Category: Interesting - Page 2

21 April 20238 April 2024

Planning for Solar Panels part 5 – Cable sizes, lengths and resistance.

“Why do cables matter, they are all just the same, just copper in a plastic tube” this used to be my attitude when prototyping small electronic devices running at 5V on my bench and for 99% of the time I was right, it was not until I got involved in low power DC systems i.e. Power Over Ethernet and fibre that 48V being used as a power distribution system for networking and domestic DC power then voltage drop become an issue.

For your Solar Panel wiring, we need stranded wire so it is flexible, it needs to be tin coated so that it does not corrode, especially useful on a boat, then the jacket needs to be UV safe as it is going to be left out in the sun, while also being tough and durable as it will be bent and flexed multiple times.

Solar or PV cabling being tin coated means we are not looking at strands of copper that are directly touching each other, each wire in this case has a thin coating of the less conductive Tin, Tin is only 15% as conductive as copper. This has a very slight effect on the resistance of the wire, also being stranded we have a minimal air gap between the strands so 4mm² of stranded copper wire is not as conducive as 4mm² of solid copper wire.

When I have calculated the resistance of copper wire in the past I have only used the resistance of pure copper in my calculations, but now things are getting a little more complex.

When planning your install you also have to remember there is a positive and a negative cable in the circuit, so a 20m distance between your solar panels and your MPPT or PWM solar controller is really 40m of cable, so double the resistance.

To enable me to see how much the resistance of the wire changed I originally built an excel spreadsheet that took into account the resistance of the solar stranded tinned copper wire at different diameters in mm², lucky the manufacturers only make a limited range of diameters of cable for solar systems with 2.5, 4, 6 and 10mm² being commonly available and they provide the resistance per Km for these.

For your use, I have converted the spreadsheet into some javascript code for this page, so you can input your requirements and see the effect of that cable resistance in terms of Voltage Drop as a value and as a % will have on your plans.

Typically for this kind of DC installation you want a voltage drop of less that 2% is best.

Have a play about and see what is the best cable size/cost is best for your installation

Prototype Voltage Drop Calculator

Select cable size in mm²

Enter cost per meter of your cable

Enter the system nominal DC voltage in volts

Enter the current draw in amps

Enter the one-way circuit length in meters

NOTE: This Voltage drop calculator is designed for Solar PV Cable, with Class 5 stranded tinned copper wire, which has a slightly higher resistance than solid copper wire.

Safety Disclaimer

16 April 20238 April 2024

Planning for Solar Panels Part 4 – MPPT vs PWM solar controllers

Lots of acronyms floating about in the solar industry, and as we are looking at Solar Panel controllers we have two more to look at PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking) these acronyms actually describe how each of these controller works.

Why do we need a solar panel controller, a 12V solar panel actually outputs between 18-20 volts so to stop your solar panels overcooking your battery we need to control the output to match the best charging voltage for your battery. Lets look at each type of controller see how they work and what the advantages and disadvantages of each have…

What is PWM – Pulse Width Modulation

PWM is an robust technology that is used in many devices around your home, I have used this to control motors, dim lights etc etc.

The idea behind PWM is that we take an incoming supply and switch it on and off quickly the difference between the on and the off parts can vary so for example a motor or light that gets a quickly pulsed (25+ timesaver second) at 50% on 50% off a motor will run at ½ speed or a lights output would be 50% dimmer, in these cases the speed of the modulation needs to fast so that the output looks smooth.

PWM Pulse Width Modulation diagram 25,50 and 75 Percent. — Example of PWM at 25, 50 and 75% pulse width.

PWM Controllers act like a switch, with the panels and battery connected the battery will pull the voltage down to its own level. As the battery charges that voltage increases until the batteries are fully charged. At this point the controller starts its float stage, switching on and off the supply from the solar to in effect trickle charge the batteries as required.

The inefficiency of the PWM controller comes from the way it the battery pulls down the supply voltage from the solar panels reducing the available power.

E.G. 100W Solar panel provides 20V at 5A

20V x 5A = 100W^*

When pulled down to the battery voltage it is only now supplying 13V at 5A

13V X 5A = 65W^*

A loss of potentially 35W or 35%

^*These are simplified figures to make the calculation simpler to understand

One of the other major drawbacks of this method is that you are working at lower voltages on the solar panel side of the system as they must match the battery requirement. With lower voltages the cable losses start to add up very quickly, meaning you will need thicker less lossy cables to connect your panels to the controller (We will cover cabling in a later post).

