Victron 30A MPPT fuse size

[MHS]

The in-line fuse between our 30A mppt and the battery bank has blown. It has happened 3 times now in 4 years. This time it may have just knackered our batteries as I was away from the boat for 2 weeks. 

 

The fuse we were recommended to use was a 30A. (presumably 60A blow) might it be an idea to swap it for a 35A or 40A?

 

Thanks

Mike

[Kudzucraft]

Fuses are sized to protect the wire. Putting a larger fuse will allow more electricity to pass through and that heats up the wire, possibly causing a fire.  Even though it is common for people to put in larger fuses it is a really bad idea.  Fuse size should match the wire size. If you want/need a larger fuse you have to make sure the wire can safely handle the amps, that probably means installing a larger wire.

[Iain_S]

On the other hand, if the cable was chosen to minimise volt drop, it may be capable of having a larger fuse fitted already.

[Chewbacka]

The question to ask is why is the fuse blowing?  So if you have lots of solar and the controller is regularly putting 30A into the batteries and if the fuse is not I tight fit or there is oxide on the connections then it does not surprise me that it is blowing the fuse.  In which case I would clean up or better yet replace the fuse holder.  A 30A fuse running at 30A can run warm and this will increase oxidation etc making the situation worse, so assuming the cables are suitable I would use a 40A fuse.

 

You have not said how many watts of solar you have, so if only (example) 100W then even with a 30A controller you will never get more than about 8A, however if you have 500W, then you will be getting 30A most sunny mornings.

 

BUT if you don't have a lot of solar panels and (example) you never exceed 10A then the 30A fuse should not blow, unless the fuse holder is no good - see above.

 

So I would monitor the current to see if close to 30A and also after running at max current check to see if the fuse gets warm/hot.  If it does and the cables are heavy enough then you probably need a 40A fuse and possibly a new fuse holder.

Edited June 17, 2019 by Chewbacka

[MHS]

Hi, sorry to be slow coming back. I have 400w of solar. I will see if I can find the cable size. 

[Alan de Enfield]

4 minutes ago, MHS said:

Hi, sorry to be slow coming back. I have 400w of solar. I will see if I can find the cable size. 

Over 31 amps at 13v on a 'sunny day'

[MHS]

It’s 4mm2 with a cable run from mppt to batteries of 1.4m on the negative and 2m on the positive. 

An online calculator seems to suggest that might give a 5% loss but is ok. 

3 minutes ago, Alan de Enfield said:

Over 31 amps at 13v on a 'sunny day'

So maybe I should go for 35A or 40A as per Chewbacka. 

Edited June 17, 2019 by MHS

[Alan de Enfield]

Its like running your engine just above the 'red line' -t'aint going to do it any good !!

[Chewbacka]

11 minutes ago, MHS said:

It’s 4mm2 with a cable run from mppt to batteries of 1.4m on the negative and 2m on the positive. 

An online calculator seems to suggest that might give a 5% loss but is ok. 

So maybe I should go for 35A or 40A as per Chewbacka. 

As the fuse has been running hot, I really would check or change the fuse holder.

[MHS]

11 minutes ago, Chewbacka said:

As the fuse has been running hot, I really would check or change the fuse holder.

A new job to add to my list!! I will have a look in the next few days. Is a 35A fuse ok with 4mm2? The online calculator I looked at seems to rate it at 41A over the cable length. 

 

I just wish we could have a few dry days so I can get on. 

 

[Alan de Enfield]

4 minutes ago, MHS said:

 Is a 35A fuse ok with 4mm2

Yes

[MHS]

1 minute ago, Alan de Enfield said:

Yes

Thank you. Off to the chandlery in the morning. 

[MHS]

1 hour ago, Alan de Enfield said:

Its like running your engine just above the 'red line' -t'aint going to do it any good !!

One last post please before I leave you all in peace. 

 

If I upgraded the cable to say 10mm2, could I safely add a bigger fuse to say 40A or 50A? Is the fuse in this position (between mppt and battery bank) only protecting the cable? The mppt does have its own internal fuse. 

[trackman]

I had a similar problem with fuses blowing. A much beefier fuse holder & slightly heftier fuse has not only cured the problem, I'm also seeing better charging currents for given voltages on the panels.

[Alan de Enfield]

19 minutes ago, MHS said:

One last post please before I leave you all in peace. 

