12V to 24V Conversion Question

[riverbug]

Pondering another question about amperage. If I add a 12V to 24V converter to charge a 24V battery from the truck's alternator, is the following sequence and amperages correct? The primary question is whether the step up converter and the DC charger should have the same amperage rating. I want a 40 amp DC charger, so the converter needs to be capable of the same amperage I believe?

12V alternator to 12V/24V step up converter (40 amp) to 24V DC charger (40 amp) to 24V + and - buss bars to 24V battery and system.

 

[timselectric]

You will need 80a on the 12v side. To get 40a on the 24v side. (Minus efficiency losses)

 

[riverbug]

You will need 80a on the 12v side. To get 40a on the 24v side. (Minus efficiency losses)

Ah, yes. Exactly what I wanted to confirm, thanks. That's a lot of amps. Hmm. I do have a 397 amp alternator upgrade on my truck. So if I could put a waterproof converter close to the battery and then run 24V back to the RV, that might not be so bad...

Except, after a quick search it doesn't look like they make a 12/24 converter that large; will have to continue the search and see what other options there are...

 

[timselectric]

I assume that it would be very expensive, for that size.

 

[timselectric]

I would see if you have room to add a 24v alternator.
My truck can have two alternators.

 

[OffGridInTheCity]

Here's a 12v -> 24v @ 40a boost converter - https://www.amazon.com/Cllena-Waterproof-Converter-Regulator-Transformer/dp/B07P41HCV2 - but it's not a 'charger' - e.g. doesn't have CC/CV appropriate for you're target battery.

Here's a 12v -> 24v @ 45a for lead or lithium - https://www.powerstream.com/dc-input-charger-12v-24v-1305w.htm - but it's pretty expensive.

 

[HRTKD]

I think the OP has the Ford 6.7L engine that uses two alternators already. There isn't room for a third. Replacing one of the two 12v alternators would require reprogramming. Ford generally doesn't run both alternators at the same time. They alternate (sorry, bad pun) running so they stay cooler.

 

[riverbug]

I think the OP has the Ford 6.7L engine that uses two alternators already. There isn't room for a third. Replacing one of the two 12v alternators would require reprogramming. Ford generally doesn't run both alternators at the same time. They alternate (sorry, bad pun) running so they stay cooler.

Hi @HRTKD and all. Yes, it's a F350 with dual alternators. It does look like a tough find for an 80A converter. Couple of questions...
Is it possible to run two 40A 12/24V step up converters in parallel (like you would batteries) to get 80A output at 24V? I'm thinking this would not work but not really sure why. The thought would be that those are connected together near the truck battery, leaving 24V to send back to the camper where the DC charger would be housed near the RV battery.

Otherwise, I could start with one 40A converter providing 20A (at 24V) to the DC charger (40A). Then if I decide I need more I could just run a second 40A converter to provide the other 20A to the DC charger. That doubles the wiring and everything but I think that would work wouldn't it?

 

[riverbug]

Here's a 12v -> 24v @ 40a boost converter - https://www.amazon.com/Cllena-Waterproof-Converter-Regulator-Transformer/dp/B07P41HCV2 - but it's not a 'charger' - e.g. doesn't have CC/CV appropriate for you're target battery.

Here's a 12v -> 24v @ 45a for lead or lithium - https://www.powerstream.com/dc-input-charger-12v-24v-1305w.htm - but it's pretty expensive.

Thanks much. I actually have that 40A in my Amazon cart saved for later so I could find it again. It seems cheaper to go with a converter like the Cllena and a separate DC charger compared to the powerstream unit.

 

[HRTKD]

Hi @HRTKD and all. Yes, it's a F350 with dual alternators. It does look like a tough find for an 80A converter. Couple of questions...
Is it possible to run two 40A 12/24V step up converters in parallel (like you would batteries) to get 80A output at 24V? I'm thinking this would not work but not really sure why. The thought would be that those are connected together near the truck battery, leaving 24V to send back to the camper where the DC charger would be housed near the RV battery.

Otherwise, I could start with one 40A converter providing 20A (at 24V) to the DC charger (40A). Then if I decide I need more I could just run a second 40A converter to provide the other 20A to the DC charger. That doubles the wiring and everything but I think that would work wouldn't it?

You can run multiple charger/converters in parallel. Is the power going to a truck camper or a trailer?

The shorter the distance the easier/less expensive this will be. To get high amp charging to either a truck camper or a trailer can be quite expensive. The cost of cabling is usually what limits the amperage. 80 amps of 24 volt charging is 160 amps of 12 volt input. It's probably worse than that because the input amps are almost always higher than the output amps when the voltage needs to be increased (like from 12.8 to 14.2). For example, a 20 amp charger (12 volt input to 12 volt output) may require 25 amps from the alternator. You have to take into consideration the higher input amps in your design.

