Leaving 7 pin connected to TV overnight won't drain TV battery, IF......

judenray

Member
Apr 8, 2016
16
The Dalles, Oregon
Recently had to crawl around under the Kia Sorento and find why the TV was not charging the trailer battery while driving. Found a crimp-wire connection in the rear bumper had come loose, simple enough to fix. Decided to replace the crusty 7 pin male connector from the camper while I was at it. I was three beers into the job and was looking at the wrong 7 pin connection diagram (inside vs outside connections) and managed to reverse the power and ground wires (reverse connection solenoid on the battery cable saved the day, no harm done. Drinking and wiring don't mix). I've always wondered whether the 12v power pin was "hot" when the TV was turned off (so many conflicting posts on whether leaving the camper plugged in to the TV overnight will drain the TV battery). I generally disconnect the TV once I'm at camp, but leave it connected for overnight stops, etc. Poked around with my multimeter and found that on my 2017 Kia Sorento the 12v pin is not hot if the vehicle is turned off (and not on ACC). Looks like the tech who installed the brake controller and 7 pin connection at Camper World wired the 12v pin to a "switched" connection on the fuse block so no power supplied unless the ignition is either in the "on" or "acc" position. Many conflicting posts on this subject never mention this contingency.
 

Sjm9911

Super Active Member
May 31, 2018
12,457
Nj
It all depends on the tv. My old truck had a constant feed. And qould drain the battery on the road as I used the 12v on the fridge. The new one dosen't ( I also dont have a 12 v fridge anymore) but out of habbit I still disconect the line when stopping for lunch. Only way to tell is to test what you have. So its realy not conflicting advise , because if you had a diffrent tv it could ve diffrent.
 

judenray

Member
Apr 8, 2016
16
The Dalles, Oregon
It all depends on the tv. My old truck had a constant feed. And qould drain the battery on the road as I used the 12v on the fridge. The new one dosen't ( I also dont have a 12 v fridge anymore) but out of habbit I still disconect the line when stopping for lunch. Only way to tell is to test what you have. So its realy not conflicting advise , because if you had a diffrent tv it could ve diffrent.
Not so much a difference in the TV itself, but a difference in where the 12v power was sourced. Perhaps better if I said "incomplete advice" instead of conflicting advice. A lot of DIY folks just hook up to an always-hot battery feed, and in that case there is always a hot connection to the 12v pin in the 7 pin connection. In that case, leaving the 7 pin connected when the vehicle is off will leave the TV battery subjected to the power drain needs of the camper.
 

Raycfe

Waterford Ct.
Oct 3, 2007
18,440
Waterford, Ct
My jeep, Subaru and F150 were all key only power. And on my F150 the cap interior light has a limited on battery feed. It’s all about they are wired.
 

Sjm9911

Super Active Member
May 31, 2018
12,457
Nj
My older truck was wired from the factory. I wired in the controller, the wires were their from the factory. It Had a constant feed. I guess I could have rewired it? But at the time didn't know better.
 

jeepster04

Active Member
Nov 23, 2010
299
I'm not sure I would call it conflicting information. It all depends on the vehicle. My Jeeps with the factory tow package all turn the battery + off when the ignition is off.

Keep in mind, the fridge on DC will drain the camper battery very quickly when the tow vehicle is not running. It will not recover when the vehicle is started either. You may end up at the campsite with a dead camper battery.
 

Grandpa Don

Super Active Member
Sep 5, 2018
1,886
Southern California
I left my 7-pin connected to the truck overnight just one time. On the way back from a 7 day long camping trip I stopped at my Son's house to spend the night before heading on back home. In the morning the truck battery was dead. I never use the 12 volt power source to the Fridge, so I know that didn't drain the battery. The only thing that draws power in this situation is the solar controller and the Propane detector. I think the main camper controller does too. I guess that was enough to do it. I had a Quick-Start battery charger, so I was back on the road in no time.
 

Michael J

Active Member
Aug 9, 2018
201
Michigan
I'm not sure I would call it conflicting information. It all depends on the vehicle. My Jeeps with the factory tow package all turn the battery + off when the ignition is off.

