Will a Battery Isolator relay also protect the Tow vehicle alternator?

[Rob Culver]

Hello all,
I plan to upgrade my TM lead acid battery most likely to a Lithium Lifepo4 200 Amp hour setup. I understand that installing a Battery isolator in the Tow vehicle will prevent the Trailmanor from depleting the Tow vehicles battery when the TM is connected to the tow vehicle when the tow vehicle is not running.
So that's one valid reason to get one. But another valid reason according to the article I have linked is to protect the tow vehicle alternator from burning out by supplying more current to depleted lithium batteries than it can handle.

The lithium batteries I'm looking at could theoretically draw 50 amps from the Tow vehicle when depleted. I believe my Nissan Frontier 2022 can supply 130 amps if it needs to but I need to verify that. So perhaps in my case alternator burnout would be unlikely.

This linked article claimed that the SGP35 500 amp relay would act as an isolator AND limit current from the alternator if needed. But upon investigating the SGP35 they don't make the claim it will limit current so I'm not sure it really does.

I guess my overall question is: Have forum members had issues with alternators wearing out when charging 200 Amp hours of lithium batteries? If so, what was the solution to prevent that?

Search for "Battery Isolator Relay" in this article: https://rvacrossamerica.net/in-an-rv...ad-this-first/ and you'll see a section which claims that the Stinger SGP-35 will protect an alternator from overly strong current demands from the lithium batteries.

[Shane826]

The problem, in theory, is the ultra-low resistance of the lithium battery. The 12V+ wire on the tow vehicle is so small that it creates enough resistance that it’s not really an issue. If you upgrade the 12V+ circuit to a heavier wire on the tow vehicle it drops that resistance down and that can cause alternator burnout. The ideal solution then would be a DC to DC charge controller.

[Bill]

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

[Rob Culver]

Originally Posted by Bill

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 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

Thanks Bill,
10 gauge wire seems inadequate for the task. Looking at the Trailmanor electrical diagram it even showed 12 gauge wire in the charging circuit so I hope that's been upgraded.

Great tip to do your test to see if I already have an isolation relay and I might because I have the tow package.

Thanks again.

[Rob Culver]

Originally Posted by Shane826

The problem, in theory, is the ultra-low resistance of the lithium battery. The 12V+ wire on the tow vehicle is so small that it creates enough resistance that it’s not really an issue. If you upgrade the 12V+ circuit to a heavier wire on the tow vehicle it drops that resistance down and that can cause alternator burnout. The ideal solution then would be a DC to DC charge controller.

Thanks Shane,
It's hard information to come by how they wire my tow vehicle as far as wire gauges and length of cable. I suppose if one got serious about this you might have to upgrade the cable on the TM side too. I'll just have to wait and see how fast my new batteries charge after getting them... Take care.

[rickst29]

You don't need that battery isolator relay at all, you DO need a DC->DC charger.

It almost certainly DOES cut off extremely high current between battery packs of different voltages, but the "trailer battery charge" current (that's one of the wires within the bargman cable) is already limited to far less current: it's fused to "not more than 30A" in most vehicles. (Some large pickup trucks are fused at 50A, but the TM's bargman cable will probably melt if 50A is pulled through that wire for more than a fraction of a second - the wire inside the cable is small.)
- - -
In many modern "12v" vehicles, the under-hood voltage is controlled by an 'ECM' mini-computer. When the engine runs, the ECM order the alternator to produce higher voltage (more than 13.6V) for only a short time: That recharges the battery from the initial "discharge" caused by running the starter

But after that, the voltage will be maintained at only 13.4-13.6 volts. Turn the stereo way up, and the ECM makes the alternator work harder. Turn the stereo and lights off, while driving at high RPMs, and the ECM will make the alternator "relax", producing less power.

On alternators with mere "sense wires", not ECM controlled, it is possible to boost voltage under the hood by putting a bit of resistance in the sense wire. It is easiest to add a cheap diode (rather than an actual resistor), because the voltage drop across a diode is about right for "most applications" (.5 volts). But that will tend to overcharge the truck batteries, and shorten their lifespan.
- - -
In order to reach a relatively high state-of-charge while being charged from a "Tow Vehicle", "12v" LFP battery packs will need higher voltage (than 13.6 Volts) at the battery terminals, and all the L-O-N-G (and thin) wires between the tow vehicle "engine compartment" cause voltage to drop (between the tow vehicle "13.6V" and the TM's LFP battery terminals when current is flowing.

LFP batteries run at higher voltage than lead-acid, a 40% charged LFP is already at almost 13.0 volts. When you try to push more than just a few amps through the long wires from the 13.6 engine compartment, the remaining voltage becomes inadeqaute to push power into the LFP batteries at a decent rate.
- - -
So, you need a device ("DC->DC battery charger" to pull low voltage (from the engine compartment), only while the engine is actually running, and "boost it up" higher near the LFP batteries when those batteries aren't full. The basic Renogy 20A DC->DC battery charger https://www.renogy.com/12v-20a-dc-to...ttery-charger/is cheap and usually adequate, the best one (Victron Orion "Smart" 30A) https://battlebornbatteries.com/prod...dc-dc-charger/ costs more than 2x as much, but I recommend the Orion.

[rickst29]

Originally Posted by Rob Culver

Hello all,
I plan to upgrade my TM lead acid battery most likely to a Lithium Lifepo4 200 Amp hour setup.

The lithium batteries I'm looking at could theoretically draw 50 amps from the Tow vehicle when depleted. I believe my Nissan Frontier 2022 can supply 130 amps if it needs to but I need to verify that. So perhaps in my case alternator burnout would be unlikely.

Both of the DC chargers will pull reduced amps from the bargman cable (in the superior Victron, that's a cellphone-programmable parameter). They also pull nothing at all when voltage on the TBC falls too low.

But your on-board power converter might need to be upgraded, and your "200Ah battery setup" plan might be be an especially good one. Feel free to contact me via PM or post here, with your tentative plans or any questions. I'm way smarter than "rvacrossamerica" about this stuff.

[Shane826]

What’s the tow vehicle? I can probably find the wire gauge… It’s 12 gauge on the TM side according to the schematic I’m looking at.

[Rob Culver]

Originally Posted by Shane826

What’s the tow vehicle? I can probably find the wire gauge… It’s 12 gauge on the TM side according to the schematic I’m looking at.

It's a 2022 Nissan Frontier SV. (with Convenience and Technology package). Those add on packages may not make a difference but they do imply that you have the Tow package also).

Thanks Shane!

[Shane826]

Well, I guess I can’t look it up after all… Nissan doesn’t include wire size on their connector end views. Sorry!