To control the cost of cable, as well as the cost of the charger devices, we tend to recommend lower amp charging rates. For my RV, if I were to use the charge from the 7-pin circuit on my tow vehicle (also an F-350 6.7L w/ 397 amp charging system), I would aim for no more than 20 charge amps. To do that I would use one of the chargers from Victron:

https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-Smart-DC-DC-chargers-isolated-250-400W-EN.pdf

To get any more charge amps than that, I would have to run dedicated cabling. Let's say I wanted 40 amps. For my setup, the distance from alternator to charger (located close to the RV's LiFePO4 battery) would be 60' round trip. I use a conservative voltage of 12.0 volts as a worst case number. Using the Wire Size Calculator at Bay Marine Supply I get the following output:



2 AWG cable isn't large enough. We want the voltage drop to be under 3%. The more the voltage drops, the more amps the charger will pull. So we want the voltage drop as small as possible, within reason. 1 AWG is the smallest I would go with. 60' of 1 AWG cable from TEMCo Industrial would run about $157. That's welding wire, which is what I prefer to use.

However, Victron doesn't make a 12-12 charger suitable for this situation rated for 40 amps. The highest they have is 30 amps. The highest 12-24 charger is rated for only 15 amps.

If you want 30 amps of 24 volt charging, buy two chargers and run two sets of cable. The cable alone is going to cost over $300 if you use two chargers.

In general, the charger is supposed to be located as close to the target battery as possible. For that reason, the 12 volt current is run almost the entire length of the cable. If we could put the charger next to the alternator then the current would be running at 24 volts and we could use smaller wire. 4 AWG would work. However, you really don't want to put the charger in the engine compartment due to heat and moisture. Plus that violates our preference for placing the charger close to the target battery.

 

[riverbug]

You can run multiple charger/converters in parallel. Is the power going to a truck camper or a trailer?

The shorter the distance the easier/less expensive this will be. To get high amp charging to either a truck camper or a trailer can be quite expensive. The cost of cabling is usually what limits the amperage. 80 amps of 24 volt charging is 160 amps of 12 volt input. It's probably worse than that because the input amps are almost always higher than the output amps when the voltage needs to be increased (like from 12.8 to 14.2). For example, a 20 amp charger (12 volt input to 12 volt output) may require 25 amps from the alternator. You have to take into consideration the higher input amps in your design.

To control the cost of cable, as well as the cost of the charger devices, we tend to recommend lower amp charging rates. For my RV, if I were to use the charge from the 7-pin circuit on my tow vehicle (also an F-350 6.7L w/ 397 amp charging system), I would aim for no more than 20 charge amps. To do that I would use one of the chargers from Victron:

https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-Smart-DC-DC-chargers-isolated-250-400W-EN.pdf

To get any more charge amps than that, I would have to run dedicated cabling. Let's say I wanted 40 amps. For my setup, the distance from alternator to charger (located close to the RV's LiFePO4 battery) would be 60' round trip. I use a conservative voltage of 12.0 volts as a worst case number. Using the Wire Size Calculator at Bay Marine Supply I get the following output:

View attachment 137804

2 AWG cable isn't large enough. We want the voltage drop to be under 3%. The more the voltage drops, the more amps the charger will pull. So we want the voltage drop as small as possible, within reason. 1 AWG is the smallest I would go with. 60' of 1 AWG cable from TEMCo Industrial would run about $157. That's welding wire, which is what I prefer to use.

However, Victron doesn't make a 12-12 charger suitable for this situation rated for 40 amps. The highest they have is 30 amps. The highest 12-24 charger is rated for only 15 amps.

If you want 30 amps of 24 volt charging, buy two chargers and run two sets of cable. The cable alone is going to cost over $300 if you use two chargers.

In general, the charger is supposed to be located as close to the target battery as possible. For that reason, the 12 volt current is run almost the entire length of the cable. If we could put the charger next to the alternator then the current would be running at 24 volts and we could use smaller wire. 4 AWG would work. However, you really don't want to put the charger in the engine compartment due to heat and moisture. Plus that violates our preference for placing the charger close to the target battery.

Thanks for all of that information! Some clarifications...It is a travel trailer so the distance of cable I estimate to be 25-30 feet from truck battery to trailer battery. I was shooting for 40A for charging the 24V trailer battery, thus 80A @ 12V. However, perhaps 20A instead of 40A charging is more realistic. But something I'm still confused about, so appreciate if you tell me whether I'm whacked on this or not...

