Power Supply/Battery Charger

WFCO comparison

Univolt II & WFCO Converter size comparison

I like Airstream’s original equipment Univolt.  But I also appear to like doing things the hard way like lighting pilot lights & winding clocks.  Sure, the modern day power converters do a fine job of powering 12 volt circuits & keeping the battery properly charged.  But I enjoy having a “say” as to when & how much my battery is charged.  And the Univolt’s hum is not THAT loud…biggrin

My Univolt was conveniently located under the bathroom sink.  While reinstalling it during the refurb, the opportunity was taken to install a simple, external ON/OFF switch.  When camping on shore power, the Univolt was turned off unless an electric motor was running, or a lot of lights were on.  Between the switch, and the original analog battery minder, I never had any battery problems.

But alas, after our first two seasons in the Airstream, I found that my wife did not share my enthusiasm for constantly monitoring the electrical system.  So, for Christmas, I asked for, and received a WFCO, 35 amp, Converter/Charger from bestconverter.com to replace the Univolt.  Charging system topics include:


How the New Power Supply/Battery Charger Was Chosen

ThoughtProcessWhile I did not mind keeping an eye on my Univolt, my mind was made up long ago that if or when it was replaced, the new unit would be one capable of operating unattended by anyone (even me).  Face it, batteries & battery chargers have been around a long time, and keeping a battery both charged, and healthy should have been a hurdle cleared long ago.  Not necessarily in 1967, but certainly within the last decade since microprocessor controlled circuits are in just about everything nowadays.

Casually monitoring news groups during the refurbishment, the Intellipower with Charge Wizard appeared to be the best choice.  I was, however, surprised that it still had an override button if operator intervention was desired.  Randy, the proprietor of bestconverter.com, is a contributing member at Airstream Forums, and judging by his answers to a variety of 12 vdc questions, appeared to be quite familiar with the goals of successful, self-sufficient trailer operation.

Around the time Univolt replacement was seriously considered, Randy started a thread recommending the WFCO Converter/Charger explaining that, although the Intellipower is a fine unit & he would still continue to carry it, the WFCO unit was proving to be a better choice.  So I hit his site, studied the two units’ operations, and concluded for myself that the WFCO unit was indeed a better choice.

But which size?  My Univolt was rated to deliver up to 30 amps.  The WFCO unit is sold in five categories ranging from 35 to 75 amps output current.  Is more always better?  Making the small jump from my 30 amp unit to the closest equivalent unit offered was no big deal.  But, since I had the opportunity to dramatically upgrade, would more output current do anything good for me?  What would more power hurt?

My Overlander’s DC power grid, from it’s single battery forward, consists of 8 AWG wire running through 40 amp fuses powering three, 20 amp circuits protected by self-resetting breakers.  The Univolt, as it was in a remote location, was connected to the system (between the battery fuses & circuit breakers) by approximately 15 feet of 8 AWG wire.

Good design dictates that branch circuits should not pull in excess 85% of their rated capability.  Applying this to what I saw means that the distribution grid is capable of safely handling 51 amps.  With the Univolt delivering 30 amps, the battery makes up the difference if/when more power is needed.

Vandergraff0So was the Univolt meeting my present power needs?  How much power was projected to be needed?  At a campground with electrical hookups (a must for us at present), we seldom have more than a couple of light bulbs on, and prefer to run the AC powered air conditioner in lieu of vent fans.  While the TV/VCR combo & DVD player will run on 12 vdc, normal operation finds them plugged in to AC.

The AM/FM radio, sound system, and CB radio are hardwired into the DC grid, while the cell phone is plugged in as required.  None of these devices pull that much current though.  A laptop computer may one day make it’s way to the kitchen table, but neither it nor possible, future rechargeable kid’s toys will pull significant power.  All-in-all, the amount of DC power used on a typical camping has been well within the means of the 30 amp Univolt, and I could not see an appreciable increase happening anytime soon.

A high output charger would have it’s benefits charging a bank of batteries after boondocking.  At present, though, we do no boondocking.  In fact, the primary reason I have a battery in the Overlander at all is to power the brakes in case of breakaway.  Other than running the water pump occasionally while on the way to a campground when a public restroom is not convenient, the battery is not really needed.  While the battery did supply filtering for the Univolt, a bank of capacitors did the same thing for me during the refurb.

