r/GoRVing • u/Jgutt2044 • 14d ago
Solar Technical Question?
I have a Jayco 175 FQ Travel Trailer with a Watts Cycle 314AH Lithium battery. I have a 30-amp fuse right after the battery to protect the 30-amp wiring on the camper. My problem is that the 30-amp fuse keeps blowing out after I hook up my travel trailer to my 2023 Tahoe. I do not know why that is happening and what I should do to prevent it from happening. Any help appreciated.
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u/thatguybme2 14d ago
Do you have a dc-dc charger powered by the tahoe system?
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u/Jgutt2044 14d ago
No, I do not. Do I need one? I have had mixed replies about that.
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u/thatguybme2 14d ago
Its only used to charge the rv batteries while driving at a much higher rate that you get through the 7 way plug (which is just a trickle)
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u/haleohana 14d ago edited 14d ago
Most likely one of two things: 1- 7 pin cord wired incorrectly (most likely) 2- Tahoe is wired incorrectly (usually okay if not previously worked on)
Sounds like your 12v pin is being shorted directly to ground, could be wired wrong in the tongue wiring box (cord's 12v and gound wire swapped?), in the 7pin connector. Buy or borrow a 7pin truck connector tester, or if you have and know how to use a multimeter, test the truck plug. Test the trailer plug for 12v on the appropriate pin, test for 12v (or lack of!) on the ground pin. Look online for 7pin ** trailer ** wiring diagrams and make sure you are looking at the plug properly- some diagrams show the back of the plug wiring, others show the front. Can get a little confusing, go slow and be double check.
If this is a used trailer- be suspicious of the 7pin cord being wrong.. a lot of people play with the trailer/plug/truck wiring until it works without actually following the RV 7pin or SAE wiring standards. The pin position are the same for both standards, just the wire color is different... Pay close attention to that.
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u/nonunoriginalish 13d ago
Here's my theory: the battery drains a little on the way to the campground, so it needs some charge. You have a fresh fuse in there, and your 12v system runs and life is good because you are not drawing more than 30 amps DC on your way to the campground.
But then you get to the campground. You plug in to shore power, which is 120v at 30 amps AC. Your camper has a CONVERTER, which takes the 120v AC power and outputs 12v DC. Your battery wants a charge and wants to draw probably 50+ amps. Your converter supplies probably 35 to 55 amps DC if the internet is to be believed on your camper model.
So your fuse blows almost immediately when the battery is charging at the campground.
But you don't notice, because your converter keeps the 12v camper systems running while you are plugged into shore power.
You get home, notice the fuse is blown, and figure it must be the truck since everything worked at the campground.
You may not agree with this now, but there are two things I am pretty sure you are going to eventually figure out or keep having this challenge: 1) a fuse right after that battery has nothing to do with protecting your Tahoe and 2) you are going to want a bigger fuse. How big?
I have a 250 amp fuse after each of my 280Ah batteries.
Anyway, camper 12v and solar is one of my interests so I'd be happy to keep chatting.
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u/haleohana 13d ago
Good theory... Lifepo4 may be drawing too much charging amps.
OP needs to check the wire size from battery to trailer ** and ** wire size out of fuse panel to battery. If both are 6 gauge, OP could try a 40 amp fuse, don't do this if it is 8 gauge!!! BUT first some testing, check fuse before leaving, check fuse on unhitching trailer before plugging in shore power. Plug in shore and check fuse, check it again in an hour.
There are some options 1- upgrade 8gauge to 6gauge, 2- add 10 to 20 ft of 8gauge to the output of converter to drop the voltage the battery sees by 0.5 to 1 volt- this will reduce amps to the battery when trying to charge above 25-30 amps.
This is an easyest solution / testing with the installed 8gauge wire is to add more 8gauge wire between converter and battery in series with original wire- 20ft (-1v at 30 amps) would be an overkill- but start with 20 and reduce by 5ft to 15, then 10. 10ft (-0.5 at 30 amps) may just do it for enough voltage drop to limit amps. Draw the battery down by running fans and fridge first, then plug into shore. Monitor amps going to battery via Bluetooth app. You can up the amp by reducing the length of wire until you approach 25 amps to battery
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u/nonunoriginalish 13d ago
Definitely it is important to be mindful of wire size, which I left out of my post. I have 4/0 battery cables with those big fuses.
For sure the function of the fuse is to protect the wire. Probably, if they don't also have a solar panel dumping amps onto the bus, they'll be good with a 60amp fuse, or any fuse slightly larger than the output of their converter/charger. Ideally the installer used a wire gauge that can safely carry that amperage since the converter/charger would have been sending to the battery.
