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Old 07-03-2022, 06:45 PM   #1
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12 volt power not working - need help

Microlite 25BRDS

Accidentally hooked up battery backwards. Blew both 40 amp reverse polarity fuses.

Hooked up battery correctly. Replaced 40 amp fuses. 12 volt stuff still won’t work off battery.

Put on new battery.
Checked all fuses - all good.
Found reset switch on front of trailer where wires come together before going into trailer. Pressed button. Still not working?

Opened electrical fuse panel and converter is hard wired (no plug in). Checked black wire when plugged into shore power and it is “live “.

Any ideas? Did we damage the converter?
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Old 07-03-2022, 06:49 PM   #2
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Check the 40Amp fuses again. Their purpose in life is to protect from reverse polarity. Once battery wiring is correct, and fuses replaced should be OK.
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Old 07-03-2022, 07:19 PM   #3
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Check the 40Amp fuses again. Their purpose in life is to protect from reverse polarity. Once battery wiring is correct, and fuses replaced should be OK.
40’s look to be good after replacing and putting in new battery with wiring correct……
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Old 07-04-2022, 08:45 AM   #4
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On shore power the converter should power all the 12V stuff even with no battery. If that's not the case then I would check the reverse polarity fuses and converter output.

At the same time if the battery is charged and installed, it should power all the 12V stuff even with a bad converter/fuse.
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Old 07-04-2022, 08:51 AM   #5
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Originally Posted by Michfan1970 View Post
Microlite 25BRDS

Accidentally hooked up battery backwards. Blew both 40 amp reverse polarity fuses.

Hooked up battery correctly. Replaced 40 amp fuses. 12 volt stuff still won’t work off battery.

Put on new battery.
Checked all fuses - all good.
Found reset switch on front of trailer where wires come together before going into trailer. Pressed button. Still not working?

Opened electrical fuse panel and converter is hard wired (no plug in). Checked black wire when plugged into shore power and it is “live “.

Any ideas? Did we damage the converter?
you say 12 volt stuff does not work off battery. is this with the shore power cord plugged in or not? does the 12 volt stuff work when the shore power cord is plugged in?

i have no idea what the reference to the live back wire means? is this a 12 volt dc cable or a 120 bolt ac cable.

it was stated in an earlier post that the 12 volt stuff should work off the converter when plugged into shore power. i'd confirm this first. if the 12 volt stuff is working off shore power it means you converter is working.
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Old 07-04-2022, 09:26 AM   #6
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1. Verify your new battery is connected properly. Meter or test light between the positive post and the trailer frame (a convenient bolt) should slow current.

2. If the 12v stuff works on shorepower the converter is fine.

3. Check the 40a protection fuses with a meter -- they're probably fine but invisible breaks can occur.

4. Some trailers have a battery disconnect switch. Verify it's On and that power reaches both sides of the switch.

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Old 07-04-2022, 10:10 AM   #7
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1. Verify your new battery is connected properly. Meter or test light between the positive post and the trailer frame (a convenient bolt) should slow current.

2. If the 12v stuff works on shorepower the converter is fine.

3. Check the 40a protection fuses with a meter -- they're probably fine but invisible breaks can occur.

4. Some trailers have a battery disconnect switch. Verify it's On and that power reaches both sides of the switch.

-- Chuck
12v stuff works when plugged in. Based on your comments this seems to rule out the converter.

Will have to recheck the 40 amp fuses when get the trailer back from storage lot next time. Pretty sure they were good.

There is a button reset on wire going from battery to the manual breaker connector mounted on frame before wire goes inside trailer. I pressed this reset and did not fix issue. 12v stuff still didn’t work off battery (not plugged in to shore power).


There is also a battery shut off switch. Will also check this.

Any other suggestions?
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Old 07-04-2022, 10:28 AM   #8
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no need to check the 40 amp fuses if the 12 volt stuff is working when on shore power.

at this point the two most probably sources of the problem would be the disconnect switch and the resettable circuit breaker in the 12 volt cables. this circuit breaker will be pretty close to the batteries. unplug from shore power and use a multimeter to check that you have 12 volts (from the batteries) on both posts of the circuit breaker. you can check the disconnect switch by simply setting it to both the on and off positions.

there is also the possibility that you may have not connected all the batteries cables to the new batteries.

this is resolvable!
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Old 07-04-2022, 10:47 AM   #9
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I did the same thing several years ago, and while I can't help chase your problem, I'll mention a couple of other things to be aware of.
You didn't mention towing the trailer while the fuses were blown, but I did and I also blew 2 fuses in the tow vehicle.
Also blew the 5amp fuse in my trailer radio and the radio wouldn't work.
After than trip, I put red paint on the battery case next to the positive post so I wouldn't do that again
Good luck,
Mitch
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Old 07-04-2022, 10:47 AM   #10
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Is there a reset button on the resettable fuse block in your second picture? It's usually a small black button on either end.
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Old 07-04-2022, 09:34 PM   #11
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Originally Posted by Michfan1970 View Post
12v stuff works when plugged in. Based on your comments this seems to rule out the converter.

Will have to recheck the 40 amp fuses when get the trailer back from storage lot next time. Pretty sure they were good.

There is a button reset on wire going from battery to the manual breaker connector mounted on frame before wire goes inside trailer. I pressed this reset and did not fix issue. 12v stuff still didn’t work off battery (not plugged in to shore power).


There is also a battery shut off switch. Will also check this.

Any other suggestions?
I am focused on that little circuit breaker. Most have a reset button. Some are auto reset. Either way you should have 12-13 v between frame ground and both of those breaker posts. If no voltage , trace back to you battery. If only on the battery side of the breaker, either it is not reset,,,,or it has failed.

Like always. Please comeback and report the solution when you solve it.
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Old 07-05-2022, 08:58 AM   #12
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Not sure what the top photo is. If it's a breaker try cycling and resetting it.

The bottom photo is the breaker common on many if not all campers. Test it:

1. Disconnect shore power. Check the circuit breaker in the bottom photo by checking for 12vDC between both screws and ground (the trailer frame). This will determine if power is getting out of the battery to this point and if there's voltage on both pins it's getting thru the breaker. Bad breaker is a simple replacement. You can also bypass this breaker with a wire jumper if you have to while waiting for a new part. At your risk, of course.

2. If the breaker is good: Disconnect the battery (remove all wires from the negative pole) and reconnect shorepower and repeat the tests. This will determine if the wiring between the breaker and the converter (which we know is good) are connected to each other. If not the problem is in the wiring between this breaker and the converter.

#1 is more likely than #2 based on the fact you fried the reverse polarity fuses in the converter which can't happen if the battery and converter ain't connected. No point in chasing your tail if this breaker won't reset. If this is good the tail chasing may resume.

The reverse polarity fuses are good as Chickdoe notes.

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Old 07-06-2022, 07:47 PM   #13
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Thanks for all the suggestions!

Went back out to the storage lot this evening and was able to determine we have a faulty battery disconnect switch.

New switch from Amazon for 8 bucks.
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Old 07-07-2022, 07:30 AM   #14
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Glad it's working!

Stick link to the Amazon switch in the thread so we can see the bad part and solution.

-- Chuck
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