Author Topic: 12V DC routing  (Read 3506 times)

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Online paulcaTopic starter

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12V DC routing
« on: January 18, 2022, 04:05:17 pm »
Hi,

I know I asked this last year, but I didn't get a viable solution then and have decided with an alternative.

In short I have 12VDC solar power available in the garage.  I would like to use that 12VDC in the office, 10 meters away in the upper floor.  Originally I thought as the system is rated for 10amps, I would need to route 10Amps, or 120W and needed to use a boost converter to save on expensive thick cable by transmitting the DC at 50V or higher.

Now I have a plan B, which should be cheaper and easier.  Ballast.  A can put a lead acid in the office, run the 12V up over 1.5mm cable and put a CC/CV buck converter on the end, connected to the lead acid.  The CC/CV PSU will simply limit the charge current on the 10 meter wire run to 2 Amps.  So it will effiectively charge the office battery 24/7 at no more than 2Amps.  Even if I pull 20 Amps from it to run a DC/DC power supply at 20Amps.

Anyone see any problems with this approach?  It's basically running a remote CC/CV charger as a load on the existing solar system.  If it goes low battery, it will shut the load off.
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Offline tszaboo

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Re: 12V DC routing
« Reply #1 on: January 18, 2022, 04:13:01 pm »
10m 2.5mm cable is 21 EUR in the local home improvement store  :-//
 
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Offline fourtytwo42

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Re: 12V DC routing
« Reply #2 on: January 18, 2022, 04:19:07 pm »
The problem I see is hours of sunlight. Assuming your charging source is PV then for example on a short winter day you will only be able to transfer 10AH to the battery.
 

Online paulcaTopic starter

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Re: 12V DC routing
« Reply #3 on: January 18, 2022, 04:31:48 pm »
10m 2.5mm cable is 21 EUR in the local home improvement store  :-//

Yes, but if I start pulling more than a few amps the voltage drop will start to bring me well under 12V and may cause other issues.

I think 2.5mm at 10A still results in something like a 1.4V drop.

Mostly the 12V will be used to run other battery chargers, headphone amps and electronics bench supplies.  Basically a lot of low wattage stuff.  However the DC/DC bench supplies I have can buck 20Amps and boost 5Amps (input limited).  So there is a use case for temporary, periodic, short term high current draws that would not be possible with 12VDC over any civilised thickness of cable without a >1V drop.

The problem I see is hours of sunlight. Assuming your charging source is PV then for example on a short winter day you will only be able to transfer 10AH to the battery.

It's backed by a 100Ah battery.  So as long as I keep an eye on the main solar battery levels over winter and don't drain it.  It should be fine.
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Offline DenzilPenberthy

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Re: 12V DC routing
« Reply #4 on: January 18, 2022, 04:35:53 pm »
Your 12v battery will not like being kept at 12v. It will need something like 14.4v absorbtion charge and then storage at a float voltage of something like 13.8v.

This seems like a very complicated and expensive solution to a problem that's not really much of a problem.

You can work out volt-drops for yourself based on a 10m circuit and how much load you forsee drawing, but even 16sqmm wire is only £1.67/m and that will be good for <5% volt drop at 20A

https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

You can buy by the metre from here (or tonnes of other places)
https://www.tlc-direct.co.uk/Main_Index/Cable_Index/Singles_1/index.html
 

Online paulcaTopic starter

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Re: 12V DC routing
« Reply #5 on: January 18, 2022, 06:05:08 pm »
Your 12v battery will not like being kept at 12v. It will need something like 14.4v absorbtion charge and then storage at a float voltage of something like 13.8v.

That's why I'll use a basic CC/CV charger on it.  That will at least give it 14.40V boost.

This seems like a very complicated and expensive solution to a problem that's not really much of a problem.

It's not exactly expensive, I already have the battery and beyond the cable it's a £10 CC/CV buck/boost module off Amazon

even 16sqmm wire is only £1.67/m and that will be good for <5% volt drop at 20A

2 reasons.

1.  I need 2 cores, ideally brown/blue or black/red which looks like £40 for 25 meters.  Twice.  That's £80 + clips.  The 1.5mm 3 core, outdoor UV resistant, round black stuff on Amazon is £35 delivered for 25m.  (I have not measured the distance exactly).
2.  It has to go through 2 walls and be clipped to an exterior wall for routing.  I do not at all fancy trying to clip up 16mm cables to an upstairs room.

Anyway.  I figure the CC/CV charger and the lead acid are a red herring. It's still just a basic 12VDC circuit.  I can decide to put a buck/boost, charger or a funky lamp on the end of it.