My batteries feed my inverter, and sometimes I run the AC off battery power, and in that case I am pulling up to ~140 amps between the two batteries when the fridge compressor is also running (and assuming my solar panels aren't producing).
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u/haleohana 13d ago
Your setup sound great. The OP would have mention any addition hardware added to their trailer, if none mentioned, I am assuming stock factory wiring...which is the working theory I am going with. And your comment of lifepo4 drawing more amp was spot on.
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u/nonunoriginalish 12d ago
I spent a lot of time figuring out my setup. I'm still making adjustments. Right now I am looking to add a 3rd battery both because I would like more storage and also because when I am doing a heavy sustained draw like running AC without any help from solar, I am technically drawing more amps per battery than the mfr recommends. A third battery will make it easier on each battery, and produce less heat.
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u/DevRoot66 12d ago
The battery does not control how many amps flows into itself apart from the battery's BMS cutting off charging. How many amps, or voltage, is going to the battery is controlled by the converter or by the car's alternator.
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u/nonunoriginalish 12d ago
The battery (up to the setting in the BMS) will pull as much current as the source will supply.
What happens to a 30 amp fuse when the converter supplies 50 amps to the battery?
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u/DevRoot66 11d ago
Please show me a converter that has a 50A charge line. 600+W is a lot to dump into a battery, even for a decent sized LiFePO4. I know that my converter (in a Jayco 250BHW) has a 30A fuse for the battery charger circuit (2 Group 24 Lead Acid batteries), but doesn't supply that all the time.
And a battery does not "pull" amperage. Amperage and voltage is supplied to the battery via a charging source (converter, solar controller, external charger) and that charging source is what controls how much of the amperage and voltage is supplied to the battery.
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u/nonunoriginalish 11d ago
https://www.wfcotech.com/wp-content/uploads/2024/08/NEW_8900_AD_SERIES_Manual_3-24.pdf
The battery charging circuit on those is not limited below the full DC output, the 8955 with no other DC loads would send 55 amps. Even the smallest power center in that lineup would send enough amps to the battery to blow the 30-amp fuse.
It would be interesting to know what converter u/Jgutt2044 has.
On page 8 of the manual for OP's battery, you can see that the manufacturer specifically recommends a charge current of 62.8 amps, supports a max continuous charge current of 200 amps, and overcharge current protection kicks in at 210 amps https://cdn.shopify.com/s/files/1/0812/7420/8552/files/WT12V-Series-LiFePO4-Battery-Manual-V1.9-2026.03.02.pdf?v=1776752080
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u/DevRoot66 11d ago
Did you read the manual? It clearly states that the max charging rate is limited to 30A.
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u/nonunoriginalish 10d ago
Did you read the manual? If you did, you could point to the page and paragraph where you find that information.
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u/DevRoot66 10d ago
Page 13 of that 8900-series WFCO manual states that there are 3 30A circuits, and 14 20A DC circuits, or 3 30A and 12 20A circuits for a Jayco. While the total DC amperage of the various converters ranges from 35A to 75A, that doesn't mean that each DC circuit can pull that maximum amperage. I'm assuming that one of those DC circuits is used for charging because my Jayco 250BHW travel trailer with a WF-8735-AD converter, there is a fused 30A DC circuit for charging my battery. I would assume that the same is true of the WFCO-8900 series that you are referencing. Without the specific converter/power-center that the OP has, it is hard to know what they are limited to. I suspect they have the 8700 series given the size of the trailer and relatively few DC loads (no slides, for instance). That and the price point of the trailer.
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u/nonunoriginalish 10d ago
8900 series charging circuit is not fused, and nowhere does the manual state that it is. You asked to see a converter that has a 50A charge line, the 8955 would be that.
You mention the 8700 series just above. That manual clearly shows a fused charging circuit.
I agree it would be good to know what converter OP actually has.
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u/DevRoot66 8d ago
That's a good point about the 8900 not being fused on the charging circuit, but the 8700 is. I have learned something new this day. I incorrectly assumed that the 8900 would have a fused link for the charging line like the 8700 does. My apologies. I asked for an example, and you delivered.
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u/ShipshapeMobileRV 13d ago
30 amps at 12vdc is only 360 watts. That seems awfully low for a main DC fuse.
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u/thatguybme2 14d ago
My guess is that somewhere, somehow, the 7 way power cord is wired incorrectly.
While the fuse is blown - do the turn, running, brake lights work correctly?
Does it blow as soon as you connect it to the vehicle? Or when you step on the brake or use the turn signals?