PWM Advantages

• Low Cost 10-25% of MTTP
• Simple reliable technology
• Available in sizes up to ~60 amps

PWM Disadvantages

• Potentially higher cable losses
• Less efficient ~75% conversion
• Solar Panel voltage must match battery nominal voltage.

What is MTTP – Maximum Power Point Tracking

An MTTP controller is divided into two parts, input from the solar panels and output to the battery. The input can take a much higher voltage that what is required charge the batteries. The send part of the MTTP controller, the output will convert this to the correct voltage for your battery to charge correctly (it is a smart DC to DC converter)

So:-

200W Solar panel providing 40V at 5A

40V x 5A = 200W^**

This is converted to 13V at 200W increasing the output to 15A

200W / 13V = 15A^**

^**These are simplified figures to make the calculation simpler to understand and exclude any losses in the dc-dc conversion

A correctly spec’d MPPT controller can accept much higher voltages from the solar panels, so we can use these higher Voltages on the solar side of the system to increase efficiency, and it will drop this voltage to the correct level for the battery current state of charge, it also monitors the panels Maximum Power Point (MPPT = Maximum Power Point Tracking) as it varies during the day to make best use of the power( Watts) available from the solar array.

While being much more efficient for larger systems (<200w) it also enables us to put the panels in series increasing the output voltage of the array therefore allowing us to use thinner cable while maintaining efficiency.

While PWM controllers are rated in Amps as they are fixed to the voltage of the solar panels, a MPPT controller is rated in Watts as they can accept higher input voltages.

Amps X Volts = Watts

50A X 12V Solar Array = 600 Watts

50A X 48V Solar Array = 2400Watts

50A X 100V Solar Array = 5000 Watts

A Watt is the measure of the amount of energy available, as we increase the voltage, we can get more Watts for the same number of Amps supplied.

MPPT Advantages

• Efficient ~95% conversion
• Solar not limited to battery voltage
• More efficient cabling options

MPPT Disadvantages

• Cost significantly more that PWM
• Larger requires more space
• More complex design (less reliable)

Conclusion – Which one will we use?

As you can already probably guessed, we are going for the most efficient 800W+ system we can get and in our case that means using an MPPT controller. We have limited space for panels, but almost no limit on battery size and the necessary equipment storage so the advantages of the MPPT outweigh those of the PWM Controller.

While researching this we did come across a video from Australia that is directly targeted at the Van-life market, but had excellent examples of PWM and MPPT with solar.

Excellent Video on PWM vs MPPT

Safety Disclaimer

10 April 202325 May 2023

Installing MC4 Solar Connectors

The MC4 connector has become the standard for connecting to solar panels, it is IP67 rated being both water and dustproof, they are relatively simple to install with the correct tools and luckily these tools and getting cheaper and cheaper.

Most solar panels already come with a male and female MC4 connector already fitted for ease of connection.

These connectors can be used on 2.5mm, 4mm and 6mm single core solar PV cable, it is highly recommended to use the correct cable as the outer sleeve is designed to provide protection from heat, UV, oils, and solvents, it is basically a tough robust cable that is designed for outdoor use.

Today I am going to show you how to crimp on your own MC4 connectors using a cheap crimping tool kit that is available from eBay and amazon.

Click here to order your own MC4 Crimping Kit from Amazon UK

So what we get in your MC4 crimping kit

A ratchet crimping tool
2 X MC4 spanners, for assembling and disconnecting the connectors.
10 Compete MC4 plastic male/female connectors bodies.
10 Female metal ferrules and 10 Male metal ferrules

All you will probably need now is some wire cutters and strippers and you are ready to install your own connectors.

Each MC4 connector is made up from a male and female plastic parts and male and female metal ferrules, the male metal ferrule fits inside the female plastic body and vice versa.

We made a video to show you how simple this can be

How to release the male/female parts

The plastic parts normally come already clipped together, you can use your fingers to release the clips holding the male/female parts together, but it is much simpler and safer to use the two prongs on the end of the blue plastic MC4 spanners provided.

Crimping on the connector

First we need to stripped back the plastic coating to expose approximately an 1/2 inch / 12 mm of the copper wire.

We are now going to crimp on the male metal ferrule.

Push the stripped wire into the ferrule so that the insulation is pressed up to the tabs we are going to crimp.

Then depending on the wire size select the smaller position on the crimp tool for 2.5mm, the middle for 4mm and the largest for 6mm wire, there are normally marked with the correct sizes.

