Another round of the 'Using tow vehicle to charge trailer battery' question

Can I recharge (not just maintain) my trailer battery off the tow vehicle's existing system or will I need to install something like a DC input battery charger?

Hello RV forum people,

I have been reading all the posts that I can concerning using the tow vehicle to charge the trailer battery and the more responses I read the more confused I get (story of my life?).  The advice ranges from using a simple cigarette lighter adapter to a needing a second alternator just for charging the trailer batteries.  My question goes for the two different situations I face: 

1.)  Coming from the backpacking world, I have never camped with electricity and now that I'm designing a teardrop camper I'm going to install a 12V system so I can cook dinner without using my flashlight and pump water from a storage tank to do the dishes.  I assume these are fairly low draws on the trailer battery.  I usually just overnight in a spot and then move on the next day but most of these at locations don't have 'shore power'.  Here in Alaska there are many pulloffs along the road where you're welcome to stay (no water, no power, just a beautiful spot).

2.)  I have a 12V ATV sprayer from Northern Tools that draws 17 amps.  The Duracell Ultra Marine/RV Deep cycle battery that powers it is rated at 150 mins at 23 Amps.  I do an hour-long spray job at one  location and then load the machine onto a trailer and haul it 30 minutes to the next site and then do another hour-long job etc. as long as I can.  I start the day with a full charge (using my shop battery charger) and then run till I'm low on 'juice'.  I'd really like to bump up my charge while the unit is being transported from site to site to allow me to get more jobs done in a day.

I have a 2011 F-150 with a tow package.  It currently has a flat four trailer connector for the trailer lights so either way I see some tow vehicle rewiring in my future.  None of the trailers I use need trailer brakes so I'll probably keep the flat four connector and run the future trailer charging system through a separate 8 gauge wiring harness.

So, do you think I can operate off the truck's existing system or will I need to install something like a DC input battery charger to recharge (not just maintain) my trailer (or sprayer) battery?

Thanks in advance,  Jeff

The 12v battery in the ATV could be charged from the truck alternator (a DC charger). The standard 7-pin trailer plug includes a wire for that, if you have such a plug in your tow package. If not, adding one isn't difficult. RV travel trailers are routinely charged this way as they go down the road.

However, I'm not sure how much boost you will get in a short drive between gigs. 30 minutes isn't much time to push amps into a battery. They don't like to accept them rapidly, so time is more important than the size of the charger..

Gary is right.  This is easy to do - charge the ATV battery, but understand this will not full recharge the battery.  Over the course of a day, it may give you an extra hour or so.

With the teardrop, it should keep that battery fully charged because it will be used less and charge longer.

Do you use an ATV to pull the sprayer?  Can you use the ATV alternator to help charge the sprayer battery? Or is this the same battery and I am showing my ignorance?

No, I don't actually use an ATV for the ATV sprayer.  This is very tight quarter spraying so I use a 2-wheel tractor (BCS) on which I have had a special attachment fabricated to hold the sprayer.  The tiny 12V battery on the BCS is drawn down in less than 30 minutes of spraying so I had to add the deep cycle Duracell to run the pump.  The output of the Honda engines alternator is insignificant compared to the 17 amp draw of the sprayer.  Its enough to keep the starter battery charged  but not enough to run much of an additional electrical load.  The sprayer battery is not tied into the BCS's electrical system rather it simply sits next to the sprayer.  At the end of the day I just release the tie down strap and bring the battery in the shop and put it on the battery charger.  Next day I carry it out and strap it back in.

From reading the posts of similar threads I was under the impression that the voltage coming through the 7-pin connector was enough to maintain a charge in the trailer battery (or sprayer battery in my case) but that it really wasn't enough to recharge a partially depleted battery.  Building up a depleted trailer battery from the tow vehicle system was the realm of the 12V DC input chargers.  But, this is all new technology to me and that's why I'm asking the question to all of you with more experience.

Thanks for your help so far.  Jeff

It's not the voltage - its the amps that can be delivered. The truck engine alternator should have plenty of amps available, so it is only necessary to provide a sufficient wire size, and associated circuit breaker, to deliver them to the back. Heck, electric trailer brakes can draw 20-30 amps, so charging a battery is not a large load in comparison. For example, a 90-100 AH battery probably won't accept more than 12-15 amps even when near flat dead.  That's also why 30 minutes won't provide a lot of extra boost.

You might be better off carrying a spare battery and swapping them part way through the day. Or leave the truck running to charge one battery while you use the other for spraying, then swap batteries for the next round. Quick-connects on the batteries could make that simple.

Without using special "voltage boost' equipment, the tow vehicle will NOT charge the RV batteries - although the Tow Vehicle will supply power (to run the Fridge on the road) when the Trailer Batteries fall below about 50% charge. The issue IS Voltage.

