Cable sizes for battery camping system

This is a continuation of the new project, a portable lithium-battery power system for camping, see the first post here:

https://bkhome.org/news/202004/portable-lithium-power-for-camping.html

It doesn't have to be a lithium battery -- my project can easily be used with a deep-cycle AGM/GEL battery. The Amptron DC-DC charger that I am using can handle all types of batteries.

In previous camping trips, I used somewhat undersized electrical cables. I used a "6mm" automotive twin-core cable, 15m long, from solar panel to the MPPT battery regulator. From the battery to my tent I used a Narva "heavy duty" 5m cigarette-lighter extension cable (purchased from Autobarn).

Thinking about the cable run from solar panel to MPPT regulator. You need to be careful what the vendor is selling you when they claim "6mm". 6mm automotive cable is slightly less diameter than 10AWG. AWG is the same as B&S -- these are American wire standards, widely used internationally, including Australia.

I found a nice table of wire sizes, from here:

https://offroadliving.com.au/blogs/articles/q-a-series-3-what-is-b-s-and-why-is-it-so-important-to-the-output-of-your-solar-panels

The table doesn't have it, but the wire diameter of "6mm" auto cable is 2.42mm. Those diameters are actual copper wire diameter, not including the insulation.

The big question is, how much power was I losing in that fairly thin cable? There is a formula:

		Length (m) x Current (A) x 0.017
Voltage drop	=
		Area (mm2)

...that constant 0.017 is for copper wire.

My 15m cable is 30m round-trip. My "250W" panel was at best only giving about 10A. Plug the values into the formula:

Voltage drop = (30 x 10 x 0.017) / 4.59
             = 1.11V

Let's say that the panel is putting out a voltage of 16V, so the power output of the panel is:

Panel power out = 16 x 10
                = 160W

And the loss in the cable:

Power loss = 10 x 1.11
           = 11.1W

...that is a power loss of 7%

Do I want to throw away that much power? No...

Firstly, I decided on a shorter cable. I used 15m before, as there was one site where I had to place the panel a long way from my car and tent. However, can make-do with just 10m.

I purchased 10 metres of 6AWG from Big Al's on eBay. I like Big Al's, delivery has always been remarkably fast. Though, my purchases were pre-Covid-19, so I don't know about the current situation.

This is the cable I bought:

https://www.ebay.com.au/itm/124137673594

That is heavy cable! Expensive, but much cheaper than buying it locally. Plugging it into the formula:

Voltage drop = (20 x 10 x 0.017) / 13.30
             = 0.255V

So the power loss is 2.55W, which is 1.6% ...ah, much better!

Then there is the 5m cable running from the battery (in the car) into my tent, to run light, computer, fan ...and maybe even a TV. I have a fridge, but that stays in the car.

I have not done quantitative measurements yet, but did have a problem on the last camping trip. I took two laptops, my old power-hungry one, and a modern baby one. I powered them (one at a time) through a 150W inverter, and it would cutout when tried to run the old laptop -- this was because the voltage into the inverter dropped to 11.5V, which causes the inverter to turn off.

On that occasion, I was also running an LED lamp, and was charging my phone. Sometimes plugged a USB 1TB HDD into the laptop.

So the plan is to upgrade to 8AWG cable, and probably keep the length of 5m, or maybe a tad longer, say 6m. Will need to do some calculation of power load and voltage drop first. 

Tags: nomad