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

[larsdennert]

The 120V converter will also need to be able to supply 14.6V to charge Li. Some WFCO ones have a sense circuit. Most tow vehicles isolate the Bargman charge lead when the truck is not running.

[rickst29]

Originally Posted by larsdennert

The 120V converter will also need to be able to supply 14.6V to charge Li. Some WFCO ones have a sense circuit. Most tow vehicles isolate the Bargman charge lead when the truck is not running.

Charging Lithium "TM" batteries from the 120-VAC converter board has been addressed in some other Threads, I hate to see that separate topic introduced to this "charging from the tow vehicle alternator" discussion.

Some "Lithium" cell types use other chemistry, allowing them to handle higher voltage. But Lithium batteries in TM Trailers should be built using "lithium-ferro-phosphate" cells, which are less likely to catch fire and generally provide longer lifespan. Each "12v-compatible" battery pack contains 4 cells in series, and 14.6 volts of charging voltage subjects those cells to an average of 3.65 volts each.

That is the absolute maximum voltage to which a single cell should EVER be charged. Such a high voltage should be reached only once, for capacity testing (and I generally recommend not to bother with that, even one time).

The battery BMS should be be programmed to prevent current from reaching the batter pack when the sum of voltages (n the battery) has reached more than about 13.45 Volts, roughly 96% charged.

(1) It not useful to apply 14.6 volts into the "12 Volt DC" at all times (when the 120-VAC cord has been plugged in). It would be more efficient and better for the Converter's electronics to shut down the Conversion circuits, only re-activating them when voltage testing on the DC side (voltage supplied by the battery) has somewhat lower than 13.45 volts.
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For driving current into "12v" LFP battery packs, virtually nothing is accomplished by attempting to supply more than 14.2 volts anyway. (An "attempt" to reach just 14.0 volts will likely utilize all of the current which a low-powered charger can supply during "bulk" charging, before reaching that voltage, because the internal resistance of these battery cells is very low).

I have my own Converter set to see my Lithium batteries as "AGM/GEL", and that is probably the best battery-type setting to use with non-programmable controllers. To the extent that forcing a non-programmable Converter to resume "bulk" mode" when the battery is lower than I'd like, I have a programmable a Solar controller to take care of the last few % of capacity (ending no higher than 97% full, NOT near 100% full.)

With my older model "WildKat", a modified non-programmable PD controller, I'm not sure that the charge wizard push-button "mode switch" can actually push it past some non-programmable and hidden parameter, "minimum voltage at which to resume bulk charging from float". Some times it seems to work, other times it doesn't.

Some Victron Converters are are programmable, but they tend to be either lower-power or very expensive.

[Rob Culver]

Originally Posted by rickst29

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


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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.

I'm toying with the idea of putting in a DC to DC charger like the non isolated Orion one that Rick suggested. But is there any possible way to put this charger on the TM side using the existing 12 volt input from the 7 pin connector as the input in to the charger? That seems tricky due to the need to supply 12 volts to the break away line. Has anyone succeeded in putting their DC to DC charger on the TM side rather than the tow vehicle?
Thanks in advance..

[rickst29]

Originally Posted by Rob Culver

I'm toying with the idea of putting in a DC to DC charger like the non isolated Orion one that Rick suggested. But is there any possible way to put this charger on the TM side using the existing 12 volt input from the 7 pin connector as the input in to the charger? That seems tricky due to the need to supply 12 volts to the break away line. Has anyone succeeded in putting their DC to DC charger on the TM side rather than the tow vehicle?
Thanks in advance..

That charger should be installed in the TM, and preferably close to the TM batteries (electrically speaking). The Trailer battery charge wire has a 3-way junction following the break-out of individual wires from the large bargman cable end (behind the fridge). One end leads to the 12v fuse board. In original wiring, the battery cable is also connected on that board. The other end leads to the break-away switch, allowing emergency brakes to be powered by either of those 12v sources (the TM batteries OR the bargman 12v "TBC").

