DC to DC charger size questions

Mike Jordan

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Looking for info before I install a DC to DC charger for my travel trailer. Bigger may be better for quickly charging the batteries. But my concerns are over taxing the alternator or tow vehicle systems.

Where can I find info on amperages safe for my tow vehicle?
 

kenryan

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The vehicle manufacturer is the only solid source I know of. But there may be no definitive answer, because conditions such as ambient temperature and vehicle draw can vary.
 

time2roll

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If you are going to boost the existing charge line.... start with checking the fuse rating. I would go with output at about half the fuse rating.
 

Mike Jordan

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If you are going to boost the existing charge line.... start with checking the fuse rating. I would go with output at about half the fuse rating.
I am adding a DC/DC charger not "boosting" the existing charge line. There is nothing to "boost"
 

Mike Jordan

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The vehicle manufacturer is the only solid source I know of. But there may be no definitive answer, because conditions such as ambient temperature and vehicle draw can vary.
and... therefore the reason that i am asking
 

smoothJoey

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and... therefore the reason that i am asking
Get a dc clamp meter.
Measure the load of your vehicle with all the accessories that you will be using.
Subtract that from the amp rating for your alternator.
For bonus points, google for the your alternator's amps vs rpms plot.
 

rickst29

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If you are going to boost the existing charge line.... start with checking the fuse rating. I would go with output at about half the fuse rating.
I feel that to be excellent advice from time2roll.

Mike - if you go "too big", your most likely disaster will be melting insulation within the 7-wire trailer cable into the Tow Vehicle "Bargman" connector. Your second most likely disaster will be an overheated "Trailer Battery Charge" wire (TBC) within the tow vehicle itself, between the fuse block (under the hood) and the Bargman connector in the rear.

Even though the TBC circuit is usually fused at either 30A or 50A, automakers tend to put very small wires in their vehicles. In my older Toyota 4Runner, for example, it was fused for 30A - but it appeared to be #14 wire at best. It might have been even smaller than that. I rewired it to 10-AWG (as part of a complicated "Charge from the 4Runner scheme" involving TBC Voltage Boost (to 36V), the Trailer auto-detecting high voltage, and then switching the TBC from the "12v" system bus to become input to an MPPT Solar controller).

The 30A fuse would have implied 10-AWG, but Toyota used (IMO) unjustified "chassis wiring" rules in declaring the small wire to be "adequate". In my opinion, the long wire (perhaps 10-11 feet long) is not chassis wiring, able to dissipate heat at its connection ends -- it's too long for that, and generated heat (from high current levels) MUST escape through the insulation jacket. At 14 AWG with 90c insulation, and 90c, I would have rated it no better than 25A -- but again, it appeared to be even smaller than that. And the Trailer cord is much worse:. The TBC wire within your trailer cord is probably even smaller, and the cord assembly wraps all the wires in a heavy rubber cable - providing too much thermal insulation, and keeping the heat inside to start melting things.

Even without risking fires or melted and broken wires and cord assemblies, you would waste a ton of power by trying to push 30A through wires that small. So - exactly as time2roll says, I would not purchase any in-trailer DC->DC battery charger at a size which would pull more than HALF of the TV's fuse rating for the TBC wire.

That's the low voltage side, typically starting at roughly 13.6V in the engine compartment and suffering "Voltage drop" along the TBC wire and ground return from the Trailer.

Example: In order to charge an LFP battery at 20A and 14.2 Volts (284 watts), an 85% efficient DC->DC charger would need to pull 334 watts from it's input side. Since the total length of TBC (one way) is 20 feet, and the average size of TBC and Ground wire is probably not better than 14 AWG (the grounding wire is bigger, the TBC is smaller), voltage drop is almost 21%, yielding a voltage of only about 10.6V at the DC->DC charger. You simply can't do it - the 334 watts would need more than 30A at this low voltage, and it would blow a 30A fuse.
 

rickst29

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So the most important questions are (1) what is your Tow Vehicle, and (2) What sort of "charging" power do you need to obtain?
 

