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Old 06-23-2004, 08:35 PM   #1
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1996 28' Excella
Okemos , Michigan
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Question Replacing Univolt and 12V confusion

Univolt died on the last trip. Yanked it out, opened the case, desoldered the silicon rectifiers, and no volts on the secondary of the transformer. Primary has power. Boat anchor.... Now, what to replace it with? I am thinking an Intelipower, but which one? Do you replace the 50 amp Univolt with a 60 amp or a 40 amp unit? If I go with the 60 amp, do I need to increase the fuse size to the battery from 50 to 60 amps?

Now to the confusion. Reading the owner's manual and the service manual, they imply that the Univolt would charge both battery #1 and #2. Also the tow vehicle would charge both batteries when cruising down the road. BUT, looking at the wiring diagrams and the actual wires, I come to a different conclusion. It appears that the Univolt positive terminal was wired to the internal loads thru the 20 amp fuses and to Battery # 1 thru a 50 amp fuse. NOT to battery #2! Battery #2 was wired to the tow vehicle connector and to the emergency brake switch and the power jack. In case there was some hidden connection, I plugged the battery in position #1 and checked for voltage. No power to the jack, nor the breakaway switch.

So if you only install one battery, and the Univolt dies on you, you cannot charge it with the vehicle unless it is in position #2. In order to use it for the lights and water pump, you need to move it over to #1. When you get back on the road again, you have to move it to #2 to be able to use the breakaway switch. (the power jack can be run off the tow vehicle)

It appears that the manuals are misleading. With the 2nd battery "option", you had better have one installed, or be ready to swap the battery back and forth. (builds muscles).

Does any of this make sense to anyone? When I get the new converter, should I run a set of output cables to the 2nd battery? I believe it has two sets of connectors.

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Old 06-24-2004, 12:06 AM   #2
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I think I would go with the 60 amp intelepower with charge wizard.

I would not worry about increasing the fuse from 50 to 60 amps. I doubt your ever going need that much power.

As far as the two batteries would go, I don't have any experience with that as my Safari was only setup with one battery. But I would just add the two circuits you mentioned to the first battery via a fuse for each one. For example, put a 20 amp fuse for the jack, and maybe a 30 amp one for the break away switch.

Then use a 40 amp for the charge line from the tow vehcile to the trailer battery. Be sure and fuse the charge line on both sides of it.

Good luck. Sounds like you already know what ou need to do!

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Old 06-25-2004, 12:35 PM   #3
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Lightbulb More thoughts on the '79 two battery system.

After looking at isolators and pestering one of my friends who can fix anything electrical, I believe that I need one of those marine battery switches. They are rated for 350 amps with positions of "1", "2", "BOTH", and "NONE". ($30) The converter and trailer load circuits would be attached at the main tap. The batteries would be at the "1" and "2" positons, with the feed from the truck, the breakaway switch, and jack still attached thru the fuses to battery #2.

"NONE" is for storage with the batteries isolated.

"1" is the same as the trailer came from the factory, with the converter charging battery #1 and running the load. Battery #2 running the brakes and jack and being charged by the truck.

"2" would swap battery #2 for battery #1 when #1 was drained while boondocking, allowing the water pump and lights to run off #2.

"BOTH" would allow the breakaway switch to apply the brakes if only one battery is installed in the trailer and things got hairy. If two batteries are installed, this position would charge both of them from the truck while driving.

Anyone installed such a switch?
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Old 06-25-2004, 04:08 PM   #4
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Battery switches

Go the the website:

And do a search for West Advisors resource reports on battery isolators and battery switches. They show schematics for various switch and isolator arrangements.

Many boats have a similar need for battery switches.

You can try:
and for isolator parts:

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Old 06-25-2004, 11:09 PM   #5
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Sander ,the problem with your univolt is more than likely the Caps.I have fixed about 4 Univolts and all had the same problem.The secondary winding will not show a voltage as long as the one of the caps is shorted. Here is a link to my work around
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Old 06-25-2004, 11:10 PM   #6
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Why so complicated? Just run the batteries in parallel, add a disconnect if you want, that way you can use and charge both batteries evenly.
The truck would charge both batteries when it's hooked up, and so would the converter. Your amp - hours will double, and if you accidentally run down both batteries, you can always hook up the tow vehicle to run the jack etc.
Simpler high amperage wiring usually means safer conditions in the long run.
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Old 06-25-2004, 11:19 PM   #7
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I agree with UWE. I have had both systems and found that simple is better.I once had an Isolator die on me on the road, had to find a way to bypass it on it , it stopped charging the vehicle battery.
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Old 06-29-2004, 06:51 AM   #8
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Thumbs up Ol' Buzzy is back. Hmmmmmmmmmmm.

Oh the embarassment. 71Tradewind, you were right. It was the caps. I should
have checked the easiest part to replace first. That transformer in the old
Univolt is a mystery to me. How can a short in the part of the circuit with two
thin wires prevent the 50 amp secondary windings from getting power is bizarre.
Two 440VAC 4uf AC motor caps later and ol' buzzy is humming again. The
output cables have the right DC voltage (with the expected huge amount
of AC ripple).
Thanks to everyone for the advice.
I think I'll put ol' buzzy back in. I think that the cross connection issue can be
solved with a simple on-off switch joining the two battery connections. I just
need to see if the connectors have enough room for an additional cable so
I don't have to do any splices.
It would be nice to get a new quiet power supply, but we only use the trailer
for short camping trips. (<1 week or so) The Univolt is never on long enough to
boil the batteries dry.
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Old 06-29-2004, 09:09 AM   #9
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Replace the Univolt with something that won't boil your batteries (the Intellicharger with Charge Wizard is a good choice). This is particularly important if you use the trailer only for short trips and let it spend weeks sitting in wait for the next one. You should keep a battery maintainer/charger on the batteries between trips to keep them healthy longer.

Wire your batteries in parallel and keep things simple. (but do try to keep both batteries as similar as possible)

Note that the amp rating on the Univolt or its replacement is not necessarily the charge current you will get. Fusing is more a function of the wire size and length on the circuit.

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