It just comes down to the pricing and hassle of the cable.  I make that choice, install the cable, I live with it or use a battery to give local high current storage if/when needed.
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Offline Kim Christensen

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Re: 12V DC routing
« Reply #6 on: January 18, 2022, 06:45:27 pm »
What are you running in the "office" that requires 12V @ 10A?
A cheap and easy solution is to have 12V -> 240V 200W inverter (The kind designed to go in a car, caravan, or boat) and then a standard 12VDC switching supply in the office running off the 240V. Add some sort of remote ON/OFF to save on idle current when not in use.
Sure, there will be losses, but there will also with charging and discharging a 12V battery.
 

Offline BeBuLamar

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Re: 12V DC routing
« Reply #7 on: January 18, 2022, 06:49:23 pm »
How efficient is the type of converter you want to use? I much rather take the loss in the cable.
 

Online paulcaTopic starter

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Re: 12V DC routing
« Reply #8 on: January 18, 2022, 07:21:55 pm »
How efficient is the type of converter you want to use? I much rather take the loss in the cable.

It's not the loss itself I'm worried about. I mean if on 1.5mm at 10 amps it's dropping 2V and wasting 20W as heat to the outside world, it's fine... not ideal, but fine.

What isn't fine however is a battery that drops to 12V under 10Amps load, then lossing 2V on the circuit and I end up with 10V to work with.

That again is fine.  But, I will need to boost it for almost everything useful anyway.

The higher voltage alternative, or even the full AC inverter route aims to take care of the cable loss before transmission.   Although the point about converter loss +/- cable loss is taken.

I suppose it comes down to equations and data. 

Decide the current, say 10Amps max.
Decide the line working voltage range, 10-16V
For a given cable/cost/hassle what is the loss
For the same setup what is the loss in the converter, or converters.

Now it sounds like a chore, LOL

I imagine how this will work in reality.  It will spend 90% of it's time switched off at source and 9% of it's time running 1 or 2, 20W USB charge circuits and 1% of it's time running a LiPo charger at 80W or 20W of lights and a 20W laptop charger on a evening during a power cut.  So I should just cap it at 2 amps for the 99% and use mains or local battery for the remainder.
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Offline Kim Christensen

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Re: 12V DC routing
« Reply #9 on: January 18, 2022, 07:41:38 pm »
I've got another idea for you.
All you need is a single boost converter located in the garage. Since you'll know the resistance of the 12V feedline to the office, you can design the boost converter to increase it's voltage based on the amount of current being demanded. That way, the voltage in the office would stay more or less constant.

EDIT: Link to TI application note. Could be adapted for a boost converter instead of buck
https://www.ti.com/lit/an/slva657/slva657.pdf


« Last Edit: January 18, 2022, 09:33:01 pm by Kim Christensen »
 

Offline Benta

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Re: 12V DC routing
« Reply #10 on: January 18, 2022, 08:21:45 pm »
Why not just use 6 mm2 cable?
It's cheap and mandated for 16 A installations today.

Or if you need it flexible, buy automotive battery cable. 6, 10, 16... mm2 at a couple of Euros per meter.
« Last Edit: January 18, 2022, 08:26:32 pm by Benta »
 

Offline BrokenYugo

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Re: 12V DC routing
« Reply #11 on: January 18, 2022, 08:36:16 pm »
Another flexible solution is to buy 2 cheap 3 wire extension cords and tie the conductors together so each cable acts like one big wire.
 

Offline SmallCog

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Re: 12V DC routing
« Reply #12 on: January 18, 2022, 09:06:03 pm »
10m is no big deal, cable runs of this length are common in boats, busses, caravans, etc.

Size it appropriately for the expected load and fuse it appropriately for the cable rating

I haven't seen your previous thread but this may be of use to you: https://www.bluesea.com/resources/1437
 

Offline Zero999

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Re: 12V DC routing
« Reply #13 on: January 18, 2022, 09:39:35 pm »
Does the solar panel have a built-in voltage regulator?

The last time I used a 12V solar panel, I found it had a much higher output voltage when open circuit, something like 18V. If you don't intend to use the full power output, you might find the voltage is high enough. Put the solar panel at one end of a thick cable and an efficient buck converter, with a low dropout voltage, near the load.
 

Online Siwastaja

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Re: 12V DC routing
« Reply #14 on: January 19, 2022, 03:42:22 pm »
While 12V isn't optimal for transferring 120W, 10 meters is still just 10 meters! I bet just throwing more copper at the problem is the cheapest and most reliable solution.

In other words, 6mm^2 cable. If you happen to find something else with more conductors for cheaper, you can also parallel conductors.

Now for a 50-meter link, things could be quite different.
 

Online paulcaTopic starter

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Re: 12V DC routing
« Reply #15 on: January 20, 2022, 02:16:36 pm »
Does the solar panel have a built-in voltage regulator?