But that wiring allows backflow when TBC side is lower voltage than the boosted DC->DC output voltage. To solve that new issue, You should add a 5-way relay, favoring TM battery power when battery power is present, and switching to connect the bargman TBC directly if/when battery power fails. output common goes through the break-away switch.

[Rob Culver]

Wow! Thanks for that comprehensive answer. I take it you implemented that exact solution in your TM? Do you have a specific part you can recommend for this 5 way relay? And does that mount in the area behind the refrigerator I presume? Thanks much...

[rickst29]

Originally Posted by Rob Culver

Wow! Thanks for that comprehensive answer. I take it you implemented that exact solution in your TM? Do you have a specific part you can recommend for this 5 way relay? And does that mount in the area behind the refrigerator I presume? Thanks much...

My own TM has something much more complicated and expensive (which you shouldn't try to duplicate).

A 5-pin "Bosch style" 12v relay can be bought at any car parts store, or Amazon, or eBay) but you also need a socket (converting the Relay "pins" into easily connected wires. Her'es an Amazon listing for both items:
https://www.amazon.com/Augeny-Car-Re.../dp/B0CNLBW59N

The Relay can go behind the fridge. After breaking the 3-way junction of the TBC wire, the wire which goes down, out and forwards to the breakaway switch should be connected to relay pin 30 (power "common", the fat wire with light orange insulation in the harness from Amazon).

The thin TBC lead from the bargman cable needs a new "splitter" junction. One downstream wire connects to relay pin 87. That pin is connected with pin 30 when battery power fails, and the relay coil magnet loses power. The other "downstream wire" is input 12V to the DC->DC charger.

Because the back of the refrigerator gets very hot, I would not put the DC->DC battery charger in that location. So that second "downstream wire", which is new, goes somewhere else to reach the DC->DC charger unit. (maybe under the sink, or near the WFCO, or maybe alt the way back to nearly reach the batteries). Near the batteries is best. This wire does not connect the WFCO 12v fuse board anymore, it connects only as "input" to the the DC->DC boosting charger unit.

The "+12V output" of the DC-to-DC boosting charger should go a short distance, reaching a 12V battery connection. (it's usually operating at more than 12v, of course.)

This leaves one unused and loose wire end behind the fridge. (It used to go from the WFCO 12v fuse board into the old TBC junction.) it should now be connected into another "spliiter" junction, connecting both pin 87A (the big blue wire on Amazon's socket) and pin 85 (the thin yellow-insulated "12v" wire for the coil.

The thin white wire of the Relay base (the coil "ground") be be interconnected to any other 12v grounding wire or grounding lug behind the fridge.

[Rob Culver]

Originally Posted by rickst29

My own TM has something much more complicated and expensive (which you shouldn't try to duplicate).

A 5-pin "Bosch style" 12v relay can be bought at any car parts store, or Amazon, or eBay) but you also need a socket (converting the Relay "pins" into easily connected wires. Her'es an Amazon listing for both items:
https://www.amazon.com/Augeny-Car-Re.../dp/B0CNLBW59N

The Relay can go behind the fridge. After breaking the 3-way junction of the TBC wire, the wire which goes down, out and forwards to the breakaway switch should be connected to relay pin 30 (power "common", the fat wire with light orange insulation in the harness from Amazon).

The thin TBC lead from the bargman cable needs a new "splitter" junction. One downstream wire connects to relay pin 87. That pin is connected with pin 30 when battery power fails, and the relay coil magnet loses power. The other "downstream wire" is input 12V to the DC->DC charger.

Because the back of the refrigerator gets very hot, I would not put the DC->DC battery charger in that location. So that second "downstream wire", which is new, goes somewhere else to reach the DC->DC charger unit. (maybe under the sink, or near the WFCO, or maybe alt the way back to nearly reach the batteries). Near the batteries is best. This wire does not connect the WFCO 12v fuse board anymore, it connects only as "input" to the the DC->DC boosting charger unit.

