Vehicle battery power to trailer?

[KeithB]

So we were headed home from a week south. Wonderful time. We decided to split up an 8 hour drive and parked off road for a night. We don’t boondock so have never spent the night on battery power. With an evening of lights plus vent fan running all night, the battery was a bit low next morning. As we were preparing to depart I went ahead and started the truck. I noticed that the 12V monitor slowly but deliberately increased voltage…..as in showing full voltage within minutes. Begs the question, in a pinch can I run 12V power to the camper via the truck battery?

[DavidM]

Yes and no.

The truck's power circuit to the TTs batteries only works when the truck's ignition is on. It will charge at about 10 amps when the batteries are down, dropping to 5 amps or so as they get full.

So you cannot just run off of the truck's battery while camping. You wouldn't want to anyway as you could possibly be left without enough starting capacity in the battery.

Also FWIW as you noticed the voltage slowly rise when you started your truck the battery state of charge wasn't going up much. Battery voltage is a very poor indicator of battery state of charge.

David

[creativepart]

As David said, you can't trust the voltage reading.

Your truck's alternator will put 14+ volts into your TT's battery to charge it. So, that voltage is what you see when you check your battery's voltage when plugged into a charging source (shore power, a generator, solar charger or tow vehicle's alternator).

A fully charged 12v battery is 12.7v. When you have used 50% of your battery's capacity it's at 12.1v. A dead battery is about 11v.

So, let's assume your battery was at 11.7v after your night "parked off the road." You start your truck and after a few minutes you see your battery is 13v to 14v. Wow, you say, "It's already recharged!" Nope. you're just seeing the charging voltage being applied. It will literally take hours and hours of charging to get back to 12.7v.

So, you turn off your truck and you see your voltage at 12.8v - is it all charged up? Nope. You're seeing a surface charge left over from the charge being applied by the outside charger. The minute you turn on some lights the voltage will again drop down closer to 11v.

This is why it's so difficult to use voltage to determine the state of charge for your battery. The only time a voltage meter is accurate is when the battery has been totally at rest for a number of hours. No usage, no charging for as much as 12 to 24 hours.

So, learn to not trust your voltage meter and investigate other methods of checking the condition of your battery. One of the best is to install a battery monitor with a shunt. You connect one side of the shunt to your negative battery terminal and the other side to all the negative cables that would normally be attached to your battery.

The shunt-based monitor calculates all the power... in AND out... of your battery and gives you an accurate state of charge that doesn't fool you into thinking you have more power than you really do.

Hope this helps you understand your battery. The more you understand all of this more you can care for your battery and make it last longer.

As long as you're using voltage to read your battery now, you need this chart to give you an idea of what condition your battery is in.

[Wakullabob]

On some trucks the trailer charge circuit is hot whether the ignition is on or not and could run your start battery down . On my Ram I must unplug from the truck or the trailer will run my truck battery down . Not all trucks are the same !

[KeithB]

All good info! I do have a basic understanding of batteries and 12v. But I guess my main curiosity was in the event that my battery was running dangerously low, on a temporary basis (couple hours at most let’s say) could I get by running the engine on the truck , which would keep vehicle battery charged. Chances of the scenario are very slim, but at least hypothetically, was wondering others thoughts.

[creativepart]

Keith, if that's your only option then certainly. That will work. You might find it more economical to buy a small, quiet gas or propane generator to be your back up battery charger.

If you don't want to run A/C then 1000w is generally enough. If you want to run the A/C you'll need 2,200 watts to 2,500 watts. There are lots of choices and with one of these larger gensets you can run your microwave, too and not need to worry about shore power.

[Marine359]

Depending on your tv, you can expect its alternator to supply 4-6amps through the 7 pin connector. As long as you don’t have a 12v compressor fridge, that should be enough to provide almost a night’s worth of dry camping, like lot docking, after a 6 hours of driving. For a very inexpensive solution for more power, you can purchase 2x 6v gc batteries and mount them on the tongue in parallel to replace your OEM marine deep cycle battery. 2x gc would give you enough amp hrs for 2 or more days of dry camping.

[RocknRoll]

Originally Posted by Wakullabob

On some trucks the trailer charge circuit is hot whether the ignition is on or not and could run your start battery down . On my Ram I must unplug from the truck or the trailer will run my truck battery down . Not all trucks are the same !

I did not realize this. Does it override the trailer battery?

Originally Posted by Marine359

For a very inexpensive solution for more power, you can purchase 2x 6v gc batteries and mount them on the tongue in parallel to replace your OEM marine deep cycle battery. 2x gc would give you enough amp hrs for 2 or more days of dry camping.

