charging the battery - Fiberglass RV


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Old 07-05-2007, 09:08 PM   #1
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HI, I placed a deep cycle marine battery in the trailer.... now all internal lights work on the 12v battery.
my question.... how to recharge the battery?
a- connect the car battery to trailer battery? do it directly or can i use the hitch wiring?
b- connect a solar pannel to battery?
c- buy a battery charger

and either case, will the battery automatically charge when connected to any of those sources (a or ?

thanks for all your help

Mo
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Old 07-05-2007, 10:38 PM   #2
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connect the car battery to trailer battery? do it directly

You could, but not recommended. They are not the same type of battery, and the car battery was not designed to be deeply discharged like that... you'll kill it quick, not to mention risking being stuck without a way to start your car.

or can i use the hitch wiring?

If your car wiring is set up that way... but the above still applies if you are connected and the car isn't running to keep the battery charged up.

connect a solar panel to battery?

This is the way I do it most often. It takes more than the panel, you'll need a charge controller and the right configuration(s). A search on this site for "solar" will bring up more than you ever wanted to know about how to do it.

buy a battery charger

Also, do a search here.. this is a popular topic.

I use all three methods you ask about..

The vehicle charges the battery while running/driving
I use solar when "Boondocked"
I use the charger when I have an outlet available, or when at home to get things going before a trip.

I also have a generater, but it RARELY gets used.
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Old 07-06-2007, 11:40 AM   #3
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If you charge from the car via the hitch wiring, you can avoid any problems with undesirable discharge by using an isolation relay in the car: it disconnects the trailer when the engine is not running. This is a routine setup for towing, and some searching in the forum with keywords like "isolation" and "charging" should yield some good descriptions.

I agree with Gina; they're all viable options, and the best choice depends on the circumstances.
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Old 07-07-2007, 04:25 PM   #4
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I am about to embark on a two & 1/2 week trip, staying mostly at non hook up sites...I will be at Bandon for a week with hook ups, so I will charge with my on board charger and use my AC outlets without fuss.

BUT, I am leaving my large solar array at home. Because most of the places I am staying are for one nite only, I will be driving during the day which will MORE than adequately charge me up for an evenings rest and TV etc. I simply want to pull in and set up quickly, getting rest and then making a quick escape in the am. Wrestling the array around will not allow quick stops, and it will add little, to zero to the party because of the limited amount of daylight it would actually be set out.. Best to just forget it.

I have a small fold up 15 watt panel that lives tucked away in the trailer, and will help if need be, but I don't expect to even drag it out.

Having more than one way to charge can keep you versital, and you can configure for your needs on whatever type of trip you are taking.
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Old 07-07-2007, 05:18 PM   #5
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Question from the newbie...I know the battery charges while driving. If you are boondocking, and the battery runs low, does it do any good to run your car engine for 10-15 minutes to charge it up a little? I know I probably need to learn the solar panel route, but don't have the time and money before a 3-day campout next weekend!


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I am about to embark on a two & 1/2 week trip, staying mostly at non hook up sites...I will be at Bandon for a week with hook ups, so I will charge with my on board charger and use my AC outlets without fuss.

BUT, I am leaving my large solar array at home. Because most of the places I am staying are for one nite only, I will be driving during the day which will MORE than adequately charge me up for an evenings rest and TV etc. I simply want to pull in and set up quickly, getting rest and then making a quick escape in the am. Wrestling the array around will not allow quick stops, and it will add little, to zero to the party because of the limited amount of daylight it would actually be set out.. Best to just forget it.

I have a small fold up 15 watt panel that lives tucked away in the trailer, and will help if need be, but I don't expect to even drag it out.

Having more than one way to charge can keep you versital, and you can configure for your needs on whatever type of trip you are taking.
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Old 07-07-2007, 05:33 PM   #6
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I HAVE done that in a pinch. I usually run 1/2 hour or so, then let it rest, and run it again when I am running low again. It keeps lights going, but I wouldn't do it to run the fridge or TV etc.

I don't recommend this as a permanent fix, but it can get you out of a jam. It's not real efficient, and it certainly can add wear and tear on your car.

Basically, it's using your car as a big expensive generator...
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Old 07-07-2007, 09:48 PM   #7
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The reason it's not efficient is that the TV's charging system is seeing the voltage from both batteries, and one is charged, so the charge is a lower rate than if it were the only battery.

A better way to use the TV as a generator is to hook an inverter to the TV battery and run a good portable battery charger from that to the egg battery (don't just plug the inverter into the cig socket as the TV wiring may not be adequate; clamp the inverter leads directly to the TV battery).
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Old 07-07-2007, 10:47 PM   #8
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Will plugging the Casita into electrity for a week at home before heading out for the weekend give the battery enough juice to power the lights and fantastic fan for a weekend while boondocking?
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Old 07-07-2007, 10:58 PM   #9
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Although Pete's configuration of inverter and charger sounds painfully indirect, I agree that it would be the most effective way to go. The trailer battery gets the voltage it needs to charge properly, regardless of what the "TV" is doing, and that could be particularly important with the TV's alternator running really slowly (it might not be able to produce enough voltage, even without the TV battery confusing things.

If you want to charge at 12 amps (the high rate for my charger, and reasonable for common small RV batteries) you will need from 140 to 180 watts of DC power (depending on the trailer battery voltage), which means at least 155 to 200 watts of AC power, which both suggests possible inverter size and explains why the 10 amp lighter socket is not suitable - 200 watts at 12 V means 200 W / 12 V = 17 amps (and that's a bare minimum).

If you do the inverter+charger thing with the TV unplugged from the trailer, you could supply the 12V DC for the inverter from the trailer socket on the back of the TV; if it is wired to supply power to the trailer it will be suitable for up to 30 amps. Make up an adapter and save opening the hood and clipping stuff on. There should only be power there if the TV is running, but it should be running anyway. Hmm... maybe I'll make one of these for myself. Nah - too many other projects!

Another factor to keep in mind is that for efficiency and long life batteries should be charged relatively slowly. If the battery is really run down, you wouldn't want to get it fully charged in a quarter of an hour even if the direct TV connection could do it... you want several hours. That's one reason why sporadic generator use is also a bad way to supply a battery... if the battery is sized to run down to 50% of capacity over a day of use, it should have hours to get back up to 100%. At 12 amps, 15 minutes will be only 3 amp-hours, so it wouldn't help much.
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Old 07-08-2007, 07:10 PM   #10
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The 'indirect' way used to annoy the bejeepers out of technical folks on other RV groups, who didn't like to see the power going from alternator output to voltage regulator output to inverter output to battery charger output, but the reality is that a good battery charger will determine the best charging rates for the battery based on feedback from the battery and none of the other components named will do that. Recently, I've noticed that folks who were questioning the method about five years ago are now using it...

If charging the battery properly is the prime concern, then the indirect method addresses that, and may or may not also be the most fuel-efficient way to do it, regardless of electrical inefficiencies, because the fuel efficiency is most likely to be time-dependent and the charger will likely give the best time (except charging at an excessive rate for the battery).
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