Batterty Isolators - Is there a consensus?

I have seen diode isolators with 3 output terminals, two big, one small.

Here is how they work

The big ones go to battery one or battery two.. The small one to the voltage sense terminal on a voltage regulator.. Thus automatically adjusting for it being there.

The problem.

Dang few alternators have a voltage sense lead now days.

A solid well built 100% Duty Cycle mechanical solinoid with at least 200amp continous current rating (coil current is small enough to not worry about) is my choice any day over a solid state FET device. I too have had several relays last 20 plus years come to think of it never had one fail.

Mex have you taken the defective ones apart?
What caused them to die?
Did the connector assembly stop turning as they should?
Dampness rust excessive heat etc can kill them

KJINTF wrote:
What caused them to die?

That's my question. I assume Mex wants me to measure voltage drop across the relay contacts to see how bad they have gotten in 40 years. I'll do that eventually, but I pulled my batteries for the winter last weekend.

What voltage drop would be considered too much? At what charge amperage?

I don't like to let the house batteries run down for long, so while camping, I tend to keep the charge up with gen charging. I'm optimized and balanced so that I can usually get back to 90-95% SOC every couple of days with normal gen usage to run the WH and the microwave.

I tend to think of the constant duty relay as primarily serving the purpose of making it easy to start the engine by adding in the RV batteries, not as a means to recharge the RV batteries while driving.

wa8yxm wrote:
Dang few alternators have a voltage sense lead now days.

Mine does, but then again, it's not a "now days" design - it was installed in 1972 (or rather, its great, great grandaddy was :) )

Yes I autopsied too many to count. Drilled out the spot welds holding the lid to the can. Many died because they were junk to start with. A quick and dirty lesson. Solenoids must use a spring to pop the contact disc (copper) away from the two stud contact points QUICKLY after the solenoid is de-energized. If the disc lollygags it's going to get burned all to hell. OK a strong spring means a strong magnetic field is needed to overcome the spring tension then clamp the disc hard against the two copper studs.

A lot of magnetic energy is needed. Not just amps. A solenoid can be wound two ways. One is with fewer turns of lighter gauge magnet wire the second is with a lot more turns of heavier gauge magnet wire. Both may yield a resistance of let's say 7.0 ohms when measured with a meter. But electromagnetic coil winding function is a measure of AMPERE TURNS. Turns of wire. A "heavily wound" magnet coil may draw the same amount of amperes but have TWICE the magnetic pulling power. Yes, longer wire means more ohms (a higher resistance) but what happens when that "more wire" solenoid uses a larger gauge wire...the resistance difference is offset.

Think of a cheap solenoid as being "A Loose Connection In Sheep's Clothing".

Too many amps (watts) will overheat and burn out a solenoid coil winding. So this is why a Ford Type starter solenoid can carry 400 amperes - for 1 minute.

And too many amps will melt a solenoid's power contact disc and stud contact points. If the "connection" is loose as a goose because of a poor magnetic field then the solenoid went into my dumpster.

I found out the hard way that even small relays like the BOSCH (now TYCO) have many worthless junk copy-cats. Comparing a Bosch relay to a counterfeit is like comparing a Rolls Royce to a Yugo.

The newer "Tower Type" solenoids like the ESSEX are far superior to the earlier can type solenoids. They run cooler, use less current, pass more current safely and last longer.

Unless a Field Effect Transistor type power relay is manufactured incorporating a host of protective circuits (over, under voltage, transients, short circuit, reverse polarity and thermal shutdown, it cannot be as reliable and rugged as a mechanical relay. No way in hell. The insane amount of transient voltage spikes generated when a starter motor solenoid collapses the field inside a starter motor is the bane of any electronic device. DO NOT DO THE FOLLOWING! If a person should be touching a starter motor battery post connection when the ignition key is relaxed, the resultant transient voltage will knock them on their ass. It is far worse a jolt than a spark plug shock. It is not the POSITIVE transients that do damage in a starter motor circuit. It is the negative transients, and guess what...the ground side of a starter motor is not switched.

Interesting reading here and perhaps someone can suggest a proper replacement for the battery isolator solenoid that came with my Winnebago Minnie. In the ten years we have owned this MH, I have replaced the solenoid twice, with the same model obtained from Winnebago. Last time was only a year ago, and already I'm having problems again. The problem I have is that the solenoid doesn't always pull in when the ignition is turned on and the engine running. With the ignition on, I get voltage at the coil terminal, but no action from the solenoid. I can see the house battery voltage readout on our inverter display when sitting in the driver's seat, and sometimes after driving over some rough road, I can see that the solenoid has pulled in so that the alternator is then charging the house batteries. After the solenoid pulls in the first time, I can use a jumper with the ignition off to connect the coil terminal to one of the heavy battery terminals and the solenoid pulls in and drops out normally, with a loud clunk in either direction. A day or so later I can't count on the solenoid working though. It seems that it gets stuck after sitting for a while.

The Winnebago supplied solenoid is a Trombetta #936-1215-011-21. Anyone have a suggestion for a more reliable replacement?

MEXICOWANDERER wrote:
...Unless a Field Effect Transistor type power relay is manufactured incorporating a host of protective circuits (over, under voltage, transients, short circuit, reverse polarity and thermal shutdown, it cannot be as reliable and rugged as a mechanical relay. No way in hell. The insane amount of transient voltage spikes generated when a starter motor solenoid collapses the field inside a starter motor is the bane of any electronic device. DO NOT DO THE FOLLOWING! If a person should be touching a starter motor battery post connection when the ignition key is relaxed, the resultant transient voltage will knock them on their ass. It is far worse a jolt than a spark plug shock. It is not the POSITIVE transients that do damage in a starter motor circuit. It is the negative transients, and guess what...the ground side of a starter motor is not switched.

The newer FET designs are very robust, as are all of the electronics in newer vehicles.

I have had FET battery Isolator devices for years in a few different campers and never had any issues - no contacts to burnish...

CloudDriver wrote:
The Winnebago supplied solenoid is a Trombetta #936-1215-011-21. Anyone have a suggestion for a more reliable replacement?

Mex thinks I have the Delco 11880 solenoid with silver inlay contacts. I suspect he's right, but all I really recall is that it was Delco continuous duty. Mine has lasted 40 years, unless my father replaced it in the first 10 years of RV use.

ken white wrote:
I have had FET battery Isolator devices for years in a few different campers and never had any issues - no contacts to burnish...

The cost was $170-$200 for a suitable FET isolator, when I checked. I can replace a lot of mechanical solenoids for that price, particularly when mine has been running for 40 years.

DryCamper11 wrote:

ken white wrote:
I have had FET battery Isolator devices for years in a few different campers and never had any issues - no contacts to burnish...

The cost was $170-$200 for a suitable FET isolator, when I checked. I can replace a lot of mechanical solenoids for that price, particularly when mine has been running for 40 years.

Depends on what you need, or want, and are willing to pay - BTW I did not pay anywhere near your estimates.

If you are happy, then stick with solenoid technology.

I opted for lower losses and automatic operation, YMMV.