Charging deep cycle batt. with car alternator - Do I need a charge controller??? - Page 3 - Fiberglass RV


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Old 09-29-2017, 11:55 AM   #29
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Originally Posted by Buggeee View Post
Ordered one of these. Thanks for the link Fofo.
No problem, don't forget to install a 40A circuit breaker between TV battery and relay. Atkinson also recommends using at least 10G wire between TV battery and the unit.
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Old 09-29-2017, 01:05 PM   #30
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If your vehicle came factory wired, the relay may already be there. My Frontier disables the charge line when the key is off.
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Old 09-29-2017, 01:45 PM   #31
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The consensus in what I'm reading seems to be that, as a practical matter, the voltage and/or amperage drop resulting from the long length and small diameter of the wire leading from the TV alternator all the way back to the RV trailer battery all but eliminates any opportunity for the TV to damage the RV trailer battery by overcharging it. Instead, the risk would be undercharging and sulfation if someone were to rely on the TV as the only life long source for charging. Talk among yourselves
There is no risk of overcharging a lead acid battery while charging it from the TV and it has nothing to do with wire size. The trailer batteries want to see the same charging scheme as the vehicle battery. The TV also has lead acid batteries and they are always connected to the vehicle charging system. The TV charges them to 14.1 volts all the time while driving. When stopped they rest down to 12.65.

My favorite way to charge the trailer batteries (4 trojan T-105 deep cycle six volt batteries) is to hook up jumper cables to them from the TV. This bypasses the small charging current available from the TV plug and gives them around 150 amps that is regulated to 14.1 volts. It gets the job done and it can't hurt the batteries.

Then, while driving, the TV can supply them with a smaller current through the plug and eventually get them up to 14 volts. When I stop, they are fully charged if I've driven far enough. Then they also rest down to 12.65. Ready for duty.

Charging systems must take lead acid batteries to 14 volts to achieve full charge and avoid sulfation. But that it too high to hold them continuously as it will boil out the water. So, once the battery gets to 14 volts, with a smart charger, the charging voltage is reduced to about 13.2 volts for maintenance. This is still higher than full charge, but won't hurt the battery. A fully charged battery, once rested and ready to go, will register 12.65 volts.

In cars, the voltage is set at about 14.1 volts, but they are not run continuously. The batter sits most of the time not charging. This allows a long battery life without a "smart" charging scheme with a maintenance voltage. Either on at 14.1 or off and resting.
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Old 03-02-2018, 05:00 PM   #32
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Name: G
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battery charging using vehicle alternator

Hi, good question and knowing the characteristics of flooded batteries and vehicle alternators will provide some insight to help answer the question.
First the guy that sells you the battery will often tell you whatever it takes to sell you yet another device!!
Batteries are self regulating as the voltage (pressure) goes up so does the back pressure,
A one hundred amp alternator will only make close to that briefly when charging a very low battery. As battery voltage rises the amperage declines.

This is a fairly simplistic overview but it serves to explain what is happening at the battery.

Yes there are chemical differences between starting batteries and real deep cycle units, however the intent is to pack as many watt/hours as possible into those deep cyclers. The starting battery just needs topping up as your vehicle is really running just on the alternator

So feel free to use the excess output from your alternator, it greatly exceeds your converter output! AND, motor homes do it all the time without harming either their expensive deep cycle coach batteries or their SLI ( starting, lighting, ignition) chassis battery.

Real deep cycle batteries need a fair amount of attention to electrolyte levels and terminal cleanliness. Failure to do this is the most common cause of early failure

You don't need a controller and it is only additional expense and complication

Others here have suggested many ways to make the connection, just be sure to have both a switch and a suitable fuse, jobbers sell a fuse with pigtails.

To prevent arcing make sure that that connection is off before disconnecting the trailer harness. G.
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