DC to DC Charging

**Riverbug** · 11-23-2022, 01:02 PM

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.

**traveldawg** · 11-23-2022, 02:41 PM

Originally Posted by Riverbug

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.

**Riverbug** · 11-23-2022, 03:14 PM

Originally Posted by traveldawg

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.

**Riverbug** · 11-26-2022, 02:53 PM

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?

**Scott'n'Wendy** · 11-27-2022, 06:02 AM

Originally Posted by Riverbug

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.

**traveldawg** · 11-27-2022, 03:47 PM

Originally Posted by Riverbug

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?

Here is a link to my DC to DC install. (you can find most of my mods by searching "kalakamods" - without quotes).

Here is another link.

Another link.

I find these by opening a new browser tab and use google to search [mygrandrv.com "dc" "charger"] without the brackets but with the quotes.

My Renogy instructions said 4 gauge wire for the length I ran. Lots of discussion about running cable. I didn't use the truck frame as I found these aluminum Fords to have less than dependable chassis or body parts ground points.

oh - that terminal on the positive side of the driver's side battery is one I installed myself; it isn't on the Ford connector; I removed the connector, drilled a hole and used a bolt for the stud. I'm pretty sure that that battery is on the circuit side of the Ford BMS so battery charging remains under Ford's BMS control. I have dual alternators.

**Riverbug** · 11-27-2022, 04:35 PM

Originally Posted by traveldawg

Here is a link to my DC to DC install. (you can find most of my mods by searching "kalakamods" - without quotes).

Here is another link.

Another link.

I find these by opening a new browser tab and use google to search [mygrandrv.com "dc" "charger"] without the brackets but with the quotes.

My Renogy instructions said 4 gauge wire for the length I ran. Lots of discussion about running cable. I didn't use the truck frame as I found these aluminum Fords to have less than dependable chassis or body parts ground points.

oh - that terminal on the positive side of the driver's side battery is one I installed myself; it isn't on the Ford connector; I removed the connector, drilled a hole and used a bolt for the stud. I'm pretty sure that that battery is on the circuit side of the Ford BMS so battery charging remains under Ford's BMS control. I have dual alternators.

Thanks for that. I didn't see your threads when I searched for them in the forum. I'll try Google in the future. I upgraded the alternator when I ordered the truck because I was certain a DC charger would be the cat's meow. After all the comments about whether it's really necessary, I'm starting to wonder. I don't plan on solar. I have a Honda 2000i generator that I was thinking about mounting on the A frame. Perhaps I'll start with installing the Multiplus first, see how it works to charge with a generator as needed while overnighting consecutive days without shore power, and then decide how much a DC charge might improve the situation. But one question I have is about your comment on "exercising" your battery. I've always read that the main things you can do to extend the life of lithium batteries is don't charge them to 100%, don't deplete them below 20%, and the less charge cycles it goes through the longer it will last. Thoughts?

Edit: Just occurred to me that one main point about having the DC charger was so that I could run the fridge on AC while traveling (I will be installing a Multiplus) and not rely on propane. We travel 8 hours on average, then stop without shore power for consecutive days. I've got to think about this more...

**traveldawg** · 11-28-2022, 08:04 AM

Originally Posted by Riverbug

Thanks for that. I didn't see your threads when I searched for them in the forum. I'll try Google in the future. I upgraded the alternator when I ordered the truck because I was certain a DC charger would be the cat's meow. After all the comments about whether it's really necessary, I'm starting to wonder. I don't plan on solar. I have a Honda 2000i generator that I was thinking about mounting on the A frame. Perhaps I'll start with installing the Multiplus first, see how it works to charge with a generator as needed while overnighting consecutive days without shore power, and then decide how much a DC charge might improve the situation. But one question I have is about your comment on "exercising" your battery. I've always read that the main things you can do to extend the life of lithium batteries is don't charge them to 100%, don't deplete them below 20%, and the less charge cycles it goes through the longer it will last. Thoughts?

Edit: Just occurred to me that one main point about having the DC charger was so that I could run the fridge on AC while traveling (I will be installing a Multiplus) and not rely on propane. We travel 8 hours on average, then stop without shore power for consecutive days. I've got to think about this more...