Keep in mind, the fridge on DC will drain the camper battery very quickly when the tow vehicle is not running. It will not recover when the vehicle is started either. You may end up at the campsite with a dead camper battery.
Yep, both the wife's Jeep and mine both kill the main power to the 7 way when off took me tearing half the back of he 04 GC apart to find someone pulled the wiring apart, so I finally got power to the back of hers
 

radar1

New Member
Mar 9, 2008
7
I left my 7-pin connected to the truck overnight just one time. On the way back from a 7 day long camping trip I stopped at my Son's house to spend the night before heading on back home. In the morning the truck battery was dead. I never use the 12 volt power source to the Fridge, so I know that didn't drain the battery. The only thing that draws power in this situation is the solar controller and the Propane detector. I think the main camper controller does too. I guess that was enough to do it. I had a Quick-Start battery charger, so I was back on the road in no time.
Actually, the trailer battery will pull power all by itself if the state of charge is lower than the tv battery, even if nothing is running.
Some of the newer tvs won't even provide voltage at an empty 7 pin while the tv is running, since they only power up when the camper is actually plugged in, so testing at the 7 pin might not always work.
 
Jul 30, 2018
23
I intentionally ran the 12v power pin on the TV directly to the battery - if I don't want it active I can unplug the trailer. However as I confirmed recently, that's no guarantee that the TV's alternator is going to do you any good. While older vehicles had alternators that ran "dumb" and put out a voltage high enough to guarantee a charge on the battery, modern vehicles are much smarter and only supply a voltage appropriate to the current state of the battery. This means that even with the trailer's electrical connected to the TV, there's absolutely no guarantee that power is going to flow from the TV to the trailer and not the other way around, even while driving.

RedArc has a $400-500 device that acts as a boost converter between the tow cable and the trailer's battery, and ensures that no matter what voltage is being provided by the TV, the 12v bus in the trailer will at least provide power to e.g. the fridge without draining the battery, and probably also do some slow charging. The problem is that a) it's waaay bigger than a PUP needs, and b) it's expensive.

I've done a bunch of searching around for a power supply that will take in a decent automotive range and output something like 13.6 or 14V at a few hundred watts, and so far the only thing I've found is a $200 evaluation board for a controller chip (LM5177 iirc) from TI that could easily be altered to output a float charge voltage. No "packaged" power supplies seem to exist...

If anybody has found an alternate solution to this problem, please let me know. If I *HAD* to I could go ahead and design my own power supply based on that TI chip (the part number sounds familiar, I may have used that exact chip before), but that means learning a new PCB design program (I don't have an Eagle license anymore, so probably KiCAD) and digging out all my electronics tooling again. The eval board would be an easy tweak to make, but it's still $200 for something that should be half that or less in practice.
 

bigm23

New Member
Jul 5, 2022
7
Alberta
Ya. I have a Kia Sorento too. I don't think mine is working and supplying the power to charge or anything. Still working on checking that out when I have time. But, now with Fall coming in, camping season gone and have to now work on first time winterize mine. lol
 

SteveP

Super Active Member
May 21, 2015
2,649
Try turning your headlights on, that should increase the voltage enough to get a little charge. Victron, Renogy and others offer DC/DC chargers that function the same as the Redarc but watts=watts and any increase in voltage is offset by a decrease in current. Also, make sure the voltage is high enough to activate the charger.
 

stierheim

Member
Apr 22, 2013
63
I intentionally ran the 12v power pin on the TV directly to the battery - if I don't want it active I can unplug the trailer. However as I confirmed recently, that's no guarantee that the TV's alternator is going to do you any good. While older vehicles had alternators that ran "dumb" and put out a voltage high enough to guarantee a charge on the battery, modern vehicles are much smarter and only supply a voltage appropriate to the current state of the battery. This means that even with the trailer's electrical connected to the TV, there's absolutely no guarantee that power is going to flow from the TV to the trailer and not the other way around, even while driving.

RedArc has a $400-500 device that acts as a boost converter between the tow cable and the trailer's battery, and ensures that no matter what voltage is being provided by the TV, the 12v bus in the trailer will at least provide power to e.g. the fridge without draining the battery, and probably also do some slow charging. The problem is that a) it's waaay bigger than a PUP needs, and b) it's expensive.

