the quick connect plug rather than run from battery to rear?
    1/ I think connecting at the battery is probably easier.
    2/ No. As you say a lot of people run a dedicated -ve wire, but the frame is a huge -ve wire. Don't bolt it to a body panel.
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  7. #7
    Site Team xrated's Avatar
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    My thoughts on running a cable from the front of the vehicle to the rear quick connect plug.....vs....running a short cable that is bolted to the frame at the rear and then to the quick disconnect plug.

    Front to rear using a copper cable would be less total resistance for the length of the run, vs. the steel frame because the steel's conductive qualities are lower than copper. The neg. lead from the battery connects to the frame up front somewhere, so the current would have to travel through the steel frame...vs a dedicated copper conductor.

    The second factor would be an added connector in the circuit. A connector from the frame at the back of the truck, to the quick connect plug. A copper cable from the battery would go directly to the quick connect plug and eliminate one connector....the one from the frame to the quick connect plug.

    The result in all of this, in my opinion would be less voltage drop at the quick connect when there is significant current flowing for the charging. Voltage drop will show up more when there is current flow, vs. a static state of no current flow.
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  8. #8
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    Copper is a good conductor for electricity but with the amount of steel in the frame, the frame will conduct more current than pretty much any cable. Certainly easier to route and substantially cheaper.

    As for the power cable, you are stuck running a separate cable.

    It has been pointed out that the more connectors you add, the more places for voltage drops. This does apply to the positive side too. You may just remove some of the resistance if you wired the positive directly to the alternator vs the battery. The alternator should be the highest source of voltage.

    In the end, using the right size cables, both are a valid way of transferring the load. If indeed the Ford is capable of pushing almost 400A, I would make certain the cables from your alternator and sized correctly. I understand Ford probably did, but unless it had about a 4/0 gauge cable, I would question that. Probably run two or more smaller gauge cables for added flexibility in some portions.

    I am impressed, 400A is a lot.

    We are all on a budget, but you could always use two DC/DC chargers. One to each battery.
    Last edited by Butcher; 11-27-2022 at 11:45 AM.

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    Quote Originally Posted by Butcher View Post
    Copper is a good conductor for electricity but with the amount of steel in the frame, the frame will conduct more current than pretty much any cable. Certainly easier to route and substantially cheaper.

    As for the power cable, you are stuck running a separate cable.

    It has been pointed out that the more connectors you add, the more places for voltage drops. This does apply to the positive side too. You may just remove some of the resistance if you wired the positive directly to the alternator vs the battery. The alternator should be the highest source of voltage.

    In the end, using the right size cables, both are a valid way of transferring the load. If indeed the Ford is capable of pushing almost 400A, I would make certain the cables from your alternator and sized correctly. I understand Ford probably did, but unless it had about a 4/0 gauge cable, I would question that. Probably run two or more smaller gauge cables for added flexibility in some portions.

    I am impressed, 400A is a lot.

    We are all on a budget, but you could always use two DC/DC chargers. One to each battery.
    Thanks. I did confirm that it is a dual alternator setup. I haven't been able to see in well enough to follow wiring yet to identify exactly how these are wired together or to the batteries. The engine bay is packed so tight I had a heck of a time getting my arm down to reach my cell phone that I dropped while taking pictures of the alternator tags. Should be a fun project.
    Chad
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    2022 F350 Carbonized Gray 6.7L Ultimate Lariat Pkg 4WD Crew Cab Short Bed 3.55EL Axle 3,566# Payload
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  10. #10
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    It is impressive how they can pack stuff in there. What till you get a repair bill someday, that too will be impressive.

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