The charge regulator in most alternators relies on a minimum voltage, which can only occur at a particular revs. For a small petrol engine this would be around 800rpm, a diesel around 500rpm. Under those revs there is not enough volts generated to start charging. It is possible your idle revs are too low. There should be a charge indicator somewhere that is glowing, and goes out when you blip the throttle. If this is the case increase your idle revs.
...or that's what I called it, but three different mechanics used