Vannin' Home Forums General Discussion Electronics Wiring different interior LED light systems

These simple continuous duty solenoids/ relays/contactors also fail.

The more times they are switched on and off the more the contacts will wear. A circuit which triggers it when the key to on, then disengages it while cranking, and then reengages it after the key is returned to run, cycles it 2x as much as necessary, and once more than required, basically halving its lifespan before failure occurs.


The more amps they deliver into a well depleted battery, the faster the internal contacts will wear.

The most common failure mode of these, the contacts seize together, leading to NO battery isolation with engine off. They will still make the same noise as if operating correctly, but the failure is highly unlikely to be noticed by a casual user who is not looking to see if engine battery voltage also falls when Aux battery is being depleted.


Then the starting battery gets deep cycled with the Aux battery.

Since starting batteries do badly when deeply cycled, they will fail prematurely, and likely take the Aux battery with it too.

So then one needs to replace the Aux battery, Starter battery, and the Solenoid, and this became necessary, prematurely, because the solenoid's contacts seized in the closed position. The cheaper lower rated solenoid/relay/contactor/isolator could certainly be a false economy if/when this occurs.

The Cole Hersee 200 amp continuous duty solenoid I linked is not cheap, and could certainly be overkill, but it has the Silver tungsten contacts which will not only have less resistance, transferring more voltage, thus amperage to the depleted Aux battery, but are also much less likely to wear out and fail, and take out the starting battery through unintended cycling.



I use a 1/2/BOTH/OFF Manual switch rated at 350 amps continuous, when I use a secondary battery, but Since all my charging sources are optimized with adjustable voltage, and I know how much battery capacity I use, I get away with a single 90AH group 27 Northstar AGM battery and leave the switch on battery 2.

The NS battery is now 50+ months old with over 700 Deep cycles on it. Yes it is an Expensive AGM battery, but it has no issues accepting huge charge currents, and I do not require 930CCA, but it does not hurt either. When It was new, it was able to start my engine faster than 3 flooded marine group 27 batteries in parallel could.

As I type this it is 12.2v under a 7.4 amp load with 37Ah removed from it. My alternator at 2000 engine rpm can deliver over 90 amps into this battery when it is depleted to the 50% range or less. The maximum I have seen into this battery, is 112 amps at close to 3K rpm. I'd not be running an 80 amp solenoid, personally though I use batteries pretty hard, to the point they can accept these huge amperages for quite a while before the voltage at the battery terminals hits the maximum that I allow.

Some vehicles do have circuits which become live only after the engine starts. I found a badly oxidized unused spade connector hanging under my dashboard which only becomes live after the engine starts. I employ it to power my modified external voltage regulator. My 89 dodge's stock voltage regulator is internal to the engine computer. I tricked the engine computer into thinking it is still attached to the alternator with a resistor, and use an External adjustable voltage regulator to control my alternator.

I modified this VR with an external 10 turn potentiometer mounted on my dashboard, next to my digital voltmeters and ammeter, and can choose any voltage I want between 12.8 and 15.3v, but too long over 14.7v will trigger the check engine light.

Generally I choose 14.7v any time the battery is not fully charged, and 13.6v when it is, and adjust for battery temperature, more volts when cold less when hot.

About 1/3 the amps will flow into a depleted battery when 13.6v reaches battery terminals, compared to 14.7v.

I determine full charge by the amperage the AGM battery accepts at 14.7ish volts. When it tapers to 0.45A or less at 14.7v it can be considered fully charged, but with enough time amperage will taper to near Zero on this battery. 0.45amps is 0.5% of the battery capacity(20hr rating)

Flooded/wet batteries generally are full when they can accept no more than 1 to 2% of battery capacity, but those really curious can dip a temperature compensated hydrometer and see if the specific gravity in all the cells are all 1.275 or higher. The maximum specific gravity can vary from 1.260 to 1.310 among different battery brands in different intended climates, so having a baseline to compare it to on that specific battery when new and fully charged, is wise, if one cares about getting good longevity from a battery.

Getting good longevity is all about achieving a true full recharge as often as possible as soon as possible after each discharge. Achieving full charge is best done by bringing the battery to 14.4 to 14.7v and keeping it there until it accepts very little amperage.

While huge rechargeing amperages are not nearly as gentle on the battery as lesser amperages, it is harder on a battery to rest overnight at 75% charged than it is at 85% charged.

Lesser $$ AGM batteries generally say to limit chrging amps to 30% of capacity, but the higher $$ and quality AGMS like Lifeline, Odyssey and Northstar Benefit from as high a recharging amperage as can be delivered, keeping voltage below 14.7, when the batteries are regularly subjected to deep discharges.

Slightly depleted batteries are much mroe tolerant of less than Ideal charging currents, but once they are deeply discharged regularly, the charging current and its duration becomes much more important, unless one can afford to just throw money at new batteries or get them replaced under warranty, and this is not inconvenient or is found to be too immoral.

I'd hate to be a battery retailer.
What other product can be destroyed through ignorance and or abuse and replaced free of charge?

I drove my new car into a lake, gimme a new one.
I chronically overdischarged and then undercharged my battery, gimme a new one.

Same thing.

I'm guessing you've taken this conversation as some personal argument, because now you're saying something that's just not true.



It is in no way the "most common failure". I wouldn't go as far as to say it doesn't happen, but I've never heard of one doing this. The alternator in an RV is larger (amps/current) than cars or vans, and they don't have the current to weld those contacts shut. The most common failures are that the contacts get dirty and pitted, and don't make contact, or the coil fails and the relay doesn't even move.

I entered this thread to offer a simple idea. I'm sure you've heard of KISS. In my 60+ years on this rock, I've learned that simple is usually the best way to go. Mucking things up with "more" usually gives way to more things to fail.

Many RV owners trying to recharge batteries while driving,wish the alternators provided were not just the units provided with the base platform vehicle. Very little thought and effort is given to alternator recharging of house banks on RV's. Their wiring is long and thin, they use the frame as a ground yet do not upgrade the frame to engine ground.

I've diagnosed two solenoids failing in the manner I described in person, and two over the phone directing someone with a digital multimeter, and these were not latching solenoids, but the common 80 or 90 amp continuous duty solenoids available at most any autoparts store. whether their contacts wers simply stuck or welded, well I did not bother to cut them open to see. There was simply no difference in continuity when the electromagnet was engaged or disengaged, but they still made the noise like they were working properly.

IN the interests of not bickering, or causing bad blood this will be my last post in this thread.

Good day sir

Nothing kills off a thread quite like "spirited" conversation.