The last time I used a 12V solar panel, I found it had a much higher output voltage when open circuit, something like 18V. If you don't intend to use the full power output, you might find the voltage is high enough. Put the solar panel at one end of a thick cable and an efficient buck converter, with a low dropout voltage, near the load.

Yes.  The panel is buck converted down to the what the battery charge phase demands or to current limit it when the battery is fully charged.

At dawn nothing happens until the panel hits the battery voltage, then it goes to 100% duty cycle on the buck converter effectively directly connecting the panel to the battery.

When the sun comes out and the panel is able to push current at much higher voltages, the MPPT chasing algorithm kicks in to track the max power output and bucking that voltage down to the battery required charge voltage of 14.40 or 13.60.

If the battery gets to be 14.40V and is pulling less amps the buck converter will end up very short duty cycle which will cause the panel to rise towards open circuit voltage.

The "load" side is directly connected to the battery with a 10A fuse and an automatic cut out which I have set to 11V.

While 12V isn't optimal for transferring 120W, 10 meters is still just 10 meters! I bet just throwing more copper at the problem is the cheapest and most reliable solution.

You are probably right.  For most of the usage it won't be high amps and the USB chargers will usually accept auto-motive 9-16V.

The odd requirement to have 12V or greater I can use a PSU to boost it and take the losses.
« Last Edit: January 20, 2022, 02:18:41 pm by paulca »
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Online paulcaTopic starter

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Re: 12V DC routing
« Reply #16 on: January 20, 2022, 04:55:44 pm »
To save myself hassle and making a mess of it.  I phoned my rather casual spark to install 6mm2 and hopefully doesn't cost and arm and a leg.  I don't think it will knowing him.

It will be twin and earth of course.  I was thinking to parallel and get any advantage I would need 4 cores, but then I thought about it and adding the earth in parallel to the + might mean less voltage drops on the high side and overall circuit resistance will be lower. 
« Last Edit: January 20, 2022, 04:57:28 pm by paulca »
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Offline Zero999

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Re: 12V DC routing
« Reply #17 on: January 20, 2022, 05:09:41 pm »
To save myself hassle and making a mess of it.  I phoned my rather casual spark to install 6mm2 and hopefully doesn't cost and arm and a leg.  I don't think it will knowing him.

It will be twin and earth of course.  I was thinking to parallel and get any advantage I would need 4 cores, but then I thought about it and adding the earth in parallel to the + might mean less voltage drops on the high side and overall circuit resistance will be lower.
Why parallel with earth with the positive side? Is the negative side earthed? If so, parallel the negative with the earth. Otherwise, if it's not earthed, it doesn't matter whether you parallel the positive, or negative with the earth conductor.
 

Online paulcaTopic starter

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Re: 12V DC routing
« Reply #18 on: January 21, 2022, 04:19:13 pm »
To save myself hassle and making a mess of it.  I phoned my rather casual spark to install 6mm2 and hopefully doesn't cost and arm and a leg.  I don't think it will knowing him.

It will be twin and earth of course.  I was thinking to parallel and get any advantage I would need 4 cores, but then I thought about it and adding the earth in parallel to the + might mean less voltage drops on the high side and overall circuit resistance will be lower.
Why parallel with earth with the positive side? Is the negative side earthed? If so, parallel the negative with the earth. Otherwise, if it's not earthed, it doesn't matter whether you parallel the positive, or negative with the earth conductor.

No end is earthed.  The earth conductor is redundant.

Although... is it?

What happens if it get's hit by lightening?  Should I earth the cable itself?

Actually.  I think earthing it would not matter.  If it were to have the unlikely miss fortune of lightening strike, which is more likely if it IS earthed.... anything on either end of that cable or anything near it will be vaporised earth or no earth. 

EDIT: The panel/battery system is 100% floating.  Except whatever conductivity to ground the system has from any leakage via air or the bench it's sitting on.

I think I did measure it realitive to earth once and it was under a volt battery - to plug ring earth.
« Last Edit: January 21, 2022, 04:22:56 pm by paulca »
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Offline Zero999

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Re: 12V DC routing
« Reply #19 on: January 21, 2022, 04:56:28 pm »
To save myself hassle and making a mess of it.  I phoned my rather casual spark to install 6mm2 and hopefully doesn't cost and arm and a leg.  I don't think it will knowing him.

It will be twin and earth of course.  I was thinking to parallel and get any advantage I would need 4 cores, but then I thought about it and adding the earth in parallel to the + might mean less voltage drops on the high side and overall circuit resistance will be lower.
Why parallel with earth with the positive side? Is the negative side earthed? If so, parallel the negative with the earth. Otherwise, if it's not earthed, it doesn't matter whether you parallel the positive, or negative with the earth conductor.

No end is earthed.  The earth conductor is redundant.

