How to pick a battery isolator?

[protryon]

So I have a 225 Ah battery bank that I'm charging via solar, but I want to add a isolator as a backup power source. I get the concept behind them, but how do you pick a size? I don't want to send my entire alternator's current to my batteries because that'll charge them much too quickly. (130 Ah rate on a 225 Ah bank.) This raises the question: How does my 45 Ah car battery deal with this?

 

[Optimistic Paranoid]

Well, to answer your last question first, the voltage regulator - or in the case of modern cars, the engine computer - monitors the engine's battery's voltage and cuts the alternator output back to prevent overcharging the engine battery.

As far as sending 130 amps to the house battery, remember that the principle function of the alternator is to run the engine and accessories.  Ignition system, electric fuel pump, heater/ac blower motor, headlights, windshield wiper motor, etc., etc. etc. 

The only current your house battery will see is what's left over after all of those things take their share.

I would guess that at night, in a storm, with the headlights, heater, and wipers all running at once, there wouldn't be a lot left over for the house battery.

The other factor is the size of the wire you run back to the house battery.  The fatter the wire, the more power will flow. 

Most people here just pick a 100 amp continuous duty solenoid and call it good.  I'm sure considerably less than 100 amps will flow through it.  Exactly how much is something you'll have to determine by measuring it with an ammeter while it's running.

 

[John61CT]

Depends on the circuits design and placement of the unit.

Just like a wire, needs to be sized 15% higher than the greatest current that will ever flow through it.

So max alt output, unless maybe cranking starter through that circuit, which will be higher.

For the latter find out what the 5 second rating is, will be higher than the continuous one.

For a largish system 300-500A is a lot safer than 100A.

If you are following best practice and putting all charge sources including Alt direct to the bigger House bank, and only topping up the smaller Starter through the combiner, 15-20A is enough.

In that case have a look at Xantrex (ex-Heart) Echo Charger.

 

[user 37446]

Read the Amazon reviews and descriptions of various ones. The bigger the amperage, often the larger diameter studs to attach the wire terminal fittings, as well as the better quality materials. For an example, while a Cole-Hersey 200amp costs 3x a cheapo 75 amp, the studs materials and diameter are much superior.

 

[highdesertranger]

exactly Twih, that is the one I prefer,

https://www.amazon.com/Cole-Hersee-...3_1?ie=UTF8&psc=1&refRID=STHAZR80A9V37N69GE4X

highdesertranger