Solenoid question

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OrcaSalmonSeaWolf

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Hello all! I am wondering if an 85 amp continuous duty solenoid is the right size for charging my house battery. I am using this to power a fan, lights, and a fridge. Also, will my battery become overcharged with this setup? Do i need a charge controller? Any input would be helpful. Thank you.
 
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I am not sure what solenoid you are looking at but the cheap ones aren't worth the trouble of messing with, IMO. this is the one I like to recommend,

https://www.amazon.com/Cole-Hersee-...e+hersee+24213+solenoid&qid=1605026435&sr=8-3

highdesertranger
 
A 85 amp solenoid is on the low-side. But might work for 35Ah batteries.
A minimum of 125A is better. And will probably work in 35-60 Ah batteries.
But 200A+ is very much preferred.

The challenge about the amp rating for a solenoid, is the in-rush amps. When you first connect a depleted house battery, to a fully charged starter battery - then the voltage difference will want to be equalized in an instant.

So at the time of contact, there  will be a good size spark happening inside the solenoid. At the contact points.
And for the first few seconds the amps will be as high as possible.  And may continue to be at the high end of two digits some minutes.  And then things will settle at a much lower rate, for the reminder of the charge time of the house battery.

But the internal contacts of the solenoid, will need to be able to survive that big initial spark, as the contacts first connect, and 100-170+ amp starts flowing, even if for only 10 seconds.


This takes some serious copper flanges, with some serious contact points, to repeatedly survive that kind of "abuse"/"normal operating conditions".

The next thing is, that the coil needs to be able to hold these contact points together for hours on end, without the coils overheating.


So beefy, in every aspect, is the way to go, when directly connection two batteries only via a relay/solenoid.
 
The battery won't get overcharged but if there is too much voltage drop from the alternator to your house battery it might get undercharged. While charging measure voltage at terminals, it should reach 14.4 volts and stay there for a while to be fully charge. No charge controller needed, the lead acid battery only takes the amps it needs, once it gets closed to 14.4 volts it starts taking less amps.
Some newer vehicles for fuel efficiency, the alternators don't put out 14.4 volts all the time, thats why its good to check the voltage at the terminals.
 
The first "continuous duty" solenoid I bought got quite hot after the coil in it had been on for an hour.  Whatever you get, however much you paid for it, after it has been on for several hours put your hand on it.  Toasty warm is ok.  If you can't grab on and hold it tight comfortably get another one.  If it is too hot to hold on to then it is too hot.  Inside where you can't see there are parts that are too hot for long life.
 
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