 

If I upgraded the cable to say 10mm2, could I safely add a bigger fuse to say 40A or 50A? Is the fuse in this position (between mppt and battery bank) only protecting the cable? The mppt does have its own internal fuse. 

Yes.

 

50 amps at 2 metres would give 2.83% volt drop.

 

Go for it.

[MHS]

Many thanks. 

[Kudzucraft]

Nice to see knowledgeable electric advice on a forum! So many times I see people that have no clue spouting out really bad advice.

 

My hats off to you guys!

[TheBiscuits]

4 hours ago, Kudzucraft said:

Nice to see knowledgeable electric advice on a forum! So many times I see people that have no clue spouting out really bad advice.

 

My hats off to you guys!

There are a few of us that have extensive experience, and some of us can call on very serious experts for fact checking.

 

Mine is a former defence contractor.  One of the other regular posters on these threads talks frequently with the chap who invented the Smartgauge, and he used to be a forumite too!

 

There are a lot of retired electrical engineers on here.  

 

The chap that wrote all the training manuals for the RCR breakdown service is a valued regular contributor. 

 

There is a reason this is the most popular forum, and it's not because we mollycoddle people!

 

[Sea Dog]

The OP may also wish to consider fitting a small dc breaker in place of the fuse. They're inexpensive nowadays and provide a convenient switch to isolate the solar when working on the batteries. I'm on the phone so not easy to provide a link to the model I have, but it came from eBay and is trouble free.

[WotEver]

1 hour ago, Sea Dog said:

The OP may also wish to consider fitting a small dc breaker in place of the fuse. They're inexpensive nowadays and provide a convenient switch to isolate the solar when working on the batteries. I'm on the phone so not easy to provide a link to the model I have, but it came from eBay and is trouble free.

Loads about. This one looks good:

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F163349798585

[Tony Brooks]

5 hours ago, Sea Dog said:

The OP may also wish to consider fitting a small dc breaker in place of the fuse. They're inexpensive nowadays and provide a convenient switch to isolate the solar when working on the batteries. I'm on the phone so not easy to provide a link to the model I have, but it came from eBay and is trouble free.

I don't think he should. The OP specifically states "between the controller and battery" and one thing you do not want in that position is an easy means of turning that part of the circuit off.  Most, if not all, solar controllers specifically say  always connect the batteries BEFORE the panels (my italics to ensure the controller selects the correct battery voltage). Tripping a breaker between controller and batteries will in effect be connecting the  panels before battery.

 

By all means put one or a switch between panels and controller.

[Chewbacka]

1 hour ago, Tony Brooks said:

I don't think he should. The OP specifically states "between the controller and battery" and one thing you do not want in that position is an easy means of turning that part of the circuit off.  Most, if not all, solar controllers specifically say  always connect the batteries BEFORE the panels (my italics to ensure the controller selects the correct battery voltage). Tripping a breaker between controller and batteries will in effect be connecting the  panels before battery.

 

By all means put one or a switch between panels and controller.

Or use a breaker between batteries and controller together with a prominent warning label next to the breaker - "Warning - Disconnect panels BEFORE disconnecting the batteries, and connect batteries BEFORE connecting the panels" together - if you feel the need - with the normal stuff about not fiddling with leccy stuff if you don't know what you are doing.

Edited June 18, 2019 by Chewbacka

[Sea Dog]

2 hours ago, Tony Brooks said:

I don't think he should. The OP specifically states "between the controller and battery" and one thing you do not want in that position is an easy means of turning that part of the circuit off.  Most, if not all, solar controllers specifically say  always connect the batteries BEFORE the panels (my italics to ensure the controller selects the correct battery voltage). Tripping a breaker between controller and batteries will in effect be connecting the  panels before battery.

 

By all means put one or a switch between panels and controller.

Yes indeed Tony, we'll spotted. Mine is between the panels and the controller and I made the error of assuming the same here for the very reason you suggest.  

 

Would we really want a fuse in that (controller - battery) line if the controller already has one on the output side? Don't they all?

 

[WotEver]

2 minutes ago, Sea Dog said:

Would we really want a fuse in that (controller - battery) line if the controller already has one on the output side? Don't they all?

Yes, to protect the cable. 

[Sea Dog]

2 minutes ago, WotEver said:

Yes, to protect the cable. 

Which end - next to the output fuse or next to the battery terminal?