The alternator "sense" circuit in your Your Truck/SUV ("TV") adjusts the alternator output Voltage (and therefore, current) according to the State-of-Charge ("SOC") which it sees underneath the TV hood while generating low power. (All modern alternators are adjustable, with output power controlled by such "sensing" leading to movement of the magnets around the coil). When the TV battery seems to be charged, Voltage within the TV will drop to a small value - typically less than 13.5V.

That's the starting point for "Voltage Drop" along your TV "trailer battery charge" wire, Bargman connector, and Trailer wiring to reach the battery. At low amperage, it stays pretty close to that value - but 13.5 Volts isn't going to put significant charge into the Trailer batteries. You need 14.2V, or even 14.6V, to charge rapidly.
- - - - - -
You have three alternatives to make this happen.[list type=decimal]
[*] You may bring the Trailer batteries up to the TV front end, and (using heavy "booster cables") connect terminal-to-terminal with the TV battery and frame. Having done this, the alternator sense will "see" low voltage in the Trailer batteries, and raise Voltage to charge them. This is obviously awkward, and doesn't happen while you drive.
[*] You may buy a purpose built "Trailer Battery Charger/Voltage Booster from Ctek, or Redarc. They become part of the Trailer, and raise the Bargman-supplied Voltage (around 12V under load) to the amount which is appropriate for the Trailer batteries at that moment. (These are DC-to-DC "intelligent chargers.)
[*] You could boost the Voltage in the TV "battery charge wire" at the TV (to something like 24V) and connect your Trailer's Bargman "12V battery charge" wire into a Solar Controller - basically the Ctek/Redarc solution, but creating Voltage Boost in the TV (to reduce amperage on the TV and Bargman "12V battery charge" wire, when it's under load.) I'm doing this one. I have a Solar Controller already (with panels). In the TV, I have a switch to choose 12V (normal) or 24V Boost on the Bargman. In the Trailer, a special Relay (24V DC coil) detects the presence of 24V, and causes two other Relays to exchange Solar Controller input from the Panels to the Bargman wire. At night, or under clouds, my Solar Controller "sees" a 24V panel with moderately high current capabilities, and uses it to charge Trailer Batteries and run the Fridge.

It's all controlled by this switch:
[/list]

Some systems put the alternator into Charge mode when you run the Headlights. I don't know on a Ford, but GM recommends turning the Headlights on when pulling a Camper to keep the battery charged up. The best charge can be obtained by low amperage at about 14V. The problem with charging a battery with the Tow vehicle is an alternator cannot control amperage. They just pump amperage until the voltage comes up to the regulated voltage.

Also, if you are running a Deep cycle battery, it works best if you run it down to 30% then recharge slowly. If you keep it constantly charged it will fail early, use a car battery if you are going to charge it while you use it.

Also, look for a battery with a high Reserve capacity (RC) not high CCA or CA. For example a group 31 battery with 700cca gives you 180 minutes of RC. that is 180 minutes at 25 amps before the voltage falls below 10.5V

Also, if you are running a Deep cycle battery, it works best if you run it down to 30% then recharge slowly. If you keep it constantly charged it will fail early, use a car battery if you are going to charge it while you use it.

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Battery experts recommend no more than 50% discharge for optimal battery life. Nor is it necessary to always discharge deeply, no matter what percentage you choose. Light discharge cycles, e.g. less than 20%, have no significant impact on battery life, and a quality 3-stage charger won't overcharge, so there is no penalty for keeping the battery fully charged. In fact, there is a negative impact if a battery sits around at less than full charge, so you should NOT avoid charging for very long because the battery hasn't been discharged very much.

How about this deal: Using the trailer to start the tow vehicle. Few years ago we pulled in to a site, unhooked and, unbeknown to me because I was in too much of a hurry, locked the truck with the keys in the ignition and turned on but engine shutdown. Next morning truck is dead as a doornail and the keyless entry wouldn't even work and no spare key. Jumper cables and charger in the truck with no way to get to them. So I go looking in the campground for help. Neighbor says no problem, comes over and plugs my trailer electrical connection into the truck, has me use the keyless entry to get in and says start her up. Bingo!  ;D

skborders said:

Some systems put the alternator into Charge mode when you run the Headlights. I don't know on a Ford, but GM recommends turning the Headlights on when pulling a Camper to keep the battery charged up. The best charge can be obtained by low amperage at about 14V. The problem with charging a battery with the Tow vehicle is an alternator cannot control amperage.