If I placed the 12V to 24V converter next to the truck battery (using a waterproof version of course), I would only have a distance of 1.5' @ 12V. I would then have say 30' @ 24V to get back to the trailer battery location where the 24V DC to DC charger would be located. The DC to DC charger would then be within a couple of feet to the system bus bars. Would that be a sound approach? If so, I would use those figures to determine the appropriate gauge of wiring for both the 12V short run and the longer 24V run? Or is that just not the right way to do this?

 

[time2roll]

You will need 80a on the 12v side. To get 40a on the 24v side. (Minus efficiency losses)

And possibly an alternator upgrade to get that amount of power.

 

[timselectric]

And possibly an alternator upgrade to get that amount of power.

The truck has dual alternators that alternate being used.
Might be able to make them work simultaneously.
Would just need to monitor the temperatures.

 

[HRTKD]

If I placed the 12V to 24V converter next to the truck battery (using a waterproof version of course), I would only have a distance of 1.5' @ 12V. I would then have say 30' @ 24V to get back to the trailer battery location where the 24V DC to DC charger would be located. The DC to DC charger would then be within a couple of feet to the system bus bars. Would that be a sound approach? If so, I would use those figures to determine the appropriate gauge of wiring for both the 12V short run and the longer 24V run? Or is that just not the right way to do this?

Yes, this is an option. However, the heat in the engine compartment is a limiting factor. The 12-24 converter will heat up and likely derate (if you're lucky) or melt down (if you're not). The distances between each device (alternator -> converter -> charger -> battery) are what you plug into the wire gauge calculator.

This concept has been discussed a few times here on the forum. One thread even discusses going from 12v DC alternator to 120v AC inverter to AC-DC charger. The 120v AC requires much smaller cable than if it was kept as DC.

Do you have solar on the RV? I have only 640 watts on mine and it's enough I have no need for charging from the tow vehicle.

 

[riverbug]

Yes, this is an option. However, the heat in the engine compartment is a limiting factor. The 12-24 converter will heat up and likely derate (if you're lucky) or melt down (if you're not). The distances between each device (alternator -> converter -> charger -> battery) are what you plug into the wire gauge calculator.

This concept has been discussed a few times here on the forum. One thread even discusses going from 12v DC alternator to 120v AC inverter to AC-DC charger. The 120v AC requires much smaller cable than if it was kept as DC.

Do you have solar on the RV? I have only 640 watts on mine and it's enough I have no need for charging from the tow vehicle.

Well, that question about solar was going to be my next question for you. I was trying to tackle the non-solar stuff first because I know less about that than I do about 12V systems. Deciding on 24V battery made it a little more complicated for me once I started considering this DC to DC charger business.

So I was going to ask based on all this discussion, would I be better off just upgrading or replacing the solar package on the trailer? It has the Furrion 12V fridge, one 165W solar panel, and a 25A solar charger. I've looked at the cost of adding another Furrion panel and they appear to be very over priced for the quality and capability. So I assumed I would just replace it and potentially the solar charger. But I'm too new to solar yet to understand what I could put on that would compare to or exceed the DC charger I've been asking about. That would be my point of reference for now.

 

[HRTKD]

Well, that question about solar was going to be my next question for you. I was trying to tackle the non-solar stuff first because I know less about that than I do about 12V systems. Deciding on 24V battery made it a little more complicated for me once I started considering this DC to DC charger business.

So I was going to ask based on all this discussion, would I be better off just upgrading or replacing the solar package on the trailer? It has the Furrion 12V fridge, one 165W solar panel, and a 25A solar charger. I've looked at the cost of adding another Furrion panel and they appear to be very over priced for the quality and capability. So I assumed I would just replace it and potentially the solar charger. But I'm too new to solar yet to understand what I could put on that would compare to or exceed the DC charger I've been asking about. That would be my point of reference for now.

If the solar panel is very old it may be impossible to find the same model. A panel from another vendor with the same specs will do. The 25 amp solar charge controller has room for another panel. You would want to wire the two panels in parallel (2p instead of 2s). With your existing equipment you're not getting more than about 13 amps.

Here's a video that explains the issues when trying to use mismatched panels together:

How to wire mismatched solar panels. Wire in parallel or in series?

Video series testing wiring mismatched solar panels in parallel vs. wiring in series to maximize solar PV output with an MPPT charge controller.

www.altestore.com

A 24 volt system is appropriate if you're driving bigger loads. The gain often touted with a 24 volt system is that the cables can be smaller. The counter argument to that is if your components are located close enough, the monetary savings aren't much. You're already seeing that you need to step up the tow vehicles voltage. When you interface the 24 volt RV battery bank with the existing 12 volt system in the RV you need a converter. Does the cost of those two extra components outweigh the savings in cable?

