Fuses for leisure batteries in parallel

[DavieR]

I'm upgrading my cabling between my two 110amp leisure batteries.

 

Presently, both earth and load/charge are off one battery, the second simply linked. There is a 30amp fuse on the +ve link and 6mmsq cabling is used.

 

I plan to upgrade cable to 16mmsq and have load/charge off one battery, the earth off the other. Do I need two fuses, one near each battery, or just the one? Also, what size fuse? I occasionally run a 600watt inverter.

 

Thanks

[aandncaravan]

UK motorhomes tend to have a single 25amp fuse per 100Ah'ish battery.

Continental built conversions tend to employ a single 50amp fuse per pair of 90Ah batteries.

 

Either way seems to function ok, but we prefer the continental way of a single big fuse in the positive off to the Power Controller.

Suggest you run the Inverter directly to the Battery terminals and give it it's own 100amp fuse, that is assuming it's 600watt rating is continuous and the peak is a lot higher?

 

Best fuse holders for the batteries is a Maxi and a Flat Strip fuse for the Inverter. Buy 3 x 100a Flat fuses and selotape the spares to the lid of the fuse holder.

 

 

[DavieR]

Thanks Allan,

 

I have a 40amp fuse (6mmsq cable) off to the distribution box as well. I guess I was really wondering how one fuse could protect the linking cable from both batteries?

 

About the inverter. Apologies, I didn't give the correct info. It's 600watt peak and 350watt continuous.

 

Thanks for the tip about the fuses.

 

Dave

[Brian Kirby]

This may depend somewhat on whether both batteries are in the same location. If they are not, it may be sensible to fuse the interlinking cable at its (+) point of origin as well. Don't forget the fuse is there to protect the wiring, and if the interlink might be prone to damage, I think installing a fuse at each battery would probably be worthwhile.

[DavieR]

Brian Kirby - 2018-11-15 5:23 PM

 

This may depend somewhat on whether both batteries are in the same location. If they are not, it may be sensible to fuse the interlinking cable at its (+) point of origin as well. Don't forget the fuse is there to protect the wiring, and if the interlink might be prone to damage, I think installing a fuse at each battery would probably be worthwhile.

 

There is one battery under each front seat. The flooring between seems well padded but the cables are hidden out of sight. Two fuses then.

 

Thanks Brian

[Alanb]

Davie,

 

Depending of the location of the fuse in the link cable, the cable may be unprotected. This was the case on my vehicle where the linking cable was fused close to the linking point.

 

My current installation uses 16 sqmm cable and a 20A Maxi-blade fuse close to each battery. The use of a commoning point allows the cables to be connected together. However I do not use an inverter. At full load a 600W inverter will draw about 60A. Your proposed 16 sqmm cable will be rated at 110A from cable heating considerations.

 

My suggestion would be for an individual Maxi-blade fuse close to each battery. These fuses could be rated at 60A each without seriously under protecting the linking cables. Using a binding post to connect the load side of the battery fuses will allow separate fuses for your inverter, and CBE DS520. (CBE suggest a 50A fuse.)

 

You may also wish to consider copying Welted's practice of driving his inverter from the starter battery. This would allow the individual battery fuses to be downrated to 20A each, and remove the need for a separate distribution board supply fuse.

 

Alan

 

 

 

 

[aandncaravan]

DavieR - 2018-11-15 3:39 PM

 

Thanks Allan,

 

I have a 40amp fuse (6mmsq cable) off to the distribution box as well. I guess I was really wondering how one fuse could protect the linking cable from both batteries?

 

 

Dave

 

 

It doesn't protect it because it doesn't need protecting, which is why that approach is adopted by Hymer, Burstner, etc.

 

Remember a fuse is a resistor, when the whole point of 'fat' cabling is to reduce resistance so you want as few as possible and as big as is safe.

 

 

Don't build in chicanes where they are not necessary or you may risk unbalancing the set-up

 

 

 

.

 

 

[aandncaravan]

Sorry I couldn't find a picture of the right size to show the point, before the 30 minute edit timer expired on the earlier post.

 

If you look at the 'Correct' picture below, you will see that putting a fuse in the positive link cable might result in battery 'A' having a higher resistance, unbalancing the circuit.

 

Keep it simple, just put a single fuse in the primary output cable from the joined batteries going off to the Electrical equipment.

 

If you need to provide the linking cables with physical covers, then that is what the best do, but they don't use fuses.

[DavieR]

Allan, Alan and Brian,

 

Thanks for all your input, even if it is a bit conflicting! I've a lot to digest here so, please, talk amongst yourselves :-D

 

It's interesting how there are so many ways of doing one thing with electrics. In my naivety, I thought there was just the "correct" way.

 

Regarding the inverter. I use it once in a blue moon to charge a laptop when stationary, nothing else. I'd rather not wire it up to the starter battery, it's more cable to route for one thing. Accidentally leaving it on standby is another.

[Alanb]

aandncaravan - 2018-11-15 7:17 PM

 

DavieR - 2018-11-15 3:39 PM

 

Thanks Allan,

 

I have a 40amp fuse (6mmsq cable) off to the distribution box as well. I guess I was really wondering how one fuse could protect the linking cable from both batteries?

 

 

Dave

 

 

It doesn't protect it because it doesn't need protecting, which is why that approach is adopted by Hymer, Burstner, etc.................................................

 

 

 

The batteries in a Hymer or Burstner, etc. will be close together in a battery locker. The linking cable will therefore be short in length, not much more than the width of a single battery.