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Here's exactly how it works:
  All modern alternators are adjustable. Using a Controller (which is typically mounted right on the Alternator itself), and a small "sense" wire into that controller, they adjust the position of the fixed magnets (surrounding the spinning portion) to create more or less output current (at a higher voltage) than the "sense" wire finds the "+12v" battery and electrical system to have when output momentarily decreased. (The key is Voltage - whatever the truck's battery and electrical system are willing to consume at that Voltage is the output current of the alternator.
  But the "Trailer Battery Charge" wires within the Truck, through the Trailer cable, and within the Trailer to actually reach the batteries are relatively small, and unable to greatly effect the Voltage which the "sense wire" sees.
  If you turn on the headlights (and maybe the stereo system too), then Voltage of the truck's "Sense" wire will test low - and the Voltage/current of the alternator will be adjusted upwards. Far away, in the Trailer, this will create some charging potential for the Trailer batteries. But even if you're in a situation where they would LIKE to receive 14.4 Volts, wire losses and Alternator controller "programming" will not generate such a high voltage (while the truck is driving down the road), unless the Truck battery has also reached significantly discharged state.

skborders said:

Also, if you are running a Deep cycle battery, it works best if you run it down to 30% then recharge slowly. If you keep it constantly charged it will fail early, use a car battery if you are going to charge it while you use it.

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No - that's wrong. Most "Deep Cycle" Batteries are SLA, and they will suffer shortened lifespan if frequently discharging that far. And in fact, those batteries want to be stored at a high SOC, maintained by float mode "pulse" charging.(Gary is 100% right about these things, I just add some details.) After deep discharge, they would actually benefit from a SHORT "anti-sulfation" charge at high voltage. (That should also be done to well-treated batteries too, but very infrequently.) Maintaining batteries is complicated. I suggest you do an Internet search and read some of the lengthy material which has been created about "deep cycle battery maintenance", you'll learn some new stuff.

Would it be possible to have a solar charger and another battery?  You could have one battery on the charger and run tho other in the sprayer.  Once one gets low, just swap them out.

rickst29 said:

Here's exactly how it works:
  All modern alternators are adjustable. Using a Controller (which is typically mounted right on the Alternator itself), and a small "sense" wire into that controller, they adjust the position of the fixed magnets (surrounding the spinning portion) to create more or less output current (at a higher voltage) than the "sense" wire finds the "+12v" battery and electrical system to have when output momentarily decreased. (The key is Voltage - whatever the truck's battery and electrical system are willing to consume at that Voltage is the output current of the alternator.
  But the "Trailer Battery Charge" wires within the Truck, through the Trailer cable, and within the Trailer to actually reach the batteries are relatively small, and unable to greatly effect the Voltage which the "sense wire" sees.
  If you turn on the headlights (and maybe the stereo system too), then Voltage of the truck's "Sense" wire will test low - and the Voltage/current of the alternator will be adjusted upwards. Far away, in the Trailer, this will create some charging potential for the Trailer batteries. But even if you're in a situation where they would LIKE to receive 14.4 Volts, wire losses and Alternator controller "programming" will not generate such a high voltage (while the truck is driving down the road), unless the Truck battery has also reached significantly discharged state.

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I will not debate the battery stuff I do know that batteries need to be stored charged to prevent sulfation and while both car batteries and Deep cycle batteries are SLA the deep cycle can tolerate deep discharge whereas a car battery (cranking battery) can not.

However I will attempt to explain briefly how the charging systems are more complex on the vehicles of today. These systems are found on GM trucks from 2005 and newer and Ford, depending on model and engine as early as 2000. In fact most passenger cars and light trucks use some form of computer controlled or commanded charging systems

Your description of an alternator is a little strange but fairly accurate. except, the regulators are now commanded by the computer on the vehicle. They all work differently but have some commonality in that the computer calculates the state of charge of the batteries. It also monitors the systems on the vehicle as to what is turned on and turned off as well as the driving conditions of the vehicle such as speed, calculated engine loads etc. If the electrical loads are low, the computer will command the voltage low ( can be as low as 11.5Volts) at the voltage regulator or module as you call it, allowing the alternator and battery to supply current for the vehicle. If and when the battery state of charge falls below 80 percent state of charge the PCM will command the voltage back up to between 13.5 and 15V to recharge the battery then drop back down when the battery recovers to around 90 %.

There are certain conditions that cause the PCM to command the voltage up independent of the state of charge of the battery. Headlight, wipers, cooling fans etc. Thus the statement I made about turning the headlight on to keep the batteries charged in the trailer. When you turn the headlights on, the PCM will command the voltage to around 14.5 to maintain the battery. This way if the wire to the trailer is big enough and in good condition, you may be able to maintain enough voltage to keep the trailer battery charged (13.5) A battery needs 1 volt above its open circuit voltage to efficiently reverse the chemical reaction of discharging.

If you search GM RVC charging there are multiple articles online that explain it.

skborders said:

I will not debate the battery stuff I do know that batteries need to be stored charged to prevent sulfation and while both car batteries and Deep cycle batteries are SLA the deep cycle can tolerate deep discharge whereas a car battery (cranking battery) can not.

