Fuses cooking on Sargent EC328

[John4477]

I have an Sargent EC328 which I bought and fitted myself a few years ago. I have a reoccurring problem with the two inline 20A fuses between the vehicle battery and the leisure battery to the Sargent control unit.

The fuses keep cooking - not always blowing but melting the plastic outer casing of the fuses themselves.

There's obviously a high current being drawn at some point - my suspicion is that it may not be related to power coming in to the Sargent when running equipment in the van but when the van is on mains hook up and the batteries are charging via the Sargent charger.

From the tests I've done, it seems the initial period of charging the leisure batteries in particular is when the fuses are getting their hottest.

I've changed the fuse holders in case these were sub standard and causing resistance but I'm running out of ideas now.

Any thoughts?

 

[autorouter]

You have to ask why Sargent require that the fuse is 20A when the EC328 charger that provides the charger to the leisure battery is 25A. In the installation instructions they say something about 20A per battery, and a maximum of 40A for two or more batteries.

First thing to look at is whether the amps is normal and the fuses are cheap rubbish ones with low melting point plastic. Or are they reputable fuses that are getting extra hot due to high amps flow, and melting.

Not knowing the details, I think the only reason for using a 20A fuse is because the wiring won't take more than 20A. If it was my self-build, I'd be looking at checking the wiring is good for 30A, and upping the fuse to 30A. Increasing the fuse size is normally a recipe for disaster, I know, but worth considering in this case if everything is specced properly

 

[John4477]

You have to ask why Sargent require that the fuse is 20A when the EC328 charger that provides the charger to the leisure battery is 25A. In the installation instructions they say something about 20A per battery, and a maximum of 40A for two or more batteries.

First thing to look at is whether the amps is normal and the fuses are cheap rubbish ones with low melting point plastic. Or are they reputable fuses that are getting extra hot due to high amps flow, and melting.

Not knowing the details, I think the only reason for using a 20A fuse is because the wiring won't take more than 20A. If it was my self-build, I'd be looking at checking the wiring is good for 30A, and upping the fuse to 30A. Increasing the fuse size is normally a recipe for disaster, I know, but worth considering in this case if everything is specced properly

That's really interesting - many thanks. I was also wondering if it could be cheap or underrated fuses, particularly in light of the fact they are melting and not blowing. I'll check out the wiring and see if it can carry a higher load. Thanks again.

 

[John4477]

I think I have the solution! When installing the unit Sargent supply cables that are already doubled up for the second leisure battery option, but only put on their wiring diagram to use a 20 amp inline fuse! Of course mine has two leisure batteries so it needs double the capacity. It's also worth noting that the in-built charger has three stages, a slow build up, then it throws lots of power in, then it has a holding stage - this would explain why the fuses were mainly melting and not blowing as soon as it was connected up. Can't believe I didn't spot it at the build stage - seems obvious now, but hey, the eternal learning curve continues! Thanks for the help - it made me rethink what was in front of me and see the issue

 

[Cockers]

I know I have a different control unit mines the Sargent EC500 I have two leisure battery’s there is an inline fuse for both battery’s and in the EC500 fuse board there are two 20amp fuses one for each battery

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[autorouter]

It's also worth noting that the in-built charger has three stages, a slow build up, then it throws lots of power in, then it has a holding stage

If you only look at the volts, then that's a reasonable description. But looking at the amps, it's a different story. The first stage - bulk charge - is 'constant current'. The charger outputs the maximum amps that it is capable of, ie 25A. It continues to do this, at maximum amps, as the voltage slowly build up. When the voltage reaches the 'absorption voltage', the charger flips from the bulk stage to the absorption stage.

In the absorption stage, the voltage is kept constant, and the current (amps) gradually falls from the maximum (25A) until it reaches a predefined limit, usually about 10% of the maximum amps, ie about 2.5A. When it falls to that limit, the charger decides the battery is fully charged and flips to the float stage to maintain it at 100% without overcharging.

The heat produced in the wiring and the fuse depends only on the amps flowing through the wire, not the volts. You'll get exactly the same heating effect in the wire for 20A at 12V as you would for 20A at 240V. It's the same with the fuse too. And also, the voltage drop (in volts) along the wire depends only on the amps, and not the volts. Obviously there's a big difference in the percentage voltage drop. A 1 volt drop is 0.4% for 240V circuits but is 8% for 12V circuits. Normally the rule of thumb is to keep the total voltage drop below about 3%.