TrailManor Owner's Forum - View Single Post - Will a Battery Isolator relay also protect the Tow vehicle alternator?

**rickst29** · 01-23-2024, 03:45 PM

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.)
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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.
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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.
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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.

01-23-2024, 03:45 PM	#6
rickst29 yes, they hunt lions. Join Date: Aug 2005 Location: Reno, NV Posts: 1,318	That "Battery Isolator Relay" would not provide much benefit. 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. __________________ TM='06 2619 w/5K axle, 15" Maxxis "E" tires. Plumbing protector. 630 watts solar. 450AH LiFePO4 batteries, 3500 watt inverter. CR-1110 E-F/S fridge (compressor). TV = 2007 4runner sport, with a 36 volt "power boost".