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New batteries - problem


Johny London

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2 minutes ago, WotEver said:

Absolutely. 

And therefore I suggest that the “fix” (if any is needed) is to ensure that the interconnects are 50mm^2, preferably 70, and that all connections and terminals are tight and clean. If that is done, there is no need to worry too much about the topology of the connections.

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1 minute ago, nicknorman said:

And therefore I suggest that the “fix” (if any is needed) is to ensure that the interconnects are 50mm^2, preferably 70, and that all connections and terminals are tight and clean. If that is done, there is no need to worry too much about the topology of the connections.

However, if he’s going to have to rewire the batteries (which we don’t yet know of course) then he may as well do so optimally. 

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3 minutes ago, WotEver said:

However, if he’s going to have to rewire the batteries (which we don’t yet know of course) then he may as well do so optimally. 

Sure. My point is that it’s best to tackle wiring inadequacies rather than just changing the topology to compensate. But as you say, if new wires need to be made up you might as well sort the topology at the same time.

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9 minutes ago, Col_T said:

Just out of curiosity, is there any advantage in having 2 +ve connections from the batteries to the inverter?

No. It has the distinct disadvantage that it requires 2 fuses and, should one fuse blow, then it’s guaranteed that the other one then would. 

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4 minutes ago, Johny London said:

The two wires join at the fuse. The idea was that as the inverter has 90mm2 cable going to it from the fuse, then 2x50mm2 cables from the batteries to the fuse would be good. Partly for sheer capacity and partly to spread the load on the batteries evenly.

The fuse is there to protect the cable. With two cables in parallel, if one cable should become disconnected the fuse will not protect the cable which could potentially melt, setting fire to your boat.

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9 minutes ago, Johny London said:

It's no more dangerous than having a star point, which is effectively what it is.

Where do the 2 x 50mm2 cables that join at the fuse originate from on the batteries? 

If you want to parallel them onto a common connection point,  then the safest way to join them is one of these crimps, designed to accommodate 2 cables. 

https://goo.gl/images/xAS8uN

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1 hour ago, cuthound said:

Where do the 2 x 50mm2 cables that join at the fuse originate from on the batteries? 

See the layout above. They go to two different batteries!

2 hours ago, Johny London said:

The idea was that as the inverter has 90mm2 cable going to it from the fuse, then 2x50mm2 cables from the batteries to the fuse would be good.

I can appreciate that logic, but it’s flawed. 

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Reading through the last page or so, the 2 +ve links from battery to inverter are an attempt to distribute the load across the batteries more evenly. I guess this means making each inter-connect cable see the same Amp loading. If that is the case, should there not be 2 -ve links also, so that both sides of the same battery see the same load?

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19 minutes ago, Col_T said:

Reading through the last page or so, the 2 +ve links from battery to inverter are an attempt to distribute the load across the batteries more evenly. I guess this means making each inter-connect cable see the same Amp loading. If that is the case, should there not be 2 -ve links also, so that both sides of the same battery see the same load?

Yes. However the whole idea is flawed and it shouldn’t be split wired at all. 

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I guess my easiest option might be to just get one 90mm wire from the inverter fuse to the end battery +ve (opposite end from the -ve) then move the 12v +ve supply to the end also - this would achieve the second best scenario from the smartgauge page.

Alternatively, I have yet to work out exactly what cables I'd need to make it be the perfect third scenario from the smartgauge page.

I'm hoping it might make quite a difference, either way.

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To your first point - that is what option 2 on the Smartgauge is suggesting e.g. that there should be a single link from the battery pack to the inverter, and that the -ve link should be from the battery at the other end of the pack from the +ve link. As for cable size, I'd guess that having them both the same is a good starting point.

To your second point, it looks like you a single cable from each -ve to a common post, which should be 'beefy' enough to take the maximum current the inverter could draw - your existing inter-connect cables seem an obvious candidate. Then same again for the +ve side. You then need 2 cables, 1 from the common -ve to the inverter (the existing link between the battery and the inverter seems a potential candidate), and 1 from the common +ve to the fuse - you can probably guess what's coming so I won't say it!

Thise are my thoughts, someone who knows what they are talking about will, hopefully, be along soon.

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The negatives are not to a common post - perhaps you were suggesting that is what I could do, using the ones that currently interconnect the batteries? The -ve cables to the inverter and the 12v are beefy enough so that is something.

I've been busy researching my webasto install so hope to get back to this conundrum soon, it may be that just a couple new cables need making up in order to achieve scenario 3 in the smartgauge bible.

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Yes, unless I use method 3 - it shows a way of achieving an effective star terminal on both + and - with clever use of cabling direct between the batteries - I just need to figure out what I need to get - I think prolly just an extra lead each that can span two batteries.

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If you mean the third diagram on the Smartgauge page, what I posted above should give you all the necessary and doesn't need any "clever use of cabling between the batteries".

If you mean the last diagram on that page, the 'SmileyPete' option, what looks to me like the 4th option then remove the -ve and +ve connections between the second and third batteries in your bank so that you have 2 pairs of 2 batteries. Now you need a new interconnect cable to connect the -ve of the first battery in the first pair to the -ve of the first battery in the second pair, and another new interconnect to join the +ve of the second battery in the first pair to the +ve of the second battery in the second pair. The diagram on the Smartgauge page really is your guide, but the important thing to note is that these two new interconnect cables should be the same length, so if one looks significantly too long you are about to do it wrong.

With the two pairs of batteries connected you can use existing cables to connect to the inverter - if you use the +ve connection from the top battery in your diagram, that frees up the cable from +ve connection on the bottom battery in your diagram, which may be long enough to provide the cable for the new +ve interconnect. The bad news is that you will probably have to buy new cable for the -ve interconnect, unless you've got a spare length kicking around somewhere. Just so it's been said, all cables connecting battery => battery or battery => inverter should be the same size.

For what it's worth, I reckon that the 12v connections to the battery bank are pretty much correct, other than to move the +ve connection from the 2nd battery in your diagram onto the top battery.

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37 minutes ago, Col_T said:

If you mean the third diagram on the Smartgauge page, what I posted above should give you all the necessary and doesn't need any "clever use of cabling between the batteries".

If you mean the last diagram on that page, the 'SmileyPete' option, what looks to me like the 4th option then remove the -ve and +ve connections between the second and third batteries in your bank so that you have 2 pairs of 2 batteries.

I kinda wished I never invented that. :unsure:

I've never seen any real world measurements that really prove it's the problem the website makes it out to be, and people seize on these elaborate connection methods as some solution to what is very likely to be an imaginary problem.

As Nick says, for the average batt bank just use decent size cables with good crimping on the ends, and just connect them in the diagonal ladder type configuration.

  • Greenie 1
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