The "+12V output" of the DC-to-DC boosting charger should go a short distance, reaching a 12V battery connection. (it's usually operating at more than 12v, of course.)

This leaves one unused and loose wire end behind the fridge. (It used to go from the WFCO 12v fuse board into the old TBC junction.) it should now be connected into another "spliiter" junction, connecting both pin 87A (the big blue wire on Amazon's socket) and pin 85 (the thin yellow-insulated "12v" wire for the coil.

The thin white wire of the Relay base (the coil "ground") be be interconnected to any other 12v grounding wire or grounding lug behind the fridge.

Thanks very much Rick for those excellent instructions. Looks relatively straight forward as long as you can be sure which wire is which and can run the new wire perhaps all the way back to the battery compartment. Thanks again. Much appreciated!

[Wavery]

I've been running a 230ah LiFeP04 battery in our TM for almost 2-years now and well over 10K miles. I don't have any devices between our TM and the TV (2012 Silverado). I have never had an issue with the TV alternator (crappy AC Delco alternator).

Having said that. I have solar on the TM and I never let the LiFeP04 battery get below 80% while in storage (I store the trailer in a carport). Before we leave on a trip, I leave the TM in the Sun for a few days to bring the battery up to 100%.

I think that most of the people that say that you need some device to protect your alternator have something to sell. Before anyone posts, "Oh YA!!!! my alternator burned up when I switched to LiFeP04"........ alternators fail all the time, lithium battery of no lithium battery. So a burnt out alternator is little "proof" that the battery had anything to do with it.

[Rob Culver]

Originally Posted by Wavery

I've been running a 230ah LiFeP04 battery in our TM for almost 2-years now and well over 10K miles. I don't have any devices between our TM and the TV (2012 Silverado). I have never had an issue with the TV alternator (crappy AC Delco alternator).

Having said that. I have solar on the TM and I never let the LiFeP04 battery get below 80% while in storage (I store the trailer in a carport). Before we leave on a trip, I leave the TM in the Sun for a few days to bring the battery up to 100%.

I think that most of the people that say that you need some device to protect your alternator have something to sell. Before anyone posts, "Oh YA!!!! my alternator burned up when I switched to LiFeP04"........ alternators fail all the time, lithium battery of no lithium battery. So a burnt out alternator is little "proof" that the battery had anything to do with it.

Hi Wavery,
Do you know what kind of charge rate you're getting from the Tow Vehicle to the TM in amps? I'm sure that would depend on how depleted the lithium battery was to start with. I assume also that in most cases the alternator would not provide high enough voltage to top off the last 10 to 20% on the battery so that's another advantage of having a DC to DC charger.
I haven't tried mine yet but if the tow vehicle can provide 15 amps or so then it might not be worth it to have a charger if it charges only at 20 to 25. And probably wouldn't want to charge faster than that because then it would require an upgrade on the wiring in the tow vehicle, TM, or both.
So do you know what rate your Tow vehicle charges up your TM batteries?
Just curious.. Thanks.

[rickst29]

Although my earlier post #16 (https://www.trailmanorowners.com/for...5&postcount=16 described how to wire a Relay, powering the brakes from the bargman "TBC" pwer source when the on-board batteries fail, it is my opinion that the addition of that feature is not worth the trouble.

I offer a "wild guess" estimate that 99% of hitch connection failures (with the Trailer falling loose from the Tow Vehicle while traveling), the bargman cable will have ripped loose from the Tow Vehicle at the same time as the hitch.

Power from the on-board batteries will power the brakes when the breakaway cable fails. But in the case of the no voltage from the on-board batteries AND breakaway cable ripped out, the TBC wire probably has no power either. (The relay will have switched over, but the TM brakes wil not be activated.)

Rob, if you choose to skip the relay - I don''t disagree with that choice. (Even though RIAA woul not approve of such "missed" wiring in a new trailer.) i do NOT have such a relay in my own TM.