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Old 05-31-2017, 11:10 AM   #1
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Does TV charge TT?

Stupid question time. We’re first timers… Just picked up our 2018 Rockwood 2703WS and 2017 RAM 2500 CTD. A family emergency is requiring us to employ it much sooner than I’m ready.

So -- will be doing some boondocking over next week. Question is – will the RAM recharge the 2 trailer batteries while hooked up and towing down the highway? Or will only “RV park” shore power recharge the trailer batteries? RAM has optional dual upgraded alternators and trailer has optional 2nd battery (12v deep cycle).

Will travel about 1000 miles over the course of 3 days, stay 3-4 days (next to a house - no RV style 50 amp plug in – but a 5KW generator). Then travel about 1000 miles back over the course of 3 days. Not part of my newbie RV training plan, but it’s going to happen.

Understand leave the 2 ACs off, but can run maxx air fans for a while. Propane for frig. Use a couple LED lights sparingly.

So will the truck recharge the trailer batteries while traveling? Or will the trailer be electrically dead within a couple days and remain that way the whole trip?

Bob
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Old 05-31-2017, 11:17 AM   #2
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If properly installed, the 7-pin cable from the TV to the TT will provide battery charge. Now, it is NOT a high-current thing but it will do the trick over those 1,000 miles. Your dealer / installer may have put a fuse in the line to the +12 terminal, so might be good to make sure that is checked and you have a couple spares.
We absolutely do use this method to top off our battery.
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Old 05-31-2017, 11:21 AM   #3
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As said above, it will but it's not going to be a fast charge. Also as eluded to above, make sure that it's working correctly before you leave.
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Old 05-31-2017, 11:53 AM   #4
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Good article on why a generator and dedicated battery charger is needed to boondock.

Alternators make lousy Deep Cycle Battery chargers.
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Old 06-01-2017, 06:17 PM   #5
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I just posted this replay to a similar question. My answer is evern more appropriate here:

I'd like to describe what I've done to keep my batteries fully charged while traveling. We have a 31' bumper pull Rockwood. We have 2 6v Trojan T-105 batteries and a whole house inverter that keeps our outlets at 110v so when not hooked to shore power we can run everything except the A/C.

When we travel we usually spend the nights at Walmart and often go for a week without shore power. At first we would always run out of battery power after the 2nd or 3rd night. To solve that I ran a 2 gauge hot wire from my battery to a solenoid (keeps the voltage from traveling backward from the trailer to the truck), solenoid to a 100 amp breaker, from the breaker to the back of the truck. Ran a 4 gauge ground from under the truck to the back to join the hot wire in a heavy duty connector that connects to 4 gauge cables that run to my trailer batteries. Now when we travel our batteries are always 100% charged when we pull in for the night.

In our experience that small gauge wire in the 7 pin connector cannot handle the serious amperage needed to charge heavy duty battery systems.
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Old 06-01-2017, 07:32 PM   #6
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Check your manual and verify the output. My F150 came with a relay you had to install, my Silverado says the wire is not connected from the factory. So it should have the capability but you may have steps to enable it.
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Old 06-01-2017, 08:21 PM   #7
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It should. But check b 4 you go to make sure it's charging by unhooking from shore power. Plug in TT to ram check battery voltage, then start TV , voltage should increase 2-4/10s.
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Old 06-01-2017, 09:37 PM   #8
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Quote:
Originally Posted by OldBob View Post
I just posted this replay to a similar question. My answer is evern more appropriate here:

I'd like to describe what I've done to keep my batteries fully charged while traveling. We have a 31' bumper pull Rockwood. We have 2 6v Trojan T-105 batteries and a whole house inverter that keeps our outlets at 110v so when not hooked to shore power we can run everything except the A/C.

When we travel we usually spend the nights at Walmart and often go for a week without shore power. At first we would always run out of battery power after the 2nd or 3rd night. To solve that I ran a 2 gauge hot wire from my battery to a solenoid (keeps the voltage from traveling backward from the trailer to the truck), solenoid to a 100 amp breaker, from the breaker to the back of the truck. Ran a 4 gauge ground from under the truck to the back to join the hot wire in a heavy duty connector that connects to 4 gauge cables that run to my trailer batteries. Now when we travel our batteries are always 100% charged when we pull in for the night.

In our experience that small gauge wire in the 7 pin connector cannot handle the serious amperage needed to charge heavy duty battery systems.
I don't understand how doing this negates the voltage regulated current output of the alternator.

How are you determining "100%" charged?

Voltage of 12.7+ volts (4 lights on the display) could just mean that there is residual surface charge on the plates from the drive. You would need to measure the voltage after a load was placed on the battery(s) to dissipate the surface charge and then measure the voltage or wait 24 hours with no load for the batteries to equalize. 3 lights on the display could mean anything from 50% of capacity to 100% with no way to tell the actual state of charge.

Here is the voltage range of the "4 light system" (the voltages are when that light "goes out" and what the stabilized voltages mean regards to capacity.
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Old 06-01-2017, 10:09 PM   #9
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Sorry Herk, by saying 100% I only meant that it seems like 100% compared to when I didn't have my charging setup. I no longer take a look at those idiot lights as I've come to distrust them over the years. When I only relied on the 7 pin connector to charge the batteries I would have frequent alarms from my inverter and I would have to cut down on our electrical usage. Since installing this system we never get complaints from the inverter no matter how much power we use. So I am assuming from that...the batteries are fully charging while driving. We only drive between 300 and 350 per day so we are thankful that this setup has worked for us. We carry a 2000 watt Yamaha generator for when we'll be boondocking for more than a few days.
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Old 06-02-2017, 07:57 PM   #10
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Originally Posted by Herk7769 View Post
Good article on why a generator and dedicated battery charger is needed to boondock.

Alternators make lousy Deep Cycle Battery chargers.
In reality the linked article is pretty inaccurate.

There are no constant-current chargers, nor do any chargers cut back or adjust the current. This applies to car/truck alternators, converters, and stand-alone chargers.

Most vehicle alternator regulators (in the past 10+ years) are 2 stage alternators to better protect your battery. They put out a charge voltage in the low 14s to rapidly recharge the battery, than cut back the charge voltage to around 13.6 - 13.8 volts as the battery voltage builds. This avoids "boiling" the battery on a long day of driving. The charge current is determined by the battery's internal resistance, which increases with the battery charge (decreasing the charge current).

Most RVs (past 10+ years) come with 3 stage converters. They have a 14.4V boost or bulk mode, a 13.6-13.8V normal mode, and a 13.2V trickle mode. The bulk mode is for faster charging when the battery is low (50-70% state of charge). Trickle mode is entered after a couple of days in normal mode to protect your battery from "boiling" away the water.

There are 2 things that limit how much charging the TV alternator can do.

First is voltage drop from the alternator to the RV battery from the wiring. If the alternator is putting out 14.4V, but the RV battery only sees 13.6V, it's not going to charge very fast. If your RV fridge is running on DC, and the RV running lights are on, the 10+ amp draw can make these wiring voltage drops very real.

The second limit on RV battery charging by the TV alternator is the TV battery. If the TV battery is not as discharged as the RV battery, the alternator regulator will see the higher TV battery voltage, and cut back to the lower charging voltage. When combined with the voltage drops in the wiring, the RV battery may only be getting a trickle charge.

I have found that turning our fridge off while towing for the last 2 hours before we arrive at the site makes all the difference in whether or not the RV battery is fully charged when we arrive.

Fred W
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