Thursday, 14 March 2013

A movement in “The Force”

The day started clear but cold as we made our way to the bus stop to catch the Jet Black 1 to the Sainsbury’s supermarket at Calcot.  Both Jan and I had our backpacks along with three Sainsbury’s reusable shopping bags.  The novelty of such a wide range of products on the shelves still interests Jan as she browses the isles. However the shopping didn’t take long and we were back at the Calcot bus stop in time to catch the same driver on his return trip.

Back at Waiouru the supplies were stowed before Jan headed towards the boatyard to check if there was any mail.  Meanwhile I did some fault-finding on the central heating control system.  The Hurricane heater was working directly from it’s integral switch however the remote control using the Empirbus system wasn’t!  It didn’t take long to establish the 12v relay was defective.  The sun had put in an appearance and the panels were producing 2.4 amps.  It seemed a good idea to get out of Waiouru and tilt the panels towards the sun.  No sooner had I done this than all the locals felt a small movement in “The Force” which resulted in the sun disappearing for the remainder of the day.  Actually it started to snow!  Now a decision must be made.  Will we experience wind which might blow the tilted panels off the roof?  If so, then we need go back out and to lay them flat.  Tomorrow’s weather forecast is for more fine weather but it starts to get nasty after that.  Once Jan returned from the boatyard I went down to the chandlery and purchased a replacement relay (£3.49) for the heater.  It was only a five minute job to complete the exchange and we now again have remote control of the heater.

I’ve set out to identify the best domestic battery bank charging regime for our particular circumstances and intend to document that process here over the next few days.  So if the subject of charging batteries bore you just skip this section.

Caveat..I’m going to attempt to write about this subject in very basic terms so if you already have a good knowledge of electricity then you may want to skip this first part... Sorry!

Part 1 Electrical Theory

First. A battery doesn’t create any electricity. Hopefully it stores power (energy) for future use. All batteries wear out. How you treat them may dictate the speed at which that occurs.

When it comes to the domestic batteries on your boat you have two options. Either purchase cheap batteries and pretty much ignore them anticipating they will need replacing every 18-24 months. The alternative is to purchase more expensive batteries and attempt to effectively maintain them in an effort to get many years from them.

We have gone down the latter path and purchased four Rolls 6V 450A/h flooded wet cell lead acid traction batteries. They have a five year warranty and have a life up to 15 years. As they were quite expensive we want to attempt to ensure they have a long life.

The boat has two alternators and their output can be combined to provide a theoretical maximum output of 225A (amps)

In the past I’ve found some people have difficulty understanding some electrical terms so I use water and plumbing as an analogy.

  • Voltage (Volts [V]) = water pressure
  • Current (Amps [A]) = volume of water
  • Resistance (ohms [R]) = length and diameter of the pipe
  • Watts (w) = a measurement of power (W=VxA)

We want to move the water created by our alternators to the batteries and from there to the consumers (lights, pumps, etc). 12V is a very low pressure system. Overhead transmission cables carry electricity at many thousands of Volts. Domestic main power is 230-240V. The consumers in the boat (lights, fridge, pumps, TV, inverter, etc) all use power which is measured in watts (watts = volts x amps). So if we have very low pressure (12 volts) then we need a large amount of water (amps) to achieve the required level of power.

Example: Assume we need 250 watts to run a fridge. At 12V we need 20.8A (W=VxA) but at 240V we only need 1.04A

Our boat 12 volt system is almost like a water wheel. There’s very little water pressure to turn the wheel so it’s turned using a large amount of water. A 240V system is like the high pressure water blaster that is used to clean down the hull. A huge amount of pressure and little water.

The third factor is ‘Resistance’. If you have a very high pressure then it’s possible to use smaller diameter pipes (eg, power transmission lines). On Waiouru our alternators produce a large volume of water at very low pressure so the pipes (cables) have to be very large. If the cables aren’t sufficiently large the resistance will cause the pressure (voltage) to drop and the cables will start to overheat. Without protection they might eventually catch fire.  To avoid this Waiouru has very large diameter cables between the alternators and the batteries. Just as important, large cables from the batteries to the inverter. 

Summary.  A low voltage system will likely require high amperage to deliver the required level of power.  Moreover a low voltage system requires large diameter cables to minimise resistance to the flow of electricity.

Next time.  Part 2 - Lead Acid Traction Batteries


Halfie said...

Sorry to nitpick, Tom, but the analogy for current should be water flow, e.g. in litres per second, not volume of water. Volume of water corresponds more with the energy stored.

Tom and Jan said...

You are correct. Perhaps I'm just attempting to over simplify the subject.