 

The batteries in Davies vehicle a installed under the front seats. I would expect the linking cable to be between one and two metres in length. Such a length of cable potentially exposed to short circuit discharge current of the two batteries in parallel should be protected.

 

It possible to protect the whole cable by using battery terminal cube fuses, while these are expensive, installation would be simplified, and there use is worth investigating.

 

A practical alternative would be using individual Maxi-blade fuses close to each battery terminal.

 

I see no reason why adopting either approach would prevent balancing the individual battery cable resistances.

 

Alan

 

[sshortcircuit]

Alanb - 2018-11-15 9:19 PM

 

aandncaravan - 2018-11-15 7:17 PM

 

DavieR - 2018-11-15 3:39 PM

 

Thanks Allan,

 

I have a 40amp fuse (6mmsq cable) off to the distribution box as well. I guess I was really wondering how one fuse could protect the linking cable from both batteries?

 

 

Dave[/quote

 

It doesn't protect it because it doesn't need protecting, which is why that approach is adopted by Hymer, Burstner, etc.................................................

 

 

 

The batteries in a Hymer or Burstner, etc. will be close together in a battery locker. The linking cable will therefore be short in length, not much more than the width of a single battery.

 

The batteries in Davies vehicle a installed under the front seats. I would expect the linking cable to be between one and two metres in length. Such a length of cable potentially exposed to short circuit discharge current of the two batteries in parallel should be protected.

 

It possible to protect the whole cable by using battery terminal cube fuses, while these are expensive, installation would be simplified, and there use is worth investigating.

 

A practical alternative would be using individual Maxi-blade fuses close to each battery terminal.

 

I see no reason why adopting either approach would prevent balancing the individual battery cable resistances.

 

Alan

 

 

Spot on

[aandncaravan]

Sorry, but that isn't true. Battery and Elektroblock under one seat and the 2nd battery under the other linked by cables just over a metre long is quite common.

 

 

While physical cable protection is employed, exactly as I state and suggest is done, only one fuse.

The way it is all executed reduces risk to zero, so risk of short circuit is none existent. This is a motorhome not a rally car.

 

 

But Dave you are right, lots of ways to do things, but in my experience, Hymer and Co do it better than most.

 

 

 

 

[DavieR]

In theory.

 

If a fuse adds resistance on the +ve link, does a fuse on the -ve link balance the two?

 

Or, how much shorter would a -ve cable have to be to balance a fused +ve? Fuse, cable size and length would all have to be known for an answer, I guess.

 

:-D

 

Dave

[aandncaravan]

"If a fuse adds resistance on the +ve link, does a fuse on the -ve link balance the two?"

 

 

Yes it does, but if you are thinking about doing that, I don't think you will find that in any Auto electricians manual?.

 

You seem determined to make it overly complex, be careful here because the slightest resistance at one battery will cause the electricity to divert down an easier path. That will result in one battery charging before the other, and one battery supplying more than it's share of the power. Ultimately that will harm both batteries.

 

But you also create an inefficient system that will waste the energy you are so desperate to harvest.

 

Keep it simple and it is easier to understand by anyone maintaining the solution after you sell it on.

Think of the poor electrician scratching his head in 5 years time at a fuse in the negative?

 

 

[DavieR]

No, I wasn't thinking of doing it. I did Google it but only found little. I was just testing my understanding - two weeks ago if you had asked me to construct a sentence with the words "fuse" and "resistance", I would have failed miserably. I am enjoying learning, it's fascinating and it has a nice practical side to it as well.

 

I vacillate between one fuse and two fuses, swayed between balanced resistance and safety. I see the merits of both. Is a simply braided or pvc sleeve adequate protection for the cable? There are a few soft layers between my feet and the cable.

 

 

 

[aandncaravan]

Unless there are sharp edges, where the best use Grommet style protection, then there is very little that can penetrate the tough PVC outer of a 12v cable. It is tough, even on the Thin wall cable which I suggest you avoid.

The second layer physical protection of cable armoured sleeve is all the best use. This is ideal :- https://www.ebay.co.uk/itm/Black-spiral-conduit-tube-pipe-split-non-split-cable-wire-tidy-6-5mm-23mm-/282317172757

 

 

You may not have read the Posts on Alternator charging and how UK motorhomes usually have less than 13.9v Alternator charge at the batteries when driving with the long discussions about cable length, connectors, etc. but it demonstrated how attention to detail was important.

 

The way to improve charging on those motorhomes is to reduce the cable runs and connectors.

Don't forget that every Fuse has 4 connectors : two that go onto each end of the Fuseholder and two on the fuse itself as it inserts. That is a lot of potential long term trouble .on what is heavy current carrying cable in an area that can be highly acidic and prone to corrosion.

 

Hence me saying have the minimum number of fuses you can safely get away with.

 

If you look at all the best motorhomes that are professionally done, you won't find fuses in the linking cables. There must be a reason for that, don't you think?

 

If you want to go over board on safety, then use more physical safety, have a double layer of cable protector or even triple layer of physical protection, but please don't compromise the charging or electrical flow with unnecessary fuses and connectors. It defeats the object of what you are trying to achieve.

 

These might be small things, but it's the detail that makes the difference between the best electrical infrastructure designs and the worst.

 

 

But you play it your way, I am ducking out of this now.

 

[DavieR]

Thanks Allan, I appreciate your help, as well as anyone who takes time to comment.

 

So, I have ordered a 50amp fuse and conduit for the cables.

 

Dave