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Jim Foster · 05-21-2009, 03:23 PM

Don't trust fuses just because they don't look blown.

Tuesday I installed an inverter in our "new to us" '83 Excella. It has two nearly new 12v deep cycle batteries, installed in their original locations. The four cables, 2 pos, 2 neg, come to the 12v panel, where each of the four cables connects to it's own 50 amp fuse. There is a bar connecting the two positive terminals together after the fuses. This should cause the batteries to be used about equally, and charge about equally, any difference being battery condition. In escence, one battery.
The trailer has a 55w solar panel with the regulator connected directly to battery #1, which is linked to battery #2 via the bar on the fuse panel after the fuses. This should charge the batteries as one.

I connected the inverter positive cable to the terminal where the bar connects the two batteries together. I fired off the inverter and the flat screen and all worked fine.

A couple of hours later, the inverter low voltage signal went off. The solar charger was not showing that it was charging, and the battery condition on the solar regulator showed almost 13 volts. I shut everything down.

I checked the battery voltage at each battery. Battery #1 showed just over 13 volts, which shut down the solar. Battery #2 showed 10.9. which shut down the inverter.

I pulled out all four 50 amp fuses and checked them with an ohm meter. Three of the four showed no resistance. The fourth, which was the battery #2 positive fuse, showed lots of resistance. It was not blown, not discolored, or anything like that. Just BAD. I replaced that fuse, and now both batteries are being charged and used together.

The previous owner never had a problem because he always went to RV parks with hookups. When the trailer was parked next to our home, there was a good enough connection in that fuse, that over time, both batteries got equalized and charged with the solar.

Keep a volt/ohm meter handy.

VIKING · 05-21-2009, 04:00 PM

Good post Jim.
This is common enough, but a lot of people still check their fuses with their eyes

and think they have actually checked them!

Thanks, Rich

Beginner · 05-21-2009, 04:08 PM

Fuses can and do develop a resistance, usually this creates enough heat that the element will melt. When it maintains the resistance it can be a real pain to find. The easiest way is to check them with a volt meter with every thing turned on. Place one meter lead on one end of the fuse and the other meter lead on the other end of the fuse (the fuse is of course still in its holder). If you are showing more that .25 volts either the fuse is dirty or has developed an internal resistance. If the fuse is completely blown the volt meter will read, in this case 12 volts.
If it a 110 VAC fuse and it is blown it will read 110 VAC.
This saves from having to destroy fuses snapping them in and out of the holders.
Beginner

Jim Foster · 05-21-2009, 05:20 PM

Quote:

Originally Posted by Beginner

Fuses can and do develop a resistance, usually this creates enough heat that the element will melt. When it maintains the resistance it can be a real pain to find. The easiest way is to check them with a volt meter with every thing turned on. Place one meter lead on one end of the fuse and the other meter lead on the other end of the fuse (the fuse is of course still in its holder). If you are showing more that .25 volts either the fuse is dirty or has developed an internal resistance. If the fuse is completely blown the volt meter will read, in this case 12 volts.
If it a 110 VAC fuse and it is blown it will read 110 VAC.
This saves from having to destroy fuses snapping them in and out of the holders.
Beginner

Sounds like a great way to check fuses. I would put that one back and check it just for fun to see the reading, but it is long gone in the trash. However, I think I will check all of the fuses this way tomorrow morning when it is a bit cooler in the "cave" where they put the panel.

Thanks, Jim