I've done a bunch of searching around for a power supply that will take in a decent automotive range and output something like 13.6 or 14V at a few hundred watts, and so far the only thing I've found is a $200 evaluation board for a controller chip (LM5177 iirc) from TI that could easily be altered to output a float charge voltage. No "packaged" power supplies seem to exist...

If anybody has found an alternate solution to this problem, please let me know. If I *HAD* to I could go ahead and design my own power supply based on that TI chip (the part number sounds familiar, I may have used that exact chip before), but that means learning a new PCB design program (I don't have an Eagle license anymore, so probably KiCAD) and digging out all my electronics tooling again. The eval board would be an easy tweak to make, but it's still $200 for something that should be half that or less in practice.
A couple of answers (at least IMHO):

1. Regarding a power supply: There are several DC-DC boost charging converters available which solve the low voltage problem at the trailer and which are ruggedized for RV/Marine use. You need about 120W output capability (10 Amps) to charge the RV battery and, if you also want to run a 3-way fridge which draws about 9 Amps or so, then you need around 240W capability.

For example, the Victron Orion-Tr 12/12-18 is a 12V-12V 220W boost charger converter designed for 3-stage battery charging and it currently runs around $170 on Amazon. To be effective, the DC-DC converter must be installed on the RV, not in the engine compartment of the TV. If installed in the engine compartment, then wire/connector losses will cause a considerable voltage drop at the RV end.

Some TVs have a small gauge wire, e.g., 14GA or 12GA, feeding the charge line to the 7-Pin and the resulting voltage drop may cause the DC-DC converter to shutdown. Depending on the model, most 12V DC-DC converters have a shutdown input voltage rating of 8 - 10 VDC or so.

Line/connector losses can considerably increase the current a boost converter draws from the TV. For example: You are powering a 3-way fridge at 9 amps and the RV battery is charging at 10 amps with a boost voltage of 13.6 V. Due to losses between the TV alternator and the boost converter on the RV, the TV input voltage at the RV has dropped to 10V. Under these conditions the current draw in the 12V charge line from the TV will be somewhat over 25.84 amps (13.6V * 19A = 258.4W and 258.4W / 10V = 25.84A). The efficiency of the boost converter will increase this input current draw by some amount. You have to consider the impact this will have on the TV as you may have a 20A fuse on the 12V charge line from the TV. This heavy current draw may also cause overheating of the wiring or at any poor connection point in the line. Most TVs I have used (with a factory tow package) will support 30 amps on the 12V charge line.

2. Regarding the 12V charging line from the TV: The days of running alternators at 14.6V and being able to charge the RV battery are gone. 14.6V at the alternator would typically get you a usable voltage at the RV battery when driving and could keep it charged.

Modern vehicles use a Battery Management System (BMS) which monitors power (watts) in/out of the TV battery and uses sophisticated algorithms to determine battery charging voltages. They typically charge at 13.6V to 14.1V or so and reduce voltage once the battery is charged. This improves the starting battery’s life. However, 13.6V at the alternator will barely charge an RV battery as the line losses will rapidly drop the voltage at the RV to below 13V as soon as the RV draws current. And if you are running a 3-way fridge on battery, then in most cases the voltage drop from the TV will then be below the RV battery voltage and the RV battery will begin to discharge to maintain the voltage at 12.6V. However, once the BMS considers the TV battery to be fully charged, it drops charging voltage to a level which is not usable at the RV.

A significant part of the BMS is designed to improve TV MPG. To do this, the BMS drops the alternator voltage to around 13V or below once it has determined the TV battery is charged (remember it is monitoring watts in and out of the TV battery). I have monitored how my Ford Explorer and Subaru Forester does this on long trips using a recording voltmeter and ammeter:

Ford Explorer (2016): Initially charges the TV battery at 14.6V. This tapers down to 12.7V over time as the battery becomes fully charged. At an alternator voltage of 12.7V, it is impossible to charge an RV battery using the 12V charge line as any current draw will drop the voltage to 12.6V and the RV battery will begin to discharge. After several hours at 12.7V, the BMS starts to float the TV battery at between 13.1V to 13.3V and stays at that voltage. This voltage is still too low to effectively charge the RV battery because of line losses.