MiddleOfNowhere1There is a realistic possibility, though, of the Boyz & me boondocking in the future, and the thought of upgrading for this occasion crossed my mind.  Although the present wiring between the charger & distribution point was not rated for the task, it could be upgraded to carry more current.

But I am uncomfortable with letting one big charger maintain two or more batteries.  Charging is fine, but unless the two or more batteries are in the exact same condition, I do not feel the charger’s microprocessor will necessarily be able to determine the best maintenance voltage to deliver to more than one battery.  An example would be two batteries wired in parallel, and one cell of one battery shorts out.  Since this would lower the overall voltage output, the microprocessor would think the batteries need charging, and boost it’s output.  Unless detected soon enough, damage to the good battery could result.

Let’s say the charger could act accordingly hooked to multiple batteries.  My Overlander carries it’s one battery inside the trailer underneath the street-side bed.  Although there is room for another battery, for a variety of reasons there are no plans to fill that available space with a battery.  The plan will be to carry an additional battery(ies) in the tow vehicle, and connect them through the trailer’s electrical umbilical.  When not camping, the battery would sit in my shop alongside the batteries for my other toys, and be maintained with a trickle charger.  With this scheme a bigger charger would not benefit my camping activities.

But to make sure nothing had been overlooked, I contacted Randy and outlined the above.  He agreed that the 35 amp WFCO unit was adequate for my present needs.  With his confirmation of suitability, the WFCO model WF-9835 was asked for, and received as a Christmas gift from my in-laws.

In early Overlander production runs, Airstream had the Univolt mounted near the battery.  Probably in response to customer complaints about hum while trying to sleep, it was moved to underneath the bathroom sink.  Due to the Univolt’s size & dead-space-for-ventilation requirements, it pretty much dominated the space as far as storing anything like towels.

Since the WFCO unit is extremely quiet, thought was given to locating the WFCO unit next to the battery, but a warning label on it’s cover advised of internal components that could spark on occasion – Not a good thing if the battery was busy off-gassing hydrogen.

Deciding to keep the new charger under the bathroom sink, the only modification necessary was to add a 120 vac outlet.  The Univolt had been hard-wired whereas the WFCO unit came with a cord attached.  No big deal there other than purchasing a Cu/Al  receptacle to work with my Overlander’s aluminum wiring.  The small size & fan-cooled operation of the new unit allowed for much better placement, and freed space for storage of bathroom items.  Worth mentioning is how much lighter the WFCO unit is in that there is now less cantilevered weight to stress the trailer aft of the axles.

So far so good;  The WFCO charger appears to be working as advertised.  This page will be updated should anything change.  [back to top]

The Univolt


A Univolt is an amazingly simple device: A big transformer to step down the voltage, diodes to convert AC to DC, and depending on the version, either capacitors to tune the circuit or a Silicon Controlled Rectifier to…do something.  The Univolt schematics I have seen do not regulate the voltage they deliver. They were designed to output one single voltage with as much current as available.

Univolt II schematic cropped   Univolt I schematic cropped 

My Overlander came with a Univolt II manufactured by Newmark Products Inc.  The left-hand schematic above is my interpretation of what was revealed after the case was cracked open.  Not understanding what was in front of me, the schematic was shared with three different electrical engineers, and posted on a circuits forum.  Nobody seems to understand what the SCR is doing for the circuit especially since neither D3 nor D4 are Zener diodes.  The “13.5 vdc @30 amps” declaration is what is stickered on the case.  Although all the individual components tested good, i.e. the device is operating as designed, the device’s output was more like 14.2 vdc even under load.  If you have insight into the circuit’s design goal or would like more as-measured values, please drop me an email, and I can provide more details.

[Update 01/23/10: Reader Cliff B advised me, “FYI diode D-3 in your Univolt II (Sencor LMZX-13A) is a 13V zener diode @5% tolerence it is regulating diode that sets the reference voltage to SCR1 (D-4 is a reverse voltage protection diode for the SCR's gate).”]

