Using TV to charge the TT battery-alternative method

Taranwanderer · 11-11-2011, 11:31 PM

So, I'm thinking that I'd like to have my TV charge the TT's battery when I'm en route somewhere, and not rely on the puny little charging wire in the 7-pin. So I'm going to lay this out and someone tell my why it's a bad idea: first off, my TV already has a set of 6GA battery cables running from one of the batteries all the way to the rear bumper and terminated with a heavy, "plow-type" plug. I did this a few years ago so I could plug in my portable winch when I have it slid into the receiver. So now I'm thinking that another set of plugs, some more cable, and some cable lugs and I could use this to plug in the TT battery and charge any time the TV is running (with heavier cable.) Is this a bad idea? I don't think the TV's alternator would overcharge the TT battery, but are there any other considerations I'm missing? My thought is to connect the cable lugs to either the battery terminals directly or to the terminal on the disconnect switch and a good ground, then a short run of wire ending in a plug (that would be permanently attached to the TT,) and then a longer run of wire with plugs at both ends that can be plugged in when needed and stowed when not. Anyone see any issues with this idea? Please talk me out of it...

**Herk7769** · 11-11-2011, 11:52 PM

I am not too sure that you would get more amps out of the alternator than is available with either method. The alternator (even an upgraded one) is basically a trickle charger for your starting battery and does not provide enough charging juice to do much more than replace the current used to start your truck in the course of an average hour's drive. In fact if you use your headlights in that drive you are at a "net loss" of battery charge. Folks up north can back me up on that one since hooking up an external battery charger every month or so in the "dark times" is common practice.

The red charge wire on your alternator can't handle much more than 20 amps DC in any case and that will have to be split between charging your truck's battery and the camper's battery.

When you run your winch you are pulling the lion share of amps off your battery NOT your alternator.

wfmdfm · 11-12-2011, 12:09 AM

I agree with Lou. The alternator is only pushing out so many amps. The larger conductor would eliminate some voltage drop but does not seem to me to be a large gain.

Taranwanderer · 11-12-2011, 12:10 AM

Thanks for the info, Lou. I knew that the winch was pulling from the battery, not the alternator...I've done many days of 4-wheeling with my Jeep staged at an obstacle that needed many other Jeeps to be winched over, and I've killed my share of both Optima red tops and Wal-mart NeverStarts! So even if you have a 100 amp or a 140 amp alternator, you're only going to get about 20 usable amps? That was news to me. I tend to be hard on batteries, but things always seem to work better when the engine is running. Anyway, you still didn't tell me it was a bad idea!

I guess what I'm really worried about is will this cause any damage to the TT's battery (not likely,) or to it's 12v electrical system or converter (not sure.). I think that 12 volts is 12 volts, regardless of how it's delivered, but what do I know?

LadyWindrider · 11-12-2011, 12:50 AM

While it is not a bad idea, don't go overboard with the wire size. Most modern alternators have a built in voltage regulator, set at about 14 volts max. And while the alternator will produce 100 amps, or even more, most is used in the vehicle starting battery, and that will bring the voltage up in a hurry, and the current down.

Some post on here claim hours to charge their battery with the the vehicle, I'm not sure. I get over 13.5 volts on my trailer battery when connected and the tow vehicle running. That is starting with a trailer battery fully charged. Might want to check your voltage and see what you have.

One thing to remember, when you get all this wired is, you have a parallel circuit of trailer battery (or batteries), and vehicle cranking battery (or batteries). They will be miss matched and you may not get the results you desire.

Taranwanderer · 11-13-2011, 04:38 PM

Thanks for the responses everyone. That's a good point about the batteries being different, but that's the case with every tow vehicle and every trailer that has a charging wire in it's 7-pin; it's going to charge "mismatched" batteries via that method anyway, so is it really an issue to step up to a larger wire for the task? I'm not so much worried about mismatched batteries as I am about damaging the TT's internal wiring.

**Herk7769** · 11-13-2011, 05:42 PM

Quote:

Originally Posted by Taranwanderer

So even if you have a 100 amp or a 140 amp alternator, you're only going to get about 20 usable amps? That was news to me.

That is about the "average" draw on the alternator's output. Just as the starting battery "gives" up its charge quickly when starting (Cranking amps) it also takes a recharge quickly. As the voltage goes up on the battery to meet the incoming charging voltage, the charging current drops off to near zero.

When going through the engine compartment fused line to the camper's socket to charge the deep discharge or dual purpose battery in the camper, the same thing happens. Current is limited by the 40 amp resetable limiter in the trailer's incoming 12VDC line and also by the truck's fused line (value varies by manufacturer). If you bypass the fuse in the socket with a direct line to the starting battery and then connect a discharged camper battery, the current pulled will be limited somewhat by the camper's inability to take a "fast charge" due to the much thicker plates of the house type battery. The initial charge rate may be 25 or 30 amps, but within an hour it will be unable to accept more than 5 amps or so; decreasing to zero over a period of days.

In any case you will not damage the truck's wiring unless you have a short at the terminus of the home run cable. Without a fuse you will destroy your alternator's recifier bridge for sure (and it won't do the wiring much good either).

transamz9 · 11-13-2011, 07:29 PM

I know guys that charge their trolling motor batteries on their bass boats this way and have been doing it for years with no problems. There are also systems that you can buy that will allow the alternator to charge your starting battery fully and then switch over to charging the trollers off of the alternator. I think it's called Stayin Charge.

The way that you currently have it hooked up, the trailer and TV battery are going to equalize and the alt is charging them both at the same time.

Thurman · 11-14-2011, 05:48 PM

I have a similar hook-up with my cargo/work trailer. I have a deep-cycle battery in my trailer for the lighting and to operate and inverter which charges an 18v DeWalt battery as I drive down the road, always a freshly charged battery at the job. The system on my '07 GMC Sierra with dual batteries has always been sufficient to do this. I'm thinking that having a 6 ga. battery cable at the rear of your TV/truck is not going to be anymore beneficial to you vs. the factory system.

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