Subaru Forester (2019): The Subaru BMS is more sophisticated. Initially, the BMS fully charges the battery at 14.1V or more. Once the battery is charged, the BMS monitors engine load. If the vehicle is accelerating or going uphill, the BMS shuts off the alternator and the battery begins discharging at 20 amps (20 amps is the vehicle static electrical load with AC on). When the vehicle is then cruising (light throttle) or coasting, the BMS commands the alternator to a voltage which drops the battery current to about 0 amps, neither charging or discharging the battery (voltage is about 12.7V). Note the BMS makes no attempt to return the battery to full charge at this point, rather it prefers to keep the battery somewhat undercharged - the reason for this becomes apparent when the vehicle starts braking. When the vehicle begins to brake, the BMS starts super charging the battery at 15.1V and, depending on battery charge state, charge current runs up to 60 amps. At times when cruising the BMS allows the battery to discharge for a while to ensure the battery can absorb braking effort. The effect of all this BMS action is to minimize energy used by the alternator when cruising, shut the alternator down when engine load is high so all power goes to the wheels, and finally, when braking, maximize energy recovery by using the alternator to put as much braking energy as possible into the battery (for later use), vice wasting the energy via heat through the brakes. Most of the time, the alternator is just sitting at 12.7V and this will not charge an RV battery at all.

The bottom line to all of this is: In most cases, a modern vehicle with smart charging will not be able to charge an RV battery on the road. A DC-DC charge converter on the RV can be used to solve this. I have also seen some devices used to trick a smart charging system into charging at 14V or more all the time (but this kills the MPG improvements smart charging provides and shortens TV battery life). I would not recommend tricking a smart charging system as it defeats the benefits that smart charging provides. A DC-DC boost charger on the RV is a better solution.
 
Last edited:

TSQ

Active Member
Mar 28, 2021
377
Niagara Region, ON
If anybody has found an alternate solution to this problem, please let me know.
The Victron Energy Orion-Tr Smart 12/12-18 is what you want (as @stierheim mentioned). I have one and it works great - make sure you get the "smart" version (with bluetooth) as this lets you customize everything for your system. Once it is set up correctly you don't even need to worry about unplugging your trailer - it will just turn off once the input (tow vehicle battery) voltage drops below whatever setpoint you pick. It also automatically reduces your charge current to keep your input voltage above the cut-off setpoint if your input voltage drops.

Screenshot_20220928-215325.png Screenshot_20220928-215018.png

Screenshot_20220928-214839.png Screenshot_20220928-214913.png

Screenshot_20220928-215000.png
 

ST Dog

Active Member
Feb 18, 2017
112
Rocket City, USA
Most of the time, the alternator is just sitting at 12.7V and this will not charge an RV battery at all.
That sucks for things like headlights that need 14v for full output.

That is why I've rewired the lights on many vehicles to use relays and provide direct battery/alternator voltage to the lights. Too much lost in the wiring otherwise.
 

stierheim

Member
Apr 22, 2013
63
That sucks for things like headlights that need 14v for full output.

That is why I've rewired the lights on many vehicles to use relays and provide direct battery/alternator voltage to the lights. Too much lost in the wiring otherwise.
You are correct for incandescent type headlamps. However, most vehicles today that use smart charging also use LED headlamps which do not change brightness with input voltage variations.
 

judenray

Member
Apr 8, 2016
16
The Dalles, Oregon
A couple of answers (at least IMHO):

1. Regarding a power supply: There are several DC-DC boost charging converters available which solve the low voltage problem at the trailer and which are ruggedized for RV/Marine use. You need about 120W output capability (10 Amps) to charge the RV battery and, if you also want to run a 3-way fridge which draws about 9 Amps or so, then you need around 240W capability.

