Question about wiring/charging trailer battery

[HenryJ]

Is the #10 charge wire sufficient to powere the fridge while driving without it slowly draining the trailer batteries?

This is a handy reference: Wire gauge tables Use the chart to determine the cable size for a given length and amperage.

I am currently putting together a system similar to yours. I have been out in the shop drilling , crimping and soldering today.
I ordered a nice big battery box that has a spacer for a group 78 AGM battery that I am using. That will leave room for power ports, switches and fuses. An extra power port will offer a place to attach a solar panel if one is in my future.
This battery will power the Edgestar 43qt refrigerator and a LED light strip in the RTT.
For now charging will be through a cable plugged into the trucks rear power quick connector. This same connector can plug into the winch, booster cables or the generator. It is supplied to the back of the truck with 2 gauge cable. From the connector to the trailer battery is 4 gauge. The larger cable would allow me to use the winch on the back of the trailer powered by the trailer battery. I don't ever see doing this, but the option is there. This offers lots of ways to connect in different combinations.

The parallel connection to the vehicle battery should offer a starting boost if needed. Both batteries, although not the same brand, are very similar. Both AGM group 78 compact flat plate design batteries. I chose the group 78 because that is the largest that will fit in the truck and I want to have the batteries interchangeable. My truck has already been upgraded with a 160 amp alternator. The additional loads are well within its operational capabilities. Hopefully they will play well together and charge up while traveling.

The manual quick connector is part of my KISS. Easy to connect and disconnect. The parts all interchange and can be swapped in a pinch. I don't think I would have problems if I forget to disconnect, unless is sits for a very long time. A good AGM battery can run the fridge for three days on its own.
I still need to figure out the weather proof cable access port for the trailer, as I plan to have the battery power box mounted externally. Hopefully I will be able to find something...even on black Friday.

 

[adventurebuddies]

According to that chart, #10 wire can carry 100A 12V. The Engel 60 only draws 4.5A so it #10 wire should be more than sufficient for my needs.

 

[HenryJ]

At 25' that drops to 20 amps. Still more than enough. Don't under estimate the length. You are fine being larger than needed , but will suffer if too small.
Mine will be overkill for sure, but mine is designed to be used in other ways. The fridge will be the smallest load. I hope it is the only load it ever see, but I have been known to over build and plan to for worst.

 

[R_Lefebvre]

I wonder what they assumed to generate that table. Certainly sets up for some massive voltage drop with some of those ratings.

 

[AlexJet]

This is a handy reference: Wire gauge tables Use the chart to determine the cable size for a given length and amperage.

Very nice find for the wire selection. It may become very handy. Thanks for posting.
I'm thinking of adding battery to my trailer next summer and it was an interesting read through the posts.

 

[HenryJ]

I wonder what they assumed to generate that table.

I believe it is based on the 700 circular mils per amp rule, which is very very conservative.
If you prefer to calculate the formula yourself you may find this page useful: Load carrying capacities
It includes a calculator at the bottom.
Personally I find the Wire gauge tables linked above an easier quick reference.

 

[R_Lefebvre]

The problem I have with the table is this:

It suggests you can run 100Amps through 10ga for 10 feet. I calculate the voltage drop when doing that to be 0.9989 Volts. When you have a 12V system, that's just too high of a percentage. That's crazy. If you were using that for lighting, that voltage drop would actually reduce the light output from your bulbs by a really large number... Something like 40%!

Using the calculator on the second page returns a voltage drop of 2V. So something isn't even matching up.

These "rule of thumb" wire ampacity charts are based on AC circuits where 1V drop is not a big deal.

 

[fyrfytr1717]

Discharging the trailer battery?

New guy here... I'm putting together an off road trailer and have been trying to figure out this whole trailer battery charging thing. I understand that by connecting your main battery to the trailer battery using a #10 wire you can charge your trailer battery while the vehicle is running due to the fact that the main and trailer batteries will try to equalize.

By this same line of reasoning, what happens when you apply a large load to your main battery while the trailer is hooked up? Like when you're starting the vehicle or operating a winch. It seems to me that when applying this large load to your main battery, ther trailer battery will again try to equalize and a large amount of current will be pulled through that #10 wire. Won't this overload the relatively small wire connecting the two?

Using an inline relay triggered by an ignition/run source would eliminate the starting issue I guess, but typically when winching you will have the vehicle running which leaves the two batteries connected. Can someone explain to me why this is not a problem?

 

[R_Lefebvre]

I think it would not be a problem due to the natural current limiting effect of a long length of #10 wire. It *would* be a problem if you have mostly large guage wire between the two batteries with a short section of #10 in there somewhere.

To really calculate it, we'd need to know how low the voltage on the truck battery is being drawn, how long the distance is, the internal resistance of the trailer battery....

 

[adventurebuddies]

By this same line of reasoning, what happens when you apply a large load to your main battery while the trailer is hooked up? Like when you're starting the vehicle or operating a winch. It seems to me that when applying this large load to your main battery, ther trailer battery will again try to equalize and a large amount of current will be pulled through that #10 wire. Won't this overload the relatively small wire connecting the two?

