I've looked at the article you mentioned, and I don't see any claim that it will actually limit current. And as you noted, they don't actually claim that it does. I apologize in advance for the techy-mathy stuff below, but it is real.
The text you are referring to reads:
Battery Isolator Relay – The Stinger SGP-35 is a highly reviewed 500 amp device that will protect your vehicle’s alternator from the strong current demand that a depleted lithium battery will cause. If you own a towable and simply connect your 7 pin plug to your trailer, you could damage your vehicles alternator if you don’t use a battery isolator relay.
The first thing to realize is that if you "simply connect your 7 pin plug to your trailer", the wire in the 7-pin cable and plug is size #10 wire. Now #10 will safely carry 30 amps. But if you try to cram 100 amps or 200 amps or 500 amps down that wire, the wire will quickly overheat and melt, opening the circuit, and NO current will flow. In other words, the wire acts as a fuse, shutting off the charge current entirely. And the SG-35 plays no part in shutting off the flow.
The second thing to notice is that there is probably 50 feet of wire in the 7-pin charge circuit. The resistance of #10 copper wire is about 1 ohm per thousand feet (1 milli-ohm per foot), so 50 feet of wire has 50 milli-ohms of resistance. If you run 100 amps of current through 50 milli-ohms of resistance, the voltage drop in the wire is about 5 volts. So if your truck's alternator is putting out 14 volts, only 9 volts makes it to the battery. The rest is lost in the wire. I doubt that your battery has much current acceptance at only 9 volts of charge voltage.
The third thing to notice is that the calculations above assume that your truck has #10 wire all the way from the alternator to the lithium battery. It doesn't. I traced the wire in the factory-installed towing package in my Ford Explorers, and found that at least half of it is #12 or smaller. That means even more resistance in the circuit, and even less available at the lithium battery.
So the SG35 is simply a conventional isolation relay with hefty contacts. It is very expensive, and you could do the same thing with a 30-amp or 40-amp relay for $15 or so from any auto supply store. I have used one for years, and it does just what you would want an isolation relay to do. In fact, your truck may already have an isolation relay as part of a factory-installed towing option, so that is the first thing to check. Just get a voltmeter or automotive test light, and press the probes onto the +12 VDC and Ground pins in the truck's 7-pin connector. If you find 12 VDC with the engine off, then you don't have an isolation relay. But if there is 12 VDC with the engine running, but it disappears when you turn the ignition off, then you already have one.
Bill
|