For example, the Victron Orion-Tr 12/12-18 is a 12V-12V 220W boost charger converter designed for 3-stage battery charging and it currently runs around $170 on Amazon. To be effective, the DC-DC converter must be installed on the RV, not in the engine compartment of the TV. If installed in the engine compartment, then wire/connector losses will cause a considerable voltage drop at the RV end.

Some TVs have a small gauge wire, e.g., 14GA or 12GA, feeding the charge line to the 7-Pin and the resulting voltage drop may cause the DC-DC converter to shutdown. Depending on the model, most 12V DC-DC converters have a shutdown input voltage rating of 8 - 10 VDC or so.

Line/connector losses can considerably increase the current a boost converter draws from the TV. For example: You are powering a 3-way fridge at 9 amps and the RV battery is charging at 10 amps with a boost voltage of 13.6 V. Due to losses between the TV alternator and the boost converter on the RV, the TV input voltage at the RV has dropped to 10V. Under these conditions the current draw in the 12V charge line from the TV will be somewhat over 25.84 amps (13.6V * 19A = 258.4W and 258.4W / 10V = 25.84A). The efficiency of the boost converter will increase this input current draw by some amount. You have to consider the impact this will have on the TV as you may have a 20A fuse on the 12V charge line from the TV. This heavy current draw may also cause overheating of the wiring or at any poor connection point in the line. Most TVs I have used (with a factory tow package) will support 30 amps on the 12V charge line.

2. Regarding the 12V charging line from the TV: The days of running alternators at 14.6V and being able to charge the RV battery are gone. 14.6V at the alternator would typically get you a usable voltage at the RV battery when driving and could keep it charged.

Modern vehicles use a Battery Management System (BMS) which monitors power (watts) in/out of the TV battery and uses sophisticated algorithms to determine battery charging voltages. They typically charge at 13.6V to 14.1V or so and reduce voltage once the battery is charged. This improves the starting battery’s life. However, 13.6V at the alternator will barely charge an RV battery as the line losses will rapidly drop the voltage at the RV to below 13V as soon as the RV draws current. And if you are running a 3-way fridge on battery, then in most cases the voltage drop from the TV will then be below the RV battery voltage and the RV battery will begin to discharge to maintain the voltage at 12.6V. However, once the BMS considers the TV battery to be fully charged, it drops charging voltage to a level which is not usable at the RV.

A significant part of the BMS is designed to improve TV MPG. To do this, the BMS drops the alternator voltage to around 13V or below once it has determined the TV battery is charged (remember it is monitoring watts in and out of the TV battery). I have monitored how my Ford Explorer and Subaru Forester does this on long trips using a recording voltmeter and ammeter:

Ford Explorer (2016): Initially charges the TV battery at 14.6V. This tapers down to 12.7V over time as the battery becomes fully charged. At an alternator voltage of 12.7V, it is impossible to charge an RV battery using the 12V charge line as any current draw will drop the voltage to 12.6V and the RV battery will begin to discharge. After several hours at 12.7V, the BMS starts to float the TV battery at between 13.1V to 13.3V and stays at that voltage. This voltage is still too low to effectively charge the RV battery because of line losses.

Subaru Forester (2019): The Subaru BMS is more sophisticated. Initially, the BMS fully charges the battery at 14.1V or more. Once the battery is charged, the BMS monitors engine load. If the vehicle is accelerating or going uphill, the BMS shuts off the alternator and the battery begins discharging at 20 amps (20 amps is the vehicle static electrical load with AC on). When the vehicle is then cruising (light throttle) or coasting, the BMS commands the alternator to a voltage which drops the battery current to about 0 amps, neither charging or discharging the battery (voltage is about 12.7V). Note the BMS makes no attempt to return the battery to full charge at this point, rather it prefers to keep the battery somewhat undercharged - the reason for this becomes apparent when the vehicle starts braking. When the vehicle begins to brake, the BMS starts super charging the battery at 15.1V and, depending on battery charge state, charge current runs up to 60 amps. At times when cruising the BMS allows the battery to discharge for a while to ensure the battery can absorb braking effort. The effect of all this BMS action is to minimize energy used by the alternator when cruising, shut the alternator down when engine load is high so all power goes to the wheels, and finally, when braking, maximize energy recovery by using the alternator to put as much braking energy as possible into the battery (for later use), vice wasting the energy via heat through the brakes. Most of the time, the alternator is just sitting at 12.7V and this will not charge an RV battery at all.