Zwy

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Looking for info before I install a DC to DC charger for my travel trailer. Bigger may be better for quickly charging the batteries. But my concerns are over taxing the alternator or tow vehicle systems.

Where can I find info on amperages safe for my tow vehicle?
Look up a replacement alternator for the vehicle and that will give you it's max amps. Then use a clamp on meter to determine how many amps the vehicle requires to run and for accessories you normally run including lights and blower fan on high.

I prefer to run around 50 to 60% of rated capacity so the result will be max amps x 50 to 60% minus the amps needed to operate the vehicle. The result is the max amps you should have for a B2B.
 

time2roll

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I am adding a DC/DC charger not "boosting" the existing charge line. There is nothing to "boost"
If you are pulling new wire I would look for a rating maybe 25% of the alternator.

I said "boost" as the typical existing OEM charge connection through the existing trailer connector will suffer voltage drop and provide fairly low current at a less than adequate voltage. This translates into a very slow trickle charge. The DC-DC charger is often used close to the battery being charged to boost that voltage drop and provide proper charging.

Please post the vehicle model, trim level and year for best answers. 2020 F450 w/ dual alternators? 2005 Toyota Land Cruiser?
Otherwise need to look up the replacement alternator to get the rating.
 

kenryan

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Get a dc clamp meter.
Measure the load of your vehicle with all the accessories that you will be using.
Subtract that from the amp rating for your alternator.
For bonus points, google for the your alternator's amps vs rpms plot.
And for extra bonus points be aware that alternators are normally not designed to put out full power continuously. If your system puts your alternator under full load for extended periods of time, I would not expect the alternator to hold up.
 

smoothJoey

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And for extra bonus points be aware that alternators are normally not designed to put out full power continuously. If your system puts your alternator under full load for extended periods of time, I would not expect the alternator to hold up.
Sounds like you know more about this than I do.
What do the ratings mean then?
 

kenryan

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Normally the amperage rating on a common vehicle alternator tells you the maximum that the alternator is capable of putting out. This is different from a "maximum continuous output" rating. But like I said earlier, even if you have a maximum continuous rating, that will be dependent upon a particular ambient temperature. The rating will be reduced at higher ambient temperatures. This is all because in process of producing power, the alternators produce heat. The more power, the more heat. Certain components of the alternator will fail if they get too hot.
 

Short_Shot

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If you are going to boost the existing charge line.... start with checking the fuse rating. I would go with output at about half the fuse rating.
The fuse in my truck is 40a from the factory and it's 16awg wire.

I don't recommend following this advice. Find out the wire size.

And then you have to keep in mind the DC to DC charger will pull MORE current from the tow vehicle across that wire than its putting into the battery and this is further compounded by increased voltage drop due to the still higher current.

My 18a charger will pull as much as 25a over 20 feet of 16awg.


If you're adding your own wiring, call the dealer and see if they can tell you the idle alternator output. You might be able to get this tested at a parts store as well.

And as stated before you'll need to subtract all the vehicle needs.
 

Mike Jordan

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So the most important questions are (1) what is your Tow Vehicle, and (2) What sort of "charging" power do you need to obtain?
Not the most important to me as I already know those answers. The most important to me is where to find info for the current flow safe for the alternator
 

Mike Jordan

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I feel that to be excellent advice from time2roll.

Mike - if you go "too big", your most likely disaster will be melting insulation within the 7-wire trailer cable into the Tow Vehicle "Bargman" connector. Your second most likely disaster will be an overheated "Trailer Battery Charge" wire (TBC) within the tow vehicle itself, between the fuse block (under the hood) and the Bargman connector in the rear.

Even though the TBC circuit is usually fused at either 30A or 50A, automakers tend to put very small wires in their vehicles. In my older Toyota 4Runner, for example, it was fused for 30A - but it appeared to be #14 wire at best. It might have been even smaller than that. I rewired it to 10-AWG (as part of a complicated "Charge from the 4Runner scheme" involving TBC Voltage Boost (to 36V), the Trailer auto-detecting high voltage, and then switching the TBC from the "12v" system bus to become input to an MPPT Solar controller).