 

[riverbug]

If the solar panel is very old it may be impossible to find the same model. A panel from another vendor with the same specs will do. The 25 amp solar charge controller has room for another panel. You would want to wire the two panels in parallel (2p instead of 2s). With your existing equipment you're not getting more than about 13 amps.

Here's a video that explains the issues when trying to use mismatched panels together:

How to wire mismatched solar panels. Wire in parallel or in series?

Video series testing wiring mismatched solar panels in parallel vs. wiring in series to maximize solar PV output with an MPPT charge controller.

www.altestore.com

A 24 volt system is appropriate if you're driving bigger loads. The gain often touted with a 24 volt system is that the cables can be smaller. The counter argument to that is if your components are located close enough, the monetary savings aren't much. You're already seeing that you need to step up the tow vehicles voltage. When you interface the 24 volt RV battery bank with the existing 12 volt system in the RV you need a converter. Does the cost of those two extra components outweigh the savings in cable?

Regarding the existing solar, it's a new 2023 trailer so additional panels are available but they charge $550 for a 165W Furrion if I were to get one to match. They also have proprietary connectors; not a big deal to change them out. I've read that I could get more efficient panels for about 1/3rd the price, but I haven't confirmed that as I don't know what I would be looking for yet.

Regarding the 24V: I plan to purchase a Victron Multiplus 24/3000/70 to run with two 24V 280AH batteries. From my research, it's right on the line for whether 24V is warranted. I just figured if it were more efficient than 12V I'd go 24V. But I don't have the Multiplus yet so that could be changed. I have 8 EVE LF280K cells and one 12/24V BMS, so I could just get another BMS and make it two 12V batteries instead. Then put another set together when I'm ready. We plan to boondock a fair amount and want to be able to run all the usual appliances (including A/C sometimes). 12V batteries would be more manageable weight wise, but I can deal with that aspect.

What would you do?

 

[HRTKD]

For my trailer, a 24 volt system wasn't necessary. I have two 4s 280 amp EVE DIY batteries in my trailer. It works great. I can run everything, even the air conditioner. Granted, I can't run the air conditioner for too long, but I can fire it up for an hour after sundown to cool things off. If I the extra 640 watts of ground deployed panels out, I can run the air conditioner during the day for maybe four hours. I do mostly boondocking. Since I installed the system, I haven't needed to run the on-board generator to charge the battery bank.

 

[OffGridInTheCity]

FYI - rambling

For my 7 x 14 Cargo Trailer -> Camper Trailer conversion, I installed 3000w MPP Solar 3048LV + 1,200w panels + 48v 13kwh battery - e.g. about 10kwh of useable power. We use a 9,000BTU Mini-Split as the core heat-cool. As backup, have 2 x 30lb propane bottles, heat-buddy, and propane generator. We have 1" (R5) insulation all around in the core trailer.

When winter boondocking with 45F hi / 25F lows - to keep 75F in core trailer via mini-split + 1500w heater in the 6 x 6 'tent' extension on the tailgate, we need about 22kwh over 24hrs to be perfectly warm. In moderate temps with just the core of the trailer we only need 7kwh / 24hrs. These figures include mini-split, instant hot water, k-cup, refrigerator, microwave, hot-plate, and entertainment.

I built it to be a 120v system with 120vac -> 12vdc @ 80a transformer for the 12v sub-system which powers the tongue jack, water pump, and some lights. This let's me shut-off the battery completely and use outside grid 120v only but still have a 12v sub-system.

The biggest disappointment is the panels. Every single camping spot so far - either camp ground or boondocking - is either shaded by trees or canyon (mountains) greatly reducing sun + the panels are 'flat'. Their contribution has been pretty low to overall power uses and we cannot winter boondock without generator assistance or grid or propane/heat-buddy heat or low indoor temps (e.g. 55-60F).

 

[riverbug]

For my trailer, a 24 volt system wasn't necessary. I have two 4s 280 amp EVE DIY batteries in my trailer. It works great. I can run everything, even the air conditioner. Granted, I can't run the air conditioner for too long, but I can fire it up for an hour after sundown to cool things off. If I the extra 640 watts of ground deployed panels out, I can run the air conditioner during the day for maybe four hours. I do mostly boondocking. Since I installed the system, I haven't needed to run the on-board generator to charge the battery bank.

That's pretty much our use case too. Don't want to run air all day, just cool down in the evening before bed, roadside stops when it's super hot inside the trailer, and whatever else depending on how the batteries last. Otherwise, microwave, wife's hair dryer, etc.

Thanks so much for all the insight and advice. Will give this more thought and I'm sure I'll be asking more questions.