The bottom line to all of this is: In most cases, a modern vehicle with smart charging will not be able to charge an RV battery on the road. A DC-DC charge converter on the RV can be used to solve this. I have also seen some devices used to trick a smart charging system into charging at 14V or more all the time (but this kills the MPG improvements smart charging provides and shortens TV battery life). I would not recommend tricking a smart charging system as it defeats the benefits that smart charging provides. A DC-DC boost charger on the RV is a better solution.
Thanks for the insight, was not aware of the impact of a "smart" alternator, and will consider a DC/DC charge converter since I mostly boondock, so charging to/from campsites is important. The Sorento (now that I've fixed the wiring) does appear to put some charge into the camper battery when everyting is hooked up and engine is running. With a partially discharged and connected camper battery and TV engine idling my multimeter shows 13.8v at the battery terminals. After a week of boondocking in the moist and cool Olympic Peninsula (heater and lights briefly used every day) battery was down to 12.3v. Drove 300 miles home, unplugged camper, next morning battery at posts was 12.5v (which seems to be the norm for this particular battery 24 hours after disconnecting from a charger after a 12.6v full charge). So, at least for my 2017 Sorento, the TV does charge the camper battery while on the road, though admittedly more slowly compared to hooking up to a 120v smart charger. Driving 1 hour home from my closest boondock campsite only raises the battery voltage up a little, .2v or so.
 

stierheim

Member
Apr 22, 2013
63
Thanks for the insight, was not aware of the impact of a "smart" alternator, and will consider a DC/DC charge converter since I mostly boondock, so charging to/from campsites is important. The Sorento (now that I've fixed the wiring) does appear to put some charge into the camper battery when everyting is hooked up and engine is running. With a partially discharged and connected camper battery and TV engine idling my multimeter shows 13.8v at the battery terminals. After a week of boondocking in the moist and cool Olympic Peninsula (heater and lights briefly used every day) battery was down to 12.3v. Drove 300 miles home, unplugged camper, next morning battery at posts was 12.5v (which seems to be the norm for this particular battery 24 hours after disconnecting from a charger after a 12.6v full charge). So, at least for my 2017 Sorento, the TV does charge the camper battery while on the road, though admittedly more slowly compared to hooking up to a 120v smart charger. Driving 1 hour home from my closest boondock campsite only raises the battery voltage up a little, .2v or so.
If you are able to get 13.8v at the battery (assuming you're measuring at the PUP battery and not the TV battery) with a partially discharged battery on the PUP, then you should get a decent charge. A DC-DC converter on the PUP will do better.

Note that voltage at the PUP battery will vary significantly depending on the PUP battery state of charge and if you have any high current device running on the PUP (like a 3-way fridge on 12v). Voltage at the PUP will vary with the current being supplied due to the long wire length from the alternator back to the PUP. If the PUP battery if fully charged and the PUP is not pulling any other significant load, then you should get very close to full alternator voltage at the PUP battery.
 

judenray

Member
Apr 8, 2016
16
The Dalles, Oregon
If you are able to get 13.8v at the battery (assuming you're measuring at the PUP battery and not the TV battery) with a partially discharged battery on the PUP, then you should get a decent charge. A DC-DC converter on the PUP will do better.

Note that voltage at the PUP battery will vary significantly depending on the PUP battery state of charge and if you have any high current device running on the PUP (like a 3-way fridge on 12v). Voltage at the PUP will vary with the current being supplied due to the long wire length from the alternator back to the PUP. If the PUP battery if fully charged and the PUP is not pulling any other significant load, then you should get very close to full alternator voltage at the PUP battery.
The 13.8v was at the camper battery terminals with the TV connected and the engine idling. Prior to hooking it up, the camper battery was at about 12.45, which is about where it usually is when it has sat for a month, so it was not heavily drained, but would certainly accept a decent charge. I don't have a fridge or other parasitic draws in the camper except the propane alarm and of course the camper lights and brakes when activated.
 




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