The 30A fuse would have implied 10-AWG, but Toyota used (IMO) unjustified "chassis wiring" rules in declaring the small wire to be "adequate". In my opinion, the long wire (perhaps 10-11 feet long) is not chassis wiring, able to dissipate heat at its connection ends -- it's too long for that, and generated heat (from high current levels) MUST escape through the insulation jacket. At 14 AWG with 90c insulation, and 90c, I would have rated it no better than 25A -- but again, it appeared to be even smaller than that. And the Trailer cord is much worse:. The TBC wire within your trailer cord is probably even smaller, and the cord assembly wraps all the wires in a heavy rubber cable - providing too much thermal insulation, and keeping the heat inside to start melting things.

Even without risking fires or melted and broken wires and cord assemblies, you would waste a ton of power by trying to push 30A through wires that small. So - exactly as time2roll says, I would not purchase any in-trailer DC->DC battery charger at a size which would pull more than HALF of the TV's fuse rating for the TBC wire.

That's the low voltage side, typically starting at roughly 13.6V in the engine compartment and suffering "Voltage drop" along the TBC wire and ground return from the Trailer.

Example: In order to charge an LFP battery at 20A and 14.2 Volts (284 watts), an 85% efficient DC->DC charger would need to pull 334 watts from it's input side. Since the total length of TBC (one way) is 20 feet, and the average size of TBC and Ground wire is probably not better than 14 AWG (the grounding wire is bigger, the TBC is smaller), voltage drop is almost 21%, yielding a voltage of only about 10.6V at the DC->DC charger. You simply can't do it - the 334 watts would need more than 30A at this low voltage, and it would blow a 30A fuse.
Never have considered doing charging through the existing 7 pin, nor do I have any issues with figuring wire size and voltage drop. None of those questions are in my post. I am only looking for where to find info on determining what amperages are safe for my alternator
 

Mike Jordan

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I thought I was asking a dumb question. But maybe not as it appears I may have stumped the hive. :oops: So bump for Friday eyes.

Any Friday eyes with alternator spec resources?
 

smoothJoey

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I thought I was asking a dumb question. But maybe not as it appears I may have stumped the hive. :oops: So bump for Friday eyes.

Any Friday eyes with alternator spec resources?
 

time2roll

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Never have considered doing charging through the existing 7 pin, nor do I have any issues with figuring wire size and voltage drop. None of those questions are in my post. I am only looking for where to find info on determining what amperages are safe for my alternator
OK some of the details were missing..... such as the make and model of the vehicle. Maybe even the part number for the alternator.
My old Mustang was 35 amps and some trucks go over 200 amps.

Or did I miss this? Or search for a replacement as the specs will often be given. Walk into the dealer parts dept and ask. Google is probably easier.
There is no resource that will tell the maximum accessories that you can add to a vehicle. Going to be a bit of a guess.
 
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Zwy

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I thought I was asking a dumb question. But maybe not as it appears I may have stumped the hive. :oops: So bump for Friday eyes.

Any Friday eyes with alternator spec resources?
You never gave any information that is needed. Kinda hard to answer or help with limited information. I gave you the generic version, if you want to load an alternator to 100% of output, have at it.

Your vehicle could have a 50 amp alternator or it could have a 250 amp. You are the one responsible for telling us whether you have a Yugo and you are really responsible for looking up the rating for it's alternator. If you want the duty cycle of an alternator, then contact the manufacturer of the Yugo alternator.

Once that information is related to those attempting to help you, then possibly an answer may be forthcoming.

If you want specs, contact the local parts store for the specs for the Yugo alternator. That is the easiest. You might be required to look at the Yugo build specs (little sticker somewhere which lists the equipped option codes) if asked.

If you want to play guessing games, what color is the shirt I'm wearing? It's not the same color I had on yesterday..........
 
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