The ferrules tabs should point towards the top of the crimp tool so that as they are crimped the tabs are folded over to tightly grip the exposed wire.

You can use the ratchet on the crimp tool to hold the ferrule while you insert the wire into the correct position before crimping.

Now push the male metal ferrule into the back end of the plastic female connector as until it clicks home.

If you are using the 6mm cable, you may have to unscrew the cap and removed the silicon seal with its crown clamp ring and push these onto the cable first before pushing the ferrule into the body of the connector.

Then push the silicon seal carefully in to the connector body and then lightly screw down the cap.

You should then use the MC4 spanners to tighten the cap firmly onto the body so that no screw thread is visible, at which point the connector will click, it should now fully tightened and sealed, best to give it a visual inspection to make sure you have not crossed the threads 🙁

We have now completed one end of the mc4 connector, we now repeat the process but using the male plastic connector and the female metal ferrule to make up the other side of the connecter.

Safety Disclaimer

24 November 202213 March 2023

Winter Frost Protection Year 2

Last year we set the boat up with two thermostats, a normal one that controls the Eberspächer D4W diesel heater when we need it, initially the heating was just a switch on/off. I also fitted a frost thermostat both of these have a switch so they can be taken out of circuit.

So this year we made sure the diesel tank was fully topped up switch in the frost thermostat as low as I could above freezing (about 2°C).

Update Feb 2023 – This seems to have worked well again this year, we have used a few gallons of diesel, but the boat seems to be free of any frost damage.

21 March 202224 February 2025

Batteries and Battery Monitors Part 4 – Battery Monitor & The Shunt

Our Battery Monitor

I have installed a Victron Energy BMV-712 Smart battery monitor, but there are many other options from other manufacturers, see the listing at the end of this article.

I selected this one as it gave me the opportunity to connect to my onboard Raspberry Pi which I am using to give some smart boat features and provide live location tracking for my website in the future.

The battery monitor comes in two parts, a display which provides a readout of the battery status and a means of programming/setting up the system for your configuration.

The Shunt

The second part is called a shunt, this needs to be placed, in our case, on the negative side of the battery bank, so that everything must pass through it to reach the battery, there should be no other connections to the negative side of the batteries else your measurements will never be accurate.

A shunt is a resistor of very low but known value that is placed in parallel with a voltmeter so that all the current being measured flows through it. The voltage drop across the shunt’s resistor is measured; this voltage drop across the shunt is proportional to the current flowing and can then be calculated using Ohms law (Current = Volts / Resistance).

Shunts are rated for the maximum current they can measure, in our case 500A which at 12V is 6000W, more than enough for our boat.

The battery monitor also has an extra cable that connects to the positive side of the battery bank to power the electronics and the display, but also to measure the current battery voltage, we have an extra wire connected to the starter battery, so we can monitor its voltage as well.

Peukert’s Law

To then calculate the remaining capacity of the battery, the monitor uses an adaptation of Peukert’s Law** which can be used to calculate the capacity of lead acid batteries at different rates of discharge. As we discussed earlier, the discharge rates affect the battery capacity.

** Developed by Wilhelm Peukert (1855-1932) Peukert’s law is used to calculate the batteries deliverable capacity at the current given rate of discharge, His law describes the batteries capacity at a constant discharge until it reaches its cut off voltage, below which you can damage your battery, this constant is called ‘K’, for example K=1.25 is used for our flooded lead acid batteries. There are however some limitations to this law as it does not consider the batteries temperature or age. I expect each monitor manufacturer modifies this to consider these extra factors when displaying the results, our system records each battery charge/discharge cycle.

The capacity of a battery falls at higher rates of discharge because the chemical reaction within the battery reaches its maximum speed for the given plate size and therefore the voltage drops. If left to recover, that missing capacity will return.

Using these calculations, a battery monitor can calculate the available power remaining while the battery is in use (under load) and as that load changes or even as the battery is charging, it can display the current State of Charge (SOC).

Now you know what is happening?

Armed with this information, you can then decide how you want to operate your boat and if you will need to start the engine to charge the batteries. One of our future projects is to work out the size we need for some solar panels, and we will use the data from the battery monitor to help calculate the size system we need, but that is a topic for another day

Battery Monitors Suppliers

Victron – www.victron.com
NASA Marine Instruments – www.nasamarine.com
Advanced Yacht Systems – www.advanceyacht.co.uk
Simarine – simarine.net/
Votronic – www.votronic.de

Safety Disclaimer

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