Post by Paper2002ADOpinions please - what charging amps should I expect from a 70 amp alternator
on tickover - about 950 rpm - and at my normal cruising revs of around 1200? I
seem to get around 40A at tickover, and around 60A at cruising speed.
Is this alternator, assuming the output is what I can expect, adequate to
charge five x 110 A/h domestic batteries via a Sterling 4-stage regulator, and
to keep a Sterling 3KW inverter happy? I will be sure and charge depending
upon usage - easier to keep an eye on with my new Sterling amp/hour counter!
1. It all depends upon the engine pully - alternator pully ratio, which in
turn depends upon the maximum emgine revs and the maximum alternator revs.
It will also depend upon how discharged the batteries are when you take the
readings. However assuming half discharged batteries they look a bit low to
me, but this could be the pully ratio. My 60 amp Paris-Rhone on the Bukh
delivers up to about 40 amps on tickover and maximum at a bit over 1000rpm.
Do not take this to indicate a fault on your system, the batteries could
just be well charged.
2. To answer your second question you MUST do a power audit (look in the
course notes on www.reading-college.ac.uk/marine). Remember to devide you
inverter loads by 10 (20 for 24 volt system) not 12 as you would for the 12v
loads.
Then you decide how long you are going to run for each day and see if you
can put back 110% to 120% of the figure your power audit comes up with.
Assume a steady input of about half your alternators rated output, but once
you have done the time calculation make sure that output has not dropped
much below about 10 amps by the end of the period - if it has, your
batteries will be well charged, but you may have to use less than 50% of
rated output. However if the batterioes are becomming well charged in that
time you will have the answer to your question.
Traditionally we would say limit you charge to about 55 amps (10% of 550
ah), but with modern batteries you may well be able tyo charge at 110 amps
without overheating and possibly reducing the battery life. As your
alternator is a 70 amp job you have no worries on that score.
Not too sure about the Sterling - no-one has yet explained to me how it can
control some modern alternators without modifying the alternator.
Hope thgis gives soem help, but i am sure Peter or someone else will add
their oppinion.
Tony Brooks