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Old 12-17-2010, 12:07 PM   #1
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Question Batteries being charged by tow vehicle?

I recently had one of my two hitch batteries boil out acid through a crack in the negative hollow lead terminal post. The batteries were dead before I realized what was happening while on the road. Acid was well below the plates on one batt, and the other battery's acid was almost to the plates.

After cleaning up the mess, and installing new batteries I decided to check if my tow vehicle was charging the toyhauler batteries when on the road. First I checked the battery charge without shore power (12.9V). Then with shore power (13.9V) Then disconnect shore power (12.9V). Then start and hookup tow vehicle connector (12.9V).

Since it appeared that the tow veh was not providing 14V to the batteries, I unplugged the connector and tested the TV's battery terminal (#6 @ 1 o'clock). I got a 13.9V reading on my meter.

Im not sure why the TV's 13.9V isn't getting to the hitch batteries or how it's even wired in my 2005 SandPiper Sport TH.

Could use some help diagnosing the problem.

Thanx.
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Old 12-17-2010, 03:30 PM   #2
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Did some more testing. Cleaned all the connector terminals. Vehicle pin #4 @ 1 o'clock tested out at 14.4 volts, engine running. Hookup connector, 12.8V battery pack did not change voltage when tv hooked up, running. Then I disconnected the cord and ran a single #10 wire from TV pin #4 to power wire on batt pack. No difference. Im thinking the TV will not throw additional voltage at the batteries until they get below a certain voltage? Maybe that's why it tests high at 14.4V with only a tester on the tv pin #4?

Traced the tv connector power wire (blk) into a j-box where it tied into 2 other blacks. One went to a two bolt little box (breaker or diode ?) or something. But the tv blk connected to one terminal of it and from the same bolt a wire went to the battery pack pos. One wire on that bolt also went to the break away switch. On the other side of the little box 2 wires went to the gen and the power center (I think).

When I tested the battery charging wire (disconnected from the batteries) (i.e. the TV black) I couldn't get any voltage at all. A continuity test of the trailer connector pin to the little box showed a good connection.

All I can imagine is something is happening I don't get, but it's right.

My understanding is that the TV power wire can/should go directly to the battery pack.
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Old 12-17-2010, 06:24 PM   #3
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I'm still a newbie about this too but I got a little knowledge after doing some reading...

From what I understand with the TV running at 14.4 the isolating diode is suppose to allow the 14.4 volts to charge the TT batteries in travel. But lets say you stop at WalMart for the night... Ok during the time the TV is shutdown the diode should keep from drain the TV batteries dead while you living in your TT for the night hitched up.

So I would start with the TT connector again and trace it to the isolating diode and check and see if the diode isn't blown, or a fuse isn't blown...
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Old 12-17-2010, 07:33 PM   #4
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You just have to think it works something like that, right MoparMan?

But on my TH there is nothing between the TV and the batteries except 2 wire connections. Maybe the diode is the little 2 terminal box that is between the TV power wire/battery connection and the other side which goes to the Gen and Power Center? Im told there is no isolator on my TV trailer wiring circuit.

Further testing revealed:

Connect jumper wire to #4 TV connector blade and check voltage running (14.4).

Check TH battery voltage (12.8).

Connect jumper direct to battery positive post (12.8).

Even with a direct jumper to the batteries from my truck the TH voltage remained at 12.8?

Hard to imagine the 2 little 24 series batts on my TH pulling down the voltage from my 2 big Dodge batteries.

Can anyone explain this?
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Old 12-17-2010, 08:46 PM   #5
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Did you also run a ground wire between the vehicles? If you are using a seven pin connector one pin should be ground on both the tower and the toad. The hitch itself is not a reliable ground.
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Old 12-17-2010, 09:17 PM   #6
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Quote:
Originally Posted by Dagwood View Post
Did you also run a ground wire between the vehicles? If you are using a seven pin connector one pin should be ground on both the tower and the toad. The hitch itself is not a reliable ground.
Somewhere there is a miss...

Like Dagwood suggested a ground issues is possible...

Need to do a voltage drop test... I'm not sure if the mods/admins will allow it but if they will here is the link to my page on how to do it for a engine but the principal is the same for a TT wiring...

http://mopar.mopar1973man.com/cummin...ltage-drop.htm

If any wire or connector rises above 0.2 Volts you found your weak connection...
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Old 12-17-2010, 09:23 PM   #7
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Didn't run a ground wire. But the voltage 12.8 was the same with the 7-pin connector and with the jumper (w/o the 7 pin). Also, used the battery neg post when I got 14.4 on just the jumper. So ground unlikely the problem.

The unit is in storage now. I will try any further suggestions when I bring it back out in a month. So keep the ideas a comin'
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Old 12-18-2010, 04:40 PM   #8
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Did some more research and found a thread on the Airstream forum. Guy sounded like he knew a lot about the subject so I will quote him below. All in all, Ive concluded that the lower voltage (12.8) Im seeing when hooking in the trailer batteries to the TV is normal. Also, I could reduce some of the voltage drop by cleaning or replacing my connectors, which I will do. I have no further expectations that the TV will swiftly recharge a discharged trailer battery due to the resistance in the long run of 10GA wire. Best I can expect is a slow recharge by the TV over time. Realistically I can expect the TV to keep my batteries topped off, with the typical long 10GA charge line, and several points of added connector resistance, including the ground path back to the TV.
=================================================
FWIW the Airstream thread:

tow vehicle charge line to trailer battery.. - Airstream Forums

What happens is that it just takes longer to bring the RV battery up to full charge.

The vehicle charging system senses the chassis battery and often has a separate lead for this. Once that battery is up to a proper float voltage, the system keeps it there.

So the situation is that the charging source quickly gets the chassis battery back to a float voltage yet the RV battery is still discharged because current to it has a lot of resistance to get through. The RV battery will then continue to charge because it is below the charging system's float voltage. The charge current will be limited by the resistance in the feed line plus the fact that the source voltage is not very high. That is most of the reason why it can take quite a while on the road to get the RV battery charged up.

When the battery gets up to a full charge, it will no longer accept current at the float voltage level and the very low currents nearing that point mean very low loss in the charging line. So it gets there, it just takes a while.

Note that the ground return on the RV battery is often a good source of resistance itself and a common source of problems.

================================================== =

Based on the above theory, on my next long trip I think I will test the TV and trailer batteries over numerous hours of towing and see how the voltage changes, if at all.
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Old 12-18-2010, 06:39 PM   #9
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Joe that airstream quote is accurate. I think you have a different issue, which is the circuit breaker you mentioned in your @2 post (two bolt little box). Look closely on one end, you should see a tiny black/red reset pin, push it in and re-test for charging voltage from your TV.
Hope that is the problem.
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