Amps on 7 pin cable to battery

[dedobias]

Has anyone ever measured the DC amp draw and voltage from the 7 pin cable to the battery with the batteries at 50% charge? I'm curious how many amps are pulled from the TV.

[wanderingbob]

Never measured , but judging by the wire size it must not be very much . That said , a good 4 or 5 hour drive a highway speeds will send a lot of juice to a batt.

[dedobias]

I've seen claims of 30 AMPs, that's the usual fusing on the TV. Then other places it's called a trickle charge. I'd like to quantify it better. Not that hard to put an amp meter inline from the 7 pin cable on the trailer.

[TitanMike]

My battery monitor showed up to 10 amps or so when I still had my lead acid batteries and they were discharged.

On my truck the 12 volt wire is a #10 awg and "fused" for 30 amp.

Issue I had was that as soon as the batteries received some charge, and the starting battery was fully charged, current dropped to less than 5 amps.

As said earlier, even 5 amps over a few hours of driving will charge the batteries some.

I now have LiFePo4 batteries and have installed a DC-DC charger, along with #8 awg wire directly from battery to DC-DC charger.

Batteries charge at a 20 amp rate (max output for my DC-DC charger) and they are fully charged by the time I arrive at my next campsite (boondocking).

[Chuck_S]

The alternator's voltage regulator won't supply enough volts to effectively charge a depleted trailer battery other than in an inordinate amount of time as it's only designed to put back the few amp hours the starting battery used. The long relatively thin wire back to the trailer doesn't help either. After a few minutes I only see a couple amps flowing back to my trailer. Voltage regulator stabilized about 13.7vDC and it's less at the trailer battery.

Better than nothing.

Running an auxiliary alternator to charge the house batteries is common on mid size boats and emergency vehicles.

-- Chuck

[TitanMike]

Originally Posted by Chuck_S

The alternator's voltage regulator won't supply enough volts to effectively charge a depleted trailer battery other than in an inordinate amount of time as it's only designed to put back the few amp hours the starting battery used. The long relatively thin wire back to the trailer doesn't help either. After a few minutes I only see a couple amps flowing back to my trailer. Voltage regulator stabilized about 13.7vDC and it's less at the trailer battery.

Better than nothing.

Running an auxiliary alternator to charge the house batteries is common on mid size boats and emergency vehicles.

-- Chuck

The advent of DC-DC Chargers has really improved the capability of the tow vehicle to charge trailer batteries. Even with the existing wiring. As long as the voltage on the charge wire at the 7-pin connector doesn't drop below 8 volts the DC-DC chargers will put a surprising amount of power into a trailer battery.

Also not terribly expensive with a 20 amp Renogy unit costing $127.

[pgandw]

Originally Posted by dedobias

I've seen claims of 30 AMPs, that's the usual fusing on the TV. Then other places it's called a trickle charge. I'd like to quantify it better. Not that hard to put an amp meter inline from the 7 pin cable on the trailer.

I never put an ammeter on line, but I did measure voltages when my first A-frame fridge had to be run in DC mode while towing. My Hyundai (re-badged Kia) minivan would start out at about 13.7V with fridge off. Fridge on (10 amp draw) would drop to 13.1V. Trailer running lights added another 6 amp draw and decreased voltage to 12.8V until I switched them out to LED "bulbs".

Each tow vehicle is going to be slightly different because alternators are processor controlled nowadays. In my 1960s vehicles with generators, not alternators, you could actually adjust the generator output voltage. If you did a lot of city driving, you would set the voltage up around 13.8V to keep your battery fully charged. If you did a lot of highway driving, you backed off the generator to about 13.4V to avoid "boiling" your battery.

Bottom line - I have about 10 amps to the trailer from the minivan at a useful voltage. If I stopped during the tow without turning off the fridge, I lost significant battery capacity (10+AH/hr) that could not be replaced quickly, if at all. One time, I had to wait in line 2hrs to check in at Palo Duro SP (near Amarillo). My batteries were down enough that my POS WFCO converter was complaining (loud fan noise) and the low voltage alarm on the CO/propane was chirping when I plugged in 3 hrs after arriving at the campsite (a big thunderstorm hit just as we got parked and popped up, total of 5 hrs with fridge on DC without charging).

My present A-frame can tow with the fridge on propane instead of DC, so I don't have to worry about the battery draw/charging while towing - I'll recharge 5-10 AH/hr of towing. Which is almost a normal charge rate for GC-2 batteries that start at better than 60% - 70% SOC.

When I retire and IF we are looking at going from one dry camping site to another, I will look at going the DC-DC voltage boost route to get more charge while towing. And/or add solar panel(s) to the roof. But to make use of the extra charge from the tow vehicle, I would really need to switch to LiFePo batteries.

just my experiences, yours will differ
now 2019 Flagstaff T21TBHW A-frame
prev 2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time

[Comanchecreek]

I also, (with Titan Mike's help), installed a DC to DC charger. I could drive several hours with very little increase on my trailer batteries. My older Chevy has a smaller (105 amp) alternator, and when the internal voltage regulator sensed that the truck batteries were charged it would lower the amperage output. That coupled with the small 10 gauge wire to the trailer just didn't lend itself to good charging.

