Time to say goodbye to 12v DC for boating?


I am now beginning to consider 12v power use on a boat as no longer required. There, I said it which is a rather surprising thing to hear and goes against 'conventional wisdom'. Now of course the use of DC power on board is going to be necessary, but I would much prefer to see all DC power use being run at 24v because this reduces your amperage needs, and in many cases reduces the gauge of wire needed.  When you look at everything from electric toilets, bilge pumps, lighting, heating systems and even marine electronics they all come in 24v versions - often at no more cost than their 12v cousins. 

As a society we have so much choice as to what is available to us for the home and increasingly we want these basics on board our boats.  I call them 'basics' because I do not consider it to be a luxury to have a Microwave, a Coffee Machine or a Freezer on your boat. So why do companies both when building new boats and those who supply the industry continue to be stuck in the dark ages of 12 volts and why do their products (such as a Fridge) cost so much money compared to their domestic mains powered counterpart?

Now some will say it is because of demand - there is simply not the unit sales of marine 12v fridges to cause their prices to fall in line with the colossal demand for domestic Fridges - partly this will be true, but overall it is because when one buys a boat you accept that you will just have to pay more for things to be used on it 'just because' and it frankly makes me sick!

I recently was looking at a company who supply LEDs for the marine and caravan market - they were selling 1w 12v LED 'bulbs' to fit into a 'G4' spot lamp fittings at £5.99 each! I might be able to understand this cost if the actual LED units were designed and manufactured in the UK and were of a higher quality than the 'domestic' versions one can purchase from China on Amazon or Ebay.  But when one looked into things in more detail they were the same mass produced LEDs being churned out of China, but being marked up to make someone here some money and of course, reference being made to being 'suitable for the harsh marine environment' adds to the cost. 

Now someone may say to me that it is all very well spending £80.00 on a Freezer for home but it just would not last on a boat so it is worth spending several hundred pounds more for one made 'for the marine market'.  Rubbish I say, for the cost of one marine model I could have a new Freezer every year for over six years for the cost of just one marine model.  Don't tell me a domestic Freezer will go give up after 12 months on a boat, and frankly, so what if it it did - for the cost they are to replace makes them practically disposable items these days.

But, you wonder this is all very well but you cannot expect to run a Freezer on a boat - they need 240v and that means an Inverter and the Freezer will consume a great number of Amps and you would need a massive battery bank to run the thing. Right? Let us look further and see what is needed to get a Freezer on your boat:

You would need an Inverter, and that should be and a Pure Sign Wave one too. But this is 2016 and who does not have an Inverter on a boat these days?   I’ll say for the sake of this example you’ve stuck with 12v so what you need to do is find out the wattage the unit consumes (I’ve seen a compact one on Amazon that uses 0.45Amps @ 240v for under £80.00 which is ideal for a small galley on a boat). Link: http://www.amazon.co.uk/dp/B013R42QD4  

This means your running wattage would be 108w but the compressor motor also needs a jolt of about three times its running wattage to start up, usually provided by a hefty Capacitor, this surge is needed for only a fraction of a second but the inverter needs the beans to provide this so just to start the freezer you would want to factor in say 400w which would reduce once running.  A nice  2,000w pure sign wave inverter therefore would be handy. Even nicer if it incorporated a 70Amp battery charger – so here is the beast – and it is coming in at 22KG. Link:  http://www.amazon.co.uk/dp/B00PYK573G  (You will see these branded as Sterling combi Inverters for £995.00, no kidding! go have a search)

Now, you need to know how much DC battery capacity you’ll need to have to run the Freezer.  I would personally have dedicated batteries for this task so in the worse case you lose some food not a bunch of other more vital equipment if the batteries are drained. 

So, let’s convert the Freezers AC running Watts to DC Amps. The starting wattage doesn’t last long enough for me to worry about, but we know our AC wattage is 108w because it uses .45Amps at 240v. Divide this by 12 (our DC volts) and multiple the answer by 1.1.  So 108/12x1.1= 9.9 - you would therefore need as good as makes no difference 10Amps DC to run the Freezer.  I understand that you should multiply by 1.1 to take into account the loss an Inverter (or inefficacy) it causes thus actually would require slightly more DC Amps than a ‘pure calculation’ of Watts/Volts = Amps
 
Now the above is only valid if the Freezer ran for an hour continuously, and in such time drew 10 Amps – so with this in mind  this would equate to 10Ah (Amp Hours). Now this if it were to run for 24 hours that would use rather a lot of amps - to break it down to a nice round figure I will say that the Freezer might run for 20 hours continuously (highly unlikely) which would equate to 456Ah! 

Freezers do not run consciously for 20 hours though (if your does you have an issues lol) – some say when calculating power with regard to Fridges and Freezers to work at a 50% run time per 24 hour period. I’ll go along with this for now but I am not so sure this is true. So, our 456Ah would reduce down to a more ‘manageable’ 228Ah being taken from the battery bank. 

This is still not making me happy; because the above is like saying that a Freezer will run (on and off) but effectively, none the less still be running for 10 hours in every 24 hours.  I just don’t believe a compact A+  rated appliance would run for 10 hours every 24 hours. 

So let’s dig deeper into the specifications of the Freezer and see if this helps matters. I find its Kilowatt Hour use per 24 hours is 0.4 (Kwh/24H): 0.4) or in ‘easy speak’ that is 400 Watts.  This is reference to a 240v supply, but it gives a good idea that in a 24 hour period it would consume 400w if plugged in at home. 

With this information we can now find out just how many Amps the Freezer is actually likely to sap out of your batteries in a 24 hour period and if my math is right, that means 36Amps. (400/12x1.1=36) 

On the face of matters that appears you could just use a single 110ah Battery and with careful thermostat control run the Freezer off this. Personally, I would opt for two to split the load over them and take into account the 88 – 90% efficiency an Inverter can provide and that you cannot comfortable discharge more than 50% out of a lead acid battery. Your usable capacity with just a single battery is 55Ah (110ah with two). 

So, it goes to show however, if you had a 24v system, with an Inverter that took 24v in and gave 240v out your amperage need would not be 36Amps each day for the Freezer to run but come down to something like 18Amps which is why I like more volts at DC and keeping the Amps down.

Comments

Popular posts from this blog

Brinks Belmore | The Big Tour - Day 7 (Part 1)

Brinks Rhapsody - The Review

Brinks Belmore | The Big Tour - Day 6 (Part 1)