Jim, since you're referring to a 6V configuration, I believe you probably meant series instead of parallel. (2) 12V would be wired in parallel.
https://battlebornbatteries.com/batt...ded%20together.
https://batteryuniversity.com/

My TT had a (2) GC setup when I bought it. Although you can increase power, there were distinct disadvantages to this configuration.

1. Weight - Expect to add ~80 pounds to the tongue weight

2. Discharge / Sizing - Whether you have a single 12V or (2) 6V, lead acid batteries should never be discharged below 50%. Two 6V batteries rated at 230 AH and wired in series will supply 230 total AH. This is effectively reduced to 115 AH if following the discharge rule. They are also slow to charge and require a ideal charging current of 10% of the 20 hour rated capacity, which is approximately 23A. The 4-6 AMP supply from a TV alternator is woefully short for ANY battery.

3. Maintenance - Lead acid batteries require maintenance.

There are many more advantages if you do some research. When taking into account purchasing a new battery box and cables to wire the configuration, upgrading to Lithium Iron Phosphate (LiFeP04) will not cost much more. They are now about 1/2 the price from where they were just a few years ago. This battery is not a recommendation, but an example of how inexpensive they are now, even with the current inflationary environment.

https://www.amazon.com/12V-100Ah-Pho...NsaWNrPXRydWU=

Your existing charger/controller will work, but is recommended to be replaced for a LiFeP04 profile. Cost is ~$200

[BobC]

With an evening of lights plus vent fan running all night, the battery was a bit low next morning.

A couple of questions:

1. What kind of refrigerator do you have?
2. What do you mean by the battery being a bit low?

And a couple of comments:

1. Although the prevalent rule of thumb for battery discharge is 50% depth of discharge. Going below 50% will not damage your battery unless you exceed 80% depth of discharge (20% state of charge). Discharging more than 50% but less than 80% will have some effect on your battery's life span in terms of discharge cycles but this isn't going to be significant if you only do so occasionally. For most of us, our batteries may fail for other reasons before we even get close to the discharge cycle life. This is supported by the literature of many battery manufacturers. Here's Trojan Battery's take on this topic:

1. Shallow discharges will result in a longer battery life.
2. 50% (or less) discharges are recommended.
3. 80% discharge is the maximum safe discharge.
4. Do not fully discharge flooded batteries (80% or more). This will damage (or kill) the battery.
5. Many experts recommend operating batteries only between the 50% to 85% of full charge range. A periodic equalization charge is a must when using this practice.
Do not leave batteries deeply discharged for any length of time.
Lead acid batteries do not develop a memory and do not need to be fully discharged before recharging.
6. Batteries should be charged after each period of use.

When camping without power, the best thing you can do is to "think camping, not RVing". This means turning off lights when leaving a room, opening windows instead of running a fan, etc. Running an absorption refrigerator (LPG/120V) is a minimal load but any kind of compressor refrigerator is going to be much higher, be it 12V or running off an inverter. In fact, try not to use anything that requires an inverter. Running your furnace will be a power hog as well. If you need a CPAP machine, it will use a huge amount of power overnight and it, alone can deplete a single battery.

[Marine359]

Oops, sorry. Yes I did mean 2x 6v in series.

[RocknRoll]

Originally Posted by BobC

When camping without power, the best thing you can do is to "think camping, not RVing". This means turning off lights when leaving a room, opening windows instead of running a fan, etc. Running an absorption refrigerator (LPG/120V) is a minimal load but any kind of compressor refrigerator is going to be much higher, be it 12V or running off an inverter. In fact, try not to use anything that requires an inverter.

Excellent preventative advice Bob.

I should have clarified that discharging a SLA below 50% may require a full and complete "bulk/absorption" charge (charging current of 10% of the 20 hour rated capacity) that can only be accomplished with shore power or a battery charger. This basically follows Trojan rule #6.

On another note, I'm not sure if this has been resolved by WBGO, but the WFCO OEM charger/controller has a terrible charging profile (non-multi-stage) that may damage your battery(s) prematurely by overcharging.

According to the comments, the PD converter (made in USA), besides the optimal charging profile, may also work with lithium via a jumper.
https://www.amazon.com/Progressive-D...ct_top?ie=UTF8.

Of course, double check your model # for compatibility.

[DavidM]

"On another note, I'm not sure if this has been resolved by WBGO, but the WFCO OEM charger/controller has a terrible charging profile (non-multi-stage) that may damage your battery(s) prematurely by overcharging."

Yes the standard WFCO charger is a total POS. Not only can it overcharge, it significantly undercharges, causing very slow charging, particularly frustrating while on generator power. I replaced mine with a Progressive Dynamics charger with Charge Wizard and it charges much faster. But if you have lithium batteries the Li specific PD charger is even better.

David