Although... is it?

What happens if it get's hit by lightening?  Should I earth the cable itself?

Actually.  I think earthing it would not matter.  If it were to have the unlikely miss fortune of lightening strike, which is more likely if it IS earthed.... anything on either end of that cable or anything near it will be vaporised earth or no earth. 

EDIT: The panel/battery system is 100% floating.  Except whatever conductivity to ground the system has from any leakage via air or the bench it's sitting on.

I think I did measure it realitive to earth once and it was under a volt battery - to plug ring earth.
If it gets struck by lightning then you've got bigger problems to worry about and probably not much more likely if the cable is earthed, since the voltage is so high.
 
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Online paulcaTopic starter

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Re: 12V DC routing
« Reply #20 on: January 21, 2022, 05:57:11 pm »
If it gets struck by lightning then you've got bigger problems to worry about and probably not much more likely if the cable is earthed, since the voltage is so high.

Was thinking in terms of streamers.  Would they not be far more likely to be emitted from the ends of a copper cable that connects to earth than one that doesn't?  The later has an air gap, but maybe as you mention as the potential is so great borrowing just that section of copper while going down the outside of the building would still be preferential airgap or none.   Low resistence = high current = vapourised copper.
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Offline Kim Christensen

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Re: 12V DC routing
« Reply #21 on: January 21, 2022, 06:46:42 pm »
What happens if it get's hit by lightening?  Should I earth the cable itself?
Actually.  I think earthing it would not matter.  If it were to have the unlikely miss fortune of lightening strike, which is more likely if it IS earthed.... anything on either end of that cable or anything near it will be vaporised earth or no earth.

Ask yourself this: Would you rather have the majority of the energy from a lightning strike dissipated outside your office or inside?  >:D

If you earth the negative lead to a ground stake (earth) just outside the building and also connect the +12V line to the stake via a spark-gap it'll be a lot safer. It's up to you if you want to take the risk. Personally where I live I wouldn't worry about it since lightning is very rare here. But your location may be quite different.

 

Offline Zero999

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Re: 12V DC routing
« Reply #22 on: January 21, 2022, 07:22:24 pm »
What happens if it get's hit by lightening?  Should I earth the cable itself?
Actually.  I think earthing it would not matter.  If it were to have the unlikely miss fortune of lightening strike, which is more likely if it IS earthed.... anything on either end of that cable or anything near it will be vaporised earth or no earth.

Ask yourself this: Would you rather have the majority of the energy from a lightning strike dissipated outside your office or inside?  >:D

If you earth the negative lead to a ground stake (earth) just outside the building and also connect the +12V line to the stake via a spark-gap it'll be a lot safer. It's up to you if you want to take the risk. Personally where I live I wouldn't worry about it since lightning is very rare here. But your location may be quite different.
It's true, connecting it to earth, will help to reduce the voltage a bit, but not much, as the lightning strike is very fast, so the impedance will still result in a large voltage. A screened cable will provide more protection and covering it with an earthed copper strip will be better still.
 

Online paulcaTopic starter

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Re: 12V DC routing
« Reply #23 on: January 23, 2022, 11:19:31 am »
Anyway.  Lightening strikes are rare and as pointed out, if the house gets hit by lightening my 6mm cable disintegrating and the solar system burning, along when what ever is connected at either end, I will have even worse issues at that stage.  Especially as my house has no lightening protection to speak off.  I don't know how well iron rich red bricks conduct that high voltage.  I suppose that also makes it less attractive than the huge aluminium shopping centre building 200 yards away....  and I'm the only house in the street with no TV aerial which should work in my favour.  That said  I am one of the highest houses in the neighbourhood.

Anyway....

A small floating circuit with only one short run outside the window to the solar panel is a bit different than a 10 meter 6mm antenna floating.

So maybe, I should at least ground the solar panel system to a plug socket earth.  Not to protect it from lightening, but to stop it floating to some silly ass voltage in electrical storms etc.  I mean just because the once or twice I have measured it, it was within a volt of GND, doesn't mean it's always like that.  It could get way off if there is electrical storm activity.  No?

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Offline Zero999

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Re: 12V DC routing
« Reply #24 on: January 23, 2022, 11:39:24 am »
So maybe, I should at least ground the solar panel system to a plug socket earth.  Not to protect it from lightening, but to stop it floating to some silly ass voltage in electrical storms etc.  I mean just because the once or twice I have measured it, it was within a volt of GND, doesn't mean it's always like that.  It could get way off if there is electrical storm activity.  No?
That's a good idea. It's better to earth the negative, to reduce corrosion problems. The only down side to it being earthed is increased risk of short circuits, if the positive were to touch a grounded object. Obviously a fuse is a must, both to protect the cable and the battery from fire.
 


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