However I will attempt to explain briefly how the charging systems are more complex on the vehicles of today. These systems are found on GM trucks from 2005 and newer and Ford, depending on model and engine as early as 2000. In fact most passenger cars and light trucks use some form of computer controlled or commanded charging systems

Your description of an alternator is a little strange but fairly accurate. except, the regulators are now commanded by the computer on the vehicle. They all work differently but have some commonality in that the computer calculates the state of charge of the batteries. It also monitors the systems on the vehicle as to what is turned on and turned off as well as the driving conditions of the vehicle such as speed, calculated engine loads etc. If the electrical loads are low, the computer will command the voltage low ( can be as low as 11.5Volts) at the voltage regulator or module as you call it, allowing the alternator and battery to supply current for the vehicle. If and when the battery state of charge falls below 80 percent state of charge the PCM will command the voltage back up to between 13.5 and 15V to recharge the battery then drop back down when the battery recovers to around 90 %.

There are certain conditions that cause the PCM to command the voltage up independent of the state of charge of the battery. Headlight, wipers, cooling fans etc. Thus the statement I made about turning the headlight on to keep the batteries charged in the trailer. When you turn the headlights on, the PCM will command the voltage to around 14.5 to maintain the battery. This way if the wire to the trailer is big enough and in good condition, you may be able to maintain enough voltage to keep the trailer battery charged (13.5) A battery needs 1 volt above its open circuit voltage to efficiently reverse the chemical reaction of discharging.

If you search GM RVC charging there are multiple articles online that explain it.

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Good Stuff Here..Thanks

When I was pulling my small popup trailer behind my motorcycle,I powered the trailer with a Sears 1100 watt jump starter. Not only did that give me enough power for 2-3 days,it also had a built in light,an air compressor and a small inverter if I really needed 110v.
I had a couple LED lights,and it powered my CPAP (without the humidifier).

I wired a separate 2 prong plug from the bike accessory terminal to the trailer. I can't remember for sure,but I may have included a relay.
I can't explain exactly how it worked,but can tell you that the jump starter battery would recharge while towing

Why not spend $100 at Harbor Freight tools for a cheap generator?  Heck, spend another $60 and get a "smart charger".  When you get to the campsight, fire up the gennie, hook the charger up, and charge your battery.  It will bring the battery to about 80% in a couple hours, and top it off in another 2.  This is what we do when we boondock.

https://i.ytimg.com/vi/piE8YYyTFJE/hqdefault.jpg

I posted this on the other battery thread, so reposting here in more current thread:

Very helpful info - I have the same issue with my 2004 Volvo XC90 tow vehicle ("TV") and our Teardrop trailer ("TD")

I am wondering if the following approach would help, at least for those of us with portable refrigerators in our camper.  What about running the fridge from the 12V outlet in the tow vehicle while towing, instead of having the fridge in the camper drawing on the line from the TV and the camper battery.

I also have an AGM battery, and I always leave the fridge plugged in to the camper (while towing).  Maybe putting the fridge in the TV while towing would protect the battery capacity in the camper (while keeping the fridge cold).

Would this approach cause any problems in the TV? 

(I'm guessing we wouldn't want to leave the fridge plugged in to the TV if we stop for a long break, like lunch, however, as it could drain the car battery.)

My camper is small - a teardrop - so the only draw is the fridge, the water pump, running lights, interior lights, and a car stereo and speakers (and whatever we charge from the teardrop battery, such as iPod or iPhone).  Not much draw while towing, except the fridge.

I'm trying to come up with a solution, so I don't get to the campsite with a near-empty battery.  I have a solar panel, but that's not any use if we arrive at the campsite late afternoon.

Your thoughts are MUCH appreciated.

Welcome to the Forum!

Generally, you get many more responses by starting a new thread rather than adding to a 2 year old, dead post.

First, how well insulated is the fridge?    Many fridges can go 8 - 10 hours without power and stay cold as long as they are not opened.

Second, how much power does it draw? How often?  It may only run 10 min / hr, leaving the other 50 min to keep the battery topped off.

Third, can the car 12V outlet supply sufficient amps to run the fridge?  Wire gauge is also an issue.

The big problem with using the engine alternator especially on a Tow Vehicle to charge the battery to run the inverter to run the fridge is charge rate

Even on a motor home it can be a bit anemic  But you have a LOT of fairly small wire between the alternator and the fridge.  Plus a well tarnished plug and socket  The result is that the alternator may not feed enough current to the trailer to keep up with the needs.

That said.. The key words in that are "May Not"  Implys "May" as well.

But .. Try it and keep an eye on battery voltage whenever you take a break from driving.

NOTE you can run several hours with the Fridge closed and power OFF.