Newmark Products Inc. went out of business so long ago that Google has never even heard of them.  As a consequence, it is unknown to me how long the Univolt II was used.  To be quite honest, the Univolt II’s circuit surprised me as, after researching a lot of Airstream material, I was expecting to find the ferro-resonant circuit depicted above.  The ferro-resonant circuit is cool because it will continue to deliver full current, with no adverse affect to the transformer, even if the output terminals are shorted together. cool    BTW, ferro-resonant basically means there is a capacitor (C1) hung off a tap on the output windings to tune the circuit.  This type of power supply can operate without C2 & C3 as they are there for noise suppression.

Some people appear to think that a battery must be present for the circuit to be properly tuned.  This is incorrect.  The battery serves to filter, and smooth the Univolt’s output, but is not required for operation.  However, your 12 volt lights will be dim, and your radio will have a lot of static in the absence of filtering & smoothing.  During the winter refurb, I used to keep a bank of old electrolytic capacitors out of my toy box wired up to act as a “battery simulator”.  The capacitors did all the filtering & smoothing, and allowed me to keep the battery in plain view in my Shop so I didn’t forget to check it every so often.

Update 01/11/13: Reader Art L wrote:



I own an early-80's-vintage Newmark Model DCV 40 R-A-C Power Converter and originally ran across your page a couple of years ago while looking for Newmark service literature.

Although they look somewhat similar, my Newmark is rather different. But your Univolt II was about as close as anything I could find. Since then, I've had several failures and replaced both transistors and the SCR series resistor but I never could find a schematic or any component information.

I finally decided to reverse-engineer a schematic and identify the parts so I could make sure it was operating correctly -- particularly since I had to select replacements for the unmarked transistors! The good news is that it now works great and smoothly proportions the battery charge right down to about 10 mA.

If you happen  to have (or know the whereabouts of) a converter like mine, that would be great since I could use some help confirming the identity of several parts with missing and smudged markings…

Art L

Art went on to include:

As you can see in the photo, the project's not quite complete yet. One remaining issue is the SCR resistor: this is one of the values I could learn from another operating converter.

The old resistor burned out, taking the markings with it. As best I could determine from measuring the burned-out pieces was 2 ohms. I ordered a 2 ohm replacement before I started analyzing the circuit but that hardly sounds right at this point. (2 ohms & 5 amps doesn't leave much for the battery!)

The test resistor I used is 0.5 ohm and gave me a maximum charge of only 3.3 to 3.6 amps into a fully discharged battery. I have some 0.25 ohm resistors on hand that should give something closer to the 5 amps the label specs.


During the charging process, I saw some oscillation in the gate signal. A small, 0.1μF, capacitor upstream stopped it.



  [back to top]

Speaking of battery charging

My '67 owner's manual says that the Univolt has a 100% shut off and that it is impossible to overcharge the battery. I will give Airstream the benefit of the doubt, and assume that some Univolts had a circuit breaker which would trip to effect "100% shutoff". The Univolts I have seen have no "smarts" in that they could care less if your battery is boiling, simmering, or even there.  A Univolt’s job was to supply roughly 13.5 vdc with as much current as possible.  I put it that way because, the voltage will drop as the load on the Univolt increases.  If you are actively camping and have a bunch of lights & fan motors going, the voltage difference between the battery & the Univolt will be small. In fact, the battery may even help the Univolt out if you have A LOT of stuff on.

A battery is charged by applying a DC voltage to the battery that is higher than the voltage the battery currently has. The higher the voltage difference, the greater the charge rate will be. I believe a fully charged battery will read 12.9 vdc on a volt meter.

Charging a battery is an electro-chemical reaction. As the battery charges, tiny bubbles of hydrogen gas & water vapor form on the plates, and eventually bubble off. These bubbles eventually escape from even sealed batteries. The higher the charge voltage, the more bubbles that are formed. It is this bubbling that is commonly referred to as boiling although it does not occur the same way as water boiling in a tea kettle.

But, with no lights or anything but the Univolt on, the battery will be subjected to the maximum voltage the Univolt can deliver. It will attempt to charge. After a while, the water will bubble off.  [back to top]