T12SC blows main battery fuse when first connected

[Deemar]

Hi All!

My T12SC has developed a habit of blowing the main DC fuse (20A) whenever the battery is connected.

Now, before I hear a chorus of a thousand voices saying the battery is backwards please consider the following points. It is NOT BACKWARDS.

1. It is the main 20A fuse blowing, not the reverse polarity fuse

2. When I can get the fuse to hold, all the polarity sensitive electronics (stereo, CO detector, smoke detector, LED lighting, furnace and A/C controls) work properly. The water pump and roof vent turn in the correct direction. None of these devices would work properly if the battery was backwards.

3. If plugged into 120AC first, then connect the battery, no fuse blows. If the battery was backwards, the reverse polarity fuse should still blow. It does not.

4. When I can get the fuse to hold, everything works fine even if shore power is then disconnected.

5. When I can get the fuse to hold, the charging system provides a charge to the battery. It would not if the battery was backwards.

6. I pulled ALL the fuses, except the main and reverse polarity. Still happens.

7. Breakaway switch was checked just in case. It is wired directly to the battery, so no brake current would pass through the fuse that is blowing, and the switch is open.

All devices are switched off (except smoke, CO, propane detectors and the radio memory circuit) when the battery is being connected, yet there is a bit of a spark at the battery terminal. I'm wondering if the charger has an internal capacitor and the surge current from charging up that capacitor is doing it?

Any thoughts?

Thanks!!!

[Bama Rambler]

Since you can get it to hold some of the time it appears to be a surge blowing it upon initial connection. Since you say that it will hold if you plug it into 120 and then hook the battery up it seems like the surge is because the converter is supplying a large current to the battery upon initial power up.

Have you had the battery tested?
How old is the battery?
How old is the converter?

If the battery has some age on it, I'd suggest trying a new fully charged battery and see if it blows the fuse.

[Flybob]

X2 Battery charge is very low or battery is bad. Converter has the capability to deliver more than 20A if connected to a shorted or almost dead battery.

[boondocking]

If you have pulled all the 12v fuses and it still blows the main 20a fuse when the converter is not operating then it has to be a faulty converter that is drawing a lot of current from the battery. May be some blown diodes in the converter.

[Deemar]

Thanks everybody for the input! Battery is one year old, and fully charged before being installed. The key here is what is happening and when.

Convertor off > connect battery > fuse blows
Convertor on > connect battery > everything OK
Battery connected > convertor turned on or off > everything OK.

The only time the fuse blows is when connecting the battery to an unpowered trailer, therefore charging current is zero, even if the battery was old/dead/shorted.

I think Boondocking is on the right track, I suspected the surge was from charging capacitors in the convertor, and a bad diode could be the root cause by not isolating the battery from the filter capacitors when the convertor is unpowered. That fits the symptoms exactly, because when the convertor is powered up before connecting the battery, the capacitors are already charged and won't cause a current spike. Once the battery is connected, it keeps the capacitors charged, so turning the convertor on or off wouldn't matter. Thanks Boondocking, I've been looking for a good excuse to upgrade the OEM convertor!

[Bama Rambler]

I agree that it's most likely the converter, I just wanted to make sure that you checked the other stuff that could be before just replacing it.

[boondocking]

You're welcome ! I hope you get it fixed soon, camping season is just about over, in my neck of the woods anyway.

[Grampa Jim]

A 20 amp fuse and not a 30 amp fuse. Unusual.

[Deemar]

Grampa Jim, I thought it was a bit low too, but that's what it came equipped with, and the label on the convertor is marked 20 amp. Looking into it a bit further, there is no 12 volt wiring anywhere on that trailer that can handle 30 amp, even the original battery leads were only 12 gauge.

[Tom48]

Wow. You have me. If it has been fine on a 20 amp main ( my battery main line is 40 amp ) then something has changed. If battery is well charged, it's demand should not exceed 20 a. If it does this off shore power, it can't be the battery sucking charge. It has to be a serious load in the coach. That might not be the converter, it just might be something that the converter handles before the battery come it to play, when you are on shore power. Before I pulled the converter for repair or replacement, I would disconnect it from the 12 volt system completely the try a hookup a couple times just to be sure it is not something else. Good Luck. Tom

[Grampa Jim]

I'm waiting with baited breath to find out if your converter is bad. Please let us know so all us armchair electricians can be further enlightened.

[Tom48]

Originally Posted by Grampa Jim

I'm waiting with baited breath to find out if your converter is bad. Please let us know so all us armchair electricians can be further enlightened.

Me too

[Deemar]

Yes, it is the converter. Yesterday I finally had time to do some proper diagnostics.

Test conditions:
- 120 volt AC power off (not even plugged in)
- battery negative lead connected
- battery positive lead disconnected
- all fuses pulled out except main
- all devices switched off
- ammeter probe connected to oscilloscope measuring current in battery + lead.
- voltmeter probe on battery + cable
- for test purposes the main fuse was replaced with a twelve volt, 30 amp circuit breaker

At the instant the battery is connected, there is a current spike peaking around 50 amps, lasting about 200 milliseconds and dropping to about 20 milliamps. This is what is blowing the fuse. If I wait two minutes and repeat the test the results are the same. If I repeat the test within a few seconds, the results are similar, except the spike is smaller, the faster I repeat the test, the smaller the spikes become.

The voltmeter showed voltage on the plus cable jump to 13.1 volts when connected, and when the battery was disconnected, voltage on the + cable dropped to zero over the space of about 20 seconds.

Anyone familiar with power supply electronics will recognize this as the battery trying to charge filter capacitors in the converter. A fully discharged filter capacitor (brand new, good ones, in perfect condition) normally act as a short circuit the instant they are connected to a power source. The current drawn by the capacitor diminishes as it charges up to the applied voltage. The biggest spikes come when the capacitors have had some time to discharge. Smaller spikes are generated when the capacitors are still partly charged.

On the voltmeter end, the capacitors will briefly store a charge, until it is dissipated by an external load (something else in the charging circuit) or by their own internal leakage. This is why the voltmeter shows voltage on the + cable for some time after the battery is disconnected

There should be a diode in the converter to isolate the battery from the charging circuit and filter capacitors when the charging circuit is not powered up, and that diode would prevent the sudden spike of current. Either WFCO cheaped out and omitted the diode (in which case I would think other people would have experienced this as well) or the diode is shorted as Boondocker suggested. Or WFCO screwed up on the main fuse, labelling it wrong and installing the wrong size, as Grampa Jim suggested. Maybe the fuse should have been 30 amp which would tolerate the spike better. At any rate the converter either has a design issue or an outright component failure.

Knowing that, and with the big trip of the year 3 weeks away, the temporary fix is to use a pair of fuse taps to connect a 100 ohm resistor across the main fuse terminals. Pull the fuse out, connect the battery, let it charge the capacitors through the 100 ohm resistor (which would limit current to at most a measly 120 milliamps). When the capacitors are partly charged, then install the fuse. Note that this won't fully charge the capacitors because the stereo memory and detectors are trying to draw current too. But it should get the spike down to a level where the 20 amp fuse is happy.

Long term fix is to get rid of the junk and put in an industrial 12 volt 50 amp supply, with separate chargers (shore power and solar) and automatic switchover.

Cheers all!