Battery charging times using your TV as a charger.

WilliamA

Hey all,
There's a lively discussion going on over at the T@B site and I thought that it might be worth grinding the sausage about here since it also applies to T@G's and comes up pretty often. Basically we are referring to trailer battery charge times while on the TV and what folks have been experiencing. I thought about it for a bit and it made me remember all of the ghosts and gremlins I have (needlessly) chased about in my years as an auto mechanic. It's easy to jump into a problem with both feet and overlook some of the basic "do this first" kind of stuff before throwing time and money into a problem. I'll just paste my thoughts from there in here for you to mull over.....

............Imported from T@B site thread........
Hey all,
This is proving to be an interesting thread with lots to unpack and think about. The differences in vehicle design and battery management are an ongoing issue that will no doubt prove to be a subject of ongoing research.
That said, I don't mean to add to the obfuscation here but there are some basics that need to be looked at as well. First, assuming there is a direct battery connection between the TV (tow vehicle) battery and the 7 way plug, that means that whenever the trailer plug is connected there is a parallel circuit connecting the two batteries, whether the engine is running or not. It's possible that on some vehicles, when the ignition is off the trailer battery circuit is disconnected but for this discussion anyway, that's neither here nor there. Assuming there's a parallel circuit connecting the two batteries, the batteries will work to even each other out. The higher voltage battery will push to charge the lower voltage one until they are of matched voltage. The closer they are voltage-wise, the longer that will take. Nevertheless, they will push/pull until they match voltage. As I said, this will happen regardless of whether the engine is running or not. If the TV is running, the ecm will sense the (approximate) average voltage of both batteries. In layman's terms, if the TV battery is 14 volts and the trailer battery is 12 volts the ecm will see about 13 volts and respond accordingly. Regardless of what kind of vehicle it is, the ecm is smart, but not THAT smart. It (the ecm) cannot see separate voltages for the two batteries, only the average of both. This, again, assumes they are wired in parallel and not run through a battery management system such as a smart controller.
What I've not seen discussed here that needs to be is the very basics of circuitry. If you have a 10 gauge wire going out to charge your trailer battery, then you need "at least" a 10 gauge ground wire coming back through the trailer connector. Most trailer connectors (in my experience) have quite small 16 or maybe 14 gauge wires going out to the trailer. This includes the ground wire. Remember that all of the voltage out to the trailer for lights, refrigerators, charge lines etc, must run back through the same single ground wire to the TV. If you have a 16 or even a 14 gauge ground wire in your trailer connector, then when the lights are on, that's about all the capacity that ground wire has. There are a lot of lights on a trailer and it's a long way back there. Now, in addition to the lights and other stuff, you add the current requirements of a 20 or 30 amp charging circuit and you're asking too much of the ground circuit. You've got 6 wires going out hot to the trailer and all of that potential voltage (amperage) must return on 1 ground wire. If your lights take a maximum of, say, 12 amps, then you add 20 amps from the charging line, you need a ground wire with a minimum capacity of 32 amps. It stands to reason that the total potential capacity of the ground wire must be equal to the total capacity of all the hot wires combined. If you limit the capacity of the ground circuit, you limit the amperage going out to the trailer. The voltage, as seen with a meter or gauge, will be the same or nearly so between the TV and trailer but the amperage available will be drastically less. It's the same as filling two 1 gallon jugs of water from the same pump, but one hose is 1/2" and the other is 1/8". The pressure (voltage) will be the same in both hoses but the volume (amperage) will be very different. The larger diameter hose will fill its jug first every time.
It's safe to say that many folks are experiencing problems charging because the trailer ground is too small for the capacity of the combined load of lights and charging circuit. This is easy to check without doing a whole bunch of wiring. Just add a jumper wire (10 gauge) that bolts from the trailer frame to the TV frame. I suspect you'll see a dramatic reduction in charge time.