For a while I was considering connecting the vehicle second battery to the two trailer batteries via #4 cable. This would effectively give the truck three aux batteries if the trailer is connected. Not necessarily a bad thing. But, the truck aux battery is an AGM and the trailer batteries are Blue Top gel and I've read that those should not be "connected" as the speed of power transfer to/from the AGM battery will kill a gel cell battery over time. Once the Blue Tops die, I'll buy an AGM for the trailer and go to this system.

For now though I don't want to trash two perfectly good Blue Tops (unless someone wants to buy them) so I'm going to rig up the #10 charge wire as recommended here. It makes sense to have the universally accepted setup for the tow vehicle and trailer so that my trailer interchangeble with other vehicles. I am going to put the relay from an ign/on source on the #10 charge wire though because I like the idea of the trailer batteries being separate when in camp. As for the concern about current flow through the #10 while winching, it would be very easy to disconnect the trailer harness or put in a switch to break that connection manually. I consider my winch to be a last resort option rather than a part of my plan for "forward progress" so the rare time when I need to winch and I have the trailer, I'll just unplug.

 

[Mr. Leary]

So... running it for a while with jumper cables attached would be too fast an amp draw?

If you plugged a trickle charger into a power inverter, would it work well?

 

[Toolman]

Optima batteries are AGM

A battery isolator under the hood coming off the switch will isolate the trailer from the secondary this will solve all your problems when it comes to charge and discharge.. rate of charge doesn't matter.. as long as it's coming from the alternator.. the isolator won't go bad.. but switches get dirty and fail after time. plus it's more wires under the hood you don't need. KISS employed

if you connect any cheap battery charger to any AGM or "optima" you risk ruining the batteries life span.. they take charge at a different frequency not voltage or amperage. In other words, they make special chargers for AGM batteries.

Midtronics and Snap On Tools makes these.


I found this link to be very helpful when deciding on what battery to use.

http://www.powerstridebattery.com/optima.html

 

[Toolman]

oh yeah...
keep the blue tops and wire your tow vehicle the right way... 7 pin connector..
If you're worried about another trailer with four pin they make a 7 to 4 pin adaptor for a quick conversion.

Big plus you can use the center pin for accessories, which is what I'm planning on. I'm using the center pin to turn on crawlers and flood lights for night roaming. :wings:

 

[adventurebuddies]

Optima batteries are AGM

According to what I've read - that's incorrect. My understanding is that an Optima is a gel cell battery that uses a lead grid and the acid is suspended as a thick gel in the lead grid. An AGM (Absorbed Glass Mat) uses some sort of fiberglass type weave that is saturated in acid.

See below from a solar/wind power company:

Gelled electrolyte
Gelled batteries, or "Gel Cells" contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass that looks like gooey Jell-O. The advantage of these batteries is that it is impossible to spill acid even if they are broken. However, there are several disadvantages. One is that they must be charged at a slower rate (C/20) to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger or they may be permanently damaged. This is not usually a problem with solar electric systems, but if an auxiliary generator or inverter bulk charger is used, current must be limited to the manufacturers specifications. Most better inverters commonly used in solar electric systems can be set to limit charging current to the batteries.

Some other disadvantages of gel cells is that they must be charged at a lower voltage (2/10th's less) than flooded or AGM batteries. If overcharged, voids can develop in the gel which will never heal, causing a loss in battery capacity. In hot climates, water loss can be enough over 2-4 years to cause premature battery death. It is for this and other reasons that we no longer sell any of the gelled cells except for replacement use. The newer AGM (absorbed glass mat) batteries have all the advantages (and then some) of gelled, with none of the disadvantages.

AGM, or Absorbed Glass Mat Batteries
A newer type of sealed battery uses "Absorbed Glass Mats", or AGM between the plates. This is a very fine fiber Boron-Silicate glass mat. These type of batteries have all the advantages of gelled, but can take much more abuse. We sell the Concorde (and Lifeline, made by Concorde) AGM batteries. These are also called "starved electrolyte", as the mat is about 95% saturated rather than fully soaked. That also means that they will not leak acid even if broken.

AGM batteries have several advantages over both gelled and flooded, at about the same cost as gelled:
Since all the electrolyte (acid) is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage.

Nearly all AGM batteries are "recombinant" - what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.

The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents. The Concorde (and most AGM) batteries have no charge or discharge current limits.

AGM's have a very low self-discharge - from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries. The Concorde batteries can be almost fully recharged (95% or better) even after 30 days of being totally discharged.

 

[adventurebuddies]

If, on the other hand, Toolman is correct and I am wrong and an Optima is not a gel Cell, but rather is an AGM, then that's great because I should be able to link the batteries just fine. Hard to get good info, but I'll be very pleased to confirm that Optimas are in fact AGMs.