Monday, 28 January 2013

Always more to learn - charge controllers

My last post about my endeavours received a helpful comment from Brian off NB Harnser

My observation was about voltage of the two respective battery banks as reported via the newly installed Smartgauge. The voltages were lower under charge than on Waterlily and were lower than the output reported via my multimeter off the back of the alternator.

What I forgot was that on Waterlily there is an Adverc charge controller fitted. Plus as Brian pointed out Percy would/should have a split charging controller and suggested it may be a simple diode type. From follow up reading and as Brian suggested the diode type install does consume voltage. This would explain why more was showing on the back of the alternator than was showing into the batteries. (I am still assuming that if the alternator was giving out 14.5volts then under charge the batteries would show 14.5 volts rather than their current charge state?

So it appears at this part of my learning Percy is simple (I knew that) but never considered the electrics as I had rather stupidly assumed that they were similar to Waterlily.... Alternator charges the battery bank. I forgot the charge controller/relay/diode install on both boats.

Rightly or wrongly this is how I do my learning. I could just take it into the Streethay or Mercia and say.... fit me a Smartgauge, but where would be the fun in that.

Thanks Brian for your advice (I will add you to my blog roll, not sure how I missed you off) I will hopefully get to Percy this week and have a good look at the charge controller as to date I had paid it no attention.

Question is now I know I may have an inefficient charge controller do I look to add a Smartbank or Adverc controller? I recall on two separate occasions RCR engineers advised to disconnect the Adverc unit as it 'overworked' the alternator. Sometimes you could detect the changes to the loads the unit was applying by the alternator working harder and the engine note changing...

Problem is now I know that there is in effect a loss of charging current meaning my charging is less efficient. I feel for now I need to understand the relationship between demand and supply on the batteries, I can only really do this over a period of time being on the boat.....;-)


  1. Nev - I tried to leave this comment on your latest post on Percy, but it doesn't seem to have worked:

    Nev, there are only two circumstances where the voltage at the batteries would be exactly the same as the voltage at the alternator. One: no current is flowing; two: there is zero resistance between the batteries and the alternator.

    Case one could theoretically arise if the batteries just happened to be at the same voltage as the alternator - unlikely if you are measuring 14.5V.

    Case two would imply that the connections and the cables connecting the batteries and alternator together have zero resistance, which is impossible at normal temperatures.

    Ohm's Law is essential: V = IR (voltage equals current times resistance). The voltage here is that across the resistance in question, i.e. the cables/connectors. If we assume a reasonable alternator output of 20A, then a resistance as low as 0.01 ohms would give a voltage drop of 20 x 0.01 = 0.2V. A measurable voltage drop. If the resistance of the cables/connections were to rise to 0.05 ohms then you'd get a significant drop of one volt.

    As the batteries charge up, the current flowing from the alternator falls, so that the effect of the resistance diminishes. In the example above with resistance of 0.01 ohms and a reduced current of 2A, the voltage drop is now only 2 x 0.01 = 0.02V, barely noticeable.

    Introducing a forward-biassed diode, as in a split-diode-charging system, into the circuit immediately gives you an extra voltage drop of 0.5V - 0.7V, reasonably irrespective of the amount of current flowing.

    This gives an indication of the importance of good clean connections, and as thick and short a cable as practicable (the thicker the cable the less resistance; the shorter the cable the less resistance too).

    I hope this is helpful, if not to you, Nev, then to anyone else who might chance upon this!

  2. The very useful comment above was from Halfie, there seemed to be a problem leaving the comment - let me know if you are having difficulty leaving comments?

    Thanks again John for the helpful info.


  3. Hi Nev
    RCR have said the same to me. Personally I think they just want the alternators they include in their warranty deal to have an easy and very long life.
    There must be many many boats with this set up. Choice is the boaters but I am happy with mine.7 years and still works.