I have run 10 gauge wire from the trailer plug to the battery for BOTH my charge line and my trailer ground (dedicated ground) on: My Toyota Rav4, my Jeep Cherokee and my Jeep Wrangler and have never experienced problems with my trailer battery draining while towing. I leave my refrigerator on all the time, run electric brakes and have a full compliment of lights.

WilliamA

Boyd

That is really good food for thought. Thanks

LuckyJ

Good info here! Thanks.

mreric

can you talk a bit about battery isolator units?

WilliamA

@mreric said:
can you talk a bit about battery isolator units?

Well, yes. At the basic level where most of us live, you can consider that there are two general types: The first is the dumb controller. It's basically just a very large diode or more specifically a bridge rectifier that allows current to flow one way but not the other. Put in line between your TV and trailer and it will allow the trailer battery to charge to whatever voltage your TV is at, but won't allow voltage to flow back from the trailer battery to the TV battery. It doesn't "monitor" anything. It's just an electrical one-way gate. A smart controller has a microprocessor that actually monitors voltage of each battery and turns on and off various relays to route alternator voltage around. They are very common in off-road circles and folks use them not only for dual battery management but also for lights, radios and laser beams, etc. Some of the better ones are programmable, allowing the user to set voltage minimums and maximums, charge times and many other functions. They will also shut off devices when voltage gets to high or low thresholds. Here's an example of a device that can be used as a charge controller but isn't actually sold or marketed as one:

https://www.amazon.com/Apollointech-Wrangler-2007-2018-Overhead-6-Switch/dp/B01CDTW03K

It's basically quite simple. It has a number of switches that run high-current relays. You can wire the unit so that when the ignition is off, there is no voltage going out through the relays. For example, you could wire your trailer charging circuit through one of the relays and then have a switch in the "cockpit" that controls whether you want the trailer battery to charge. If you simply leave the switch on, then anytime the ignition is on, it will connect your two batteries for charging or use. If voltage in either battery falls below a programmable threshold, it will open (disconnect) the relay isolating the two batteries from each other. A clever person could wire the digital voltmeter so that it reads either the TV battery, the trailer battery, both batteries (average voltage of both) or put a dpst switch in line to be able to switch which battery is being monitored. Keep in mind that this is a "dumb" controller in as much as it won't control exact trailer voltage but will only control when the trailer battery is connected to the TV battery. You could do the same thing with a single hi/lo relay for less money.

I'm sure that a dedicated search could find battery charge controllers that would be designed for monitoring exactly the rate and state of towed vehicle batteries. Those units won't be as adaptable as something like the apollointech unit but will be easier to install.

How do I do it? I've got my charge line for the trailer wired straight to my 7-way connector from my TV battery with a BAF (big a** fuse. ATC type 30 amp) wired inline next to my TV battery. That means that whatever my trailer battery voltage is, my TV battery voltage is the same, ignition on or off. When I camp, I disconnect the 7-way plug so I don't run the TV battery down too. I ALSO have a dedicated 10 gauge wire going from my TV battery negative terminal directly to my trailer 7-way plug ground circuit. None of the insipid "body grounds" or worse yet, just using the trailer hitch coupler as a ground. I have a digital battery voltmeter wired into my trailer. When I start my Jeep, I can open the door of my trailer and take a peek at the voltmeter and see exactly what's happening. If the voltages between my TV voltmeter and my trailer voltmeter agree (generally within .1 volts) then I forget about batteries and set about to driving and gawking.

WilliamA

TomD

William said..”When I camp, I disconnect the 7-way plug so I don't run the TV battery down too.” I have a tungsten solenoid installed in the TV such that the connection between the TV battery and the SS batteries switches off when the TV is shut down. Sometimes I don’t want to “pull the plug”.

WilliamA

Tom,
I thought about putting in a beefy relay for my charging line but decided just to add the fuse holder. I've never yet forgotten to plug my harness back in and never even unplug it for a few hours spent in a rest stop or truck stop. If it's unplugged, I'll know by my (lack of) brakes, plus my brake controller has a light on it that tells me when it's hooked up. I only unhook if I'm going offroad with my Jeep or laying up in some campground for a couple of days.