I am hardly the one to ask about battery plans, charging frequencies, levels, etc. I replace about 400ah of AGM with one 200ah Lithium and justified it for the weight savings. I just figure things work better when 'exercised' - kind of a use it or loose it mentality I guess. And I probably read someplace that using the battery is better than keeping it at full charge. For me I am still not sure I needed the DC to DC charger; still, I am glad to have it for the charge control on the battery when I do use it.

As it is I carry 2 Honda 2000's and am not shy about using them if necessary the dry camping. Usually once in the a.m. while having coffee & preparing for that day and again in the evening when appliance use is high again - all this assumes I am not running the ACs. I have no solar and doubt I will ever have it - I just don't need it.

I suppose if I were to do any amount of dry camping I would get a shunt and measure my power usage to determine the best way to manage the battery. Since you travel then dry camp for a while I think I would run the DC to DC charger while traveling since the batteries will get good use once stopped.

**SolarPoweredRV** · 12-01-2022, 11:14 PM

Originally Posted by Riverbug

But one question I have is about your comment on "exercising" your battery. I've always read that the main things you can do to extend the life of lithium batteries is don't charge them to 100%, don't deplete them below 20%, and the less charge cycles it goes through the longer it will last. Thoughts?

Edit: Just occurred to me that one main point about having the DC charger was so that I could run the fridge on AC while traveling (I will be installing a Multiplus) and not rely on propane. We travel 8 hours on average, then stop without shore power for consecutive days. I've got to think about this more...

Here is a note about charging Lithium batteries...

What you said about charging Lithium batteries is true, however, this applies to "Lithium Ion" batteries. What you have are "Lithium Iron Phosphate" batteries (LiFePo4). LiFePo4 batteries are a different chemistry and they like to be charged to 100%, however, with an expected longevity of 4,000 cycles or more, I don't think you should be too concerned about either keeping them fully charged, or draining them too much. Your BMS will keep your batteries healthy. So, use them how you like.

Running your Fridge on A/C would draw almost as much energy as traveling with your Air Conditioner running. Your fridge uses resistive heat when it is operating on A/C power and Resistive Heat uses a whole, WHOLE, LOT of Electrons. It would be okay to run your fridge on A/C if you are going through a tunnel, or are on a Ferry, but otherwise your two SOK batteries are probably going to be drained when you arrive at your destination.

One thing you haven't addressed, is how much work 80 amps are going to be placing on your Alternator. You might just burn out your Alternator because you will be placing a constant 80 amp load on it for 8 hours. Your Alternator is not designed for that much load for that many hours straight.

You may have noticed that I spoke about your Alternator as though you only have a single Alternator versus your Dual Alternator setup. This is because Ford does not share the Alternator load evenly between the two Alternators. Because of fuel economy, the second Alternator rarely gets used. Ford only wants to use that Second Alternator as a last resort.

For a detailed look into how Ford utilizes their dual Alternator system, and how charging Lithium (LiFePo4) via a DC - DC Charger works watch the following Video from YouTuber "Mortons on the Move"...

https://www.youtube.com/watch?v=uwrG3gUdDT8

**Boom 'r Bust** · 12-02-2022, 12:06 PM

Originally Posted by SolarPoweredRV

Here is a note about charging Lithium batteries...

Running your Fridge on A/C would draw almost as much energy as traveling with your Air Conditioner running. Your fridge uses resistive heat when it is operating on A/C power and Resistive Heat uses a whole, WHOLE, LOT of Electrons. It would be okay to run your fridge on A/C if you are going through a tunnel, or are on a Ferry, but otherwise your two SOK batteries are probably going to be drained when you arrive at your destination.

https://www.youtube.com/watch?v=uwrG3gUdDT8

David,

I agree with almost all of your points in the previous several posts. But the one about the frig using as much power as an air conditioner?

My 12cf frig uses 360ish watts on AC power. My air conditioning uses 1200 to 1600 watts depending on which one I'm running. My 40A DC to DC charger is a perfect match for running the frig on AC while driving.

Not sure what you are basing that opinion on...

Boomer