[Chuck_S]

The voltage regulator in even Chevy trucks regulates the voltage, not the amperage -- otherwise it'd be called an amperage regulator.

When the voltage regulator senses the truck batteries are charged it lowers the output voltage. Good battery chargers work the same way -- by regulating the voltage. They start out about 14.7vDC and drop as battery voltage increases.

It's all related, of course. The higher the alternator voltage the more current will flow to the battery. The voltage regulator keeps this voltage relatively low, typically 13.7 after a few minutes, to keep from overcharging the vehicle battery. The voltage at the end of the wires to the trailer battery will be lower.

This voltage differential effect works if you connect two batteries in parallel. The battery with the lower voltage will constantly siphon power from the stronger -- and one is always stronger. After a while both batteries are dead.

-- Chuck

[bedubya]

Originally Posted by TitanMike

The advent of DC-DC Chargers has really improved the capability of the tow vehicle to charge trailer batteries. Even with the existing wiring. As long as the voltage on the charge wire at the 7-pin connector doesn't drop below 8 volts the DC-DC chargers will put a surprising amount of power into a trailer battery.

Also not terribly expensive with a 20 amp Renogy unit costing $127.

Mike, I would love to see more info on how you set that up, i.e., Where did you mount the charger, how did you wire it, etc. Is this the charger you used?
https://www.amazon.com/Renogy-Batter.../dp/B07Q5VYPCF

[Comanchecreek]

Originally Posted by Chuck_S

The voltage regulator in even Chevy trucks regulates the voltage, not the amperage -- otherwise it'd be called an amperage regulator.

When the voltage regulator senses the truck batteries are charged it lowers the output voltage. Good battery chargers work the same way -- by regulating the voltage. They start out about 14.7vDC and drop as battery voltage increases.

It's all related, of course. The higher the alternator voltage the more current will flow to the battery. The voltage regulator keeps this voltage relatively low, typically 13.7 after a few minutes, to keep from overcharging the vehicle battery. The voltage at the end of the wires to the trailer battery will be lower. Correct?


This voltage differential effect works if you connect two batteries in parallel. The battery with the lower voltage will constantly siphon power from the stronger -- and one is always stronger. After a while both batteries are dead.

-- Chuck

Thank you Chuck for the explanation. Isn't what I said the end result? When the Voltage drops, the amperage drops. Correct? I'm not questioning what you're telling me, I'm just making sure I understand the end result. When the voltage regulator senses fully charged main truck batteries, it lowers the voltage, thus less amperage, ultimately not sending much of either to the trailer batteries even though they may need additional charging. Correct?
In other words, the alternator output is regulated based on the charge rate of the truck batteries only. The condition of the trailer batteries has no effect on the voltage regulator unless it substantially pulls down the truck batteries, which is not possible through a 20 foot long 10 gauge wire. Thanks.

[Larry-NC]

Fred mentioned Processor-Controlled Alternators. This is where it really pays to Read The Fine Manual! On our GMC Sierra 2500, the manual states (under towing) that if you want to put a higher voltage on the +12 line of the 7-pin connector while driving, turn on the headlights.

Find out if your vehicle does the same thing.

[TitanMike]

Originally Posted by bedubya

Mike, I would love to see more info on how you set that up, i.e., Where did you mount the charger, how did you wire it, etc. Is this the charger you used?
https://www.amazon.com/Renogy-Batter.../dp/B07Q5VYPCF

Yes, this is the unit I used. I mounted it inside a storage compartment where I relocated my batteries from the tongue. They are Battleborn batteries and can safely be inside without venting.

The DC-DC charger's output is connected directly to the batteries. For input i added a "Duplex" (2-conductor) #8 awg wire from my truck's battery along the wire harness that ran from under hood to the rear bumper that carries the 7-pin receptacle's wires. This was then connected via a pair of 50 amp anderson connector to the input of the DC-DC charger.

Since the DC-DC charger is switched on and off by a separate 12 volt signal that on or off according to ignition switch's position i merely ran the factory 12 volt wire to this control terminal. On my truck the OE charge wire is only hot when ignition is on.

This turns on the DC-DC charger when the engine is running and when engine is off, the DC-DC charger is now a battery isolator, preventing discharge back to the truck.

In closing, any added wire from battery to DC-DC charger should have a fuse on it capable of handling at 40-50 amps. I use a fuse as it's under the hood and circuit breakers don't like to get hot.

[TitanMike]

Originally Posted by Chuck_S

The voltage regulator in even Chevy trucks regulates the voltage, not the amperage -- otherwise it'd be called an amperage regulator.

When the voltage regulator senses the truck batteries are charged it lowers the output voltage. Good battery chargers work the same way -- by regulating the voltage. They start out about 14.7vDC and drop as battery voltage increases.

It's all related, of course. The higher the alternator voltage the more current will flow to the battery. The voltage regulator keeps this voltage relatively low, typically 13.7 after a few minutes, to keep from overcharging the vehicle battery. The voltage at the end of the wires to the trailer battery will be lower.

This voltage differential effect works if you connect two batteries in parallel. The battery with the lower voltage will constantly siphon power from the stronger -- and one is always stronger. After a while both batteries are dead.

-- Chuck

What you say is correct but there is another power management strategy in play with newer vehicles. current sensing on battery ground cable.

Current flow in or out of the battery will signal the vehicle computer the starting battery is fully charged.

[PhilFromMaine]

I have never measured it. It may vary from truck to truck, but a "PRO" article in E-trailer stated to expect 2-4 amps. Of course it also depends on the battery's state of charge. Overall, the 7 pin connector is not meant to be a battery charger.


When upgrading to lifepo4 batteries, all the manufactures recommend DC-DC chargers.

[01tundra]

I know on our GMC it would start out around 8-10 amps, but shortly after the alternator started regulating back and it would drop down to the 3A range. And we always travel with our headlights on.

The "charge" circuit for the truck is a 25' pull one way and was ran with 12 AWG . Now all the factory charge wire does is power our TPMS signal booster.

Our Victron 12/12-18 DC-DC charger and 4 AWG circuit fixed that, now we are getting 25A to the batteries at 14.4V.

[Comanchecreek]

Originally Posted by Larry-NC

Fred mentioned Processor-Controlled Alternators. This is where it really pays to Read The Fine Manual! On our GMC Sierra 2500, the manual states (under towing) that if you want to put a higher voltage on the +12 line of the 7-pin connector while driving, turn on the headlights.

Find out if your vehicle does the same thing.

I can see where that would work, as the headlights would draw enough to fool the regulator into increasing the voltage. It still doesn't help the bottleneck of the 10 gauge wire, 20 + feet long. I'm sure there's a fairly large voltage drop, but we're getting there.
As Tundra said, I always have my headlights on because of my backup camera. Still not enough to charge a big battery bank, and eliminates any possible usage for charging at idle in a campground situation. My hope is the DC to DC charger will be the big problem solver.

[Chuck_S]

It's helpful for me to think of the battery drawing or sucking power from the alternator (or whatever is charging it) rather than the alternator pushing it. Is sucks harder when the voltage differential is high and barely at all when there's less than a volt difference. (I'm sure the electrical engineers cringe at this explanation!) Automobile voltage regulators typically run at 13.7vDC and battery fully charged is 12.7vDC. Voltage will be less at the trailer connector because of the wire and distance.

Rule of thumb in battery charging is it takes the same amount of time to go from 80% to 100% charged as it did to get to 80% in the first place. This makes trying to get to 100% a long process. I read (somewhere!) that US WW2 submarines ran their batteries in the 20% to 80% range -- massive charging currents when the batteries were low and they were limited by how much time they had on the surface to charge the batteries.

I've no experience with a DC-to-DC charger but am reminded there's no free lunch. The power has to come from the charging source.

-- Chuck

[5picker]

I typically see 12.9v to 13v at the R/V battery @ around 7a with my 2017 F-250 when connected and selected in the towing screen.

[Larry-NC]

Originally Posted by Chuck_S

It's helpful for me to think of the battery drawing or sucking power from the alternator (or whatever is charging it) rather than the alternator pushing it. Is sucks harder when the voltage differential is high and barely at all when there's less than a volt difference. (I'm sure the electrical engineers cringe at this explanation!) Automobile voltage regulators typically run at 13.7vDC and battery fully charged is 12.7vDC. Voltage will be less at the trailer connector because of the wire and distance.

Well, I have two degrees in Electrical Engineering and I have no problem with your explanation. You are taking the load-side view instead of the source-side view. That's not uncommon.

One other thing that some folks have trouble with is matching replacement power supplies to loads. They worry about matching the voltage AND the current. They don't realize that the current is a function of the load, not the supply.

An analogy is connecting your trailer to a pedestal tap that's 40 psi. It doesn't matter whether you have a 1/2" hose, 5/8" hose, or 3/4" hose. The 40 psi is the right pressure for the RV. The hose diameter merely limits flow, not pressure. Since the interior plumbing is 1/2", any hose size larger than 1/2" is (loosely) overkill.

Originally Posted by Chuck_S

I've no experience with a DC-to-DC charger but am reminded there's no free lunch. The power has to come from the charging source.

You're right. The power has to come from somewhere, or as us EEs say, the power is constant. Power = volts times amps, with a little efficiency loss.

• A 120 watt inverter might convert 12 volts at 10 amps to 120 volts at 1 amp. (Two different ways to realize 120 watts)
• A 120 watt DC-DC converter might convert 12 volts at 10 amps to 14 volts at 8.5 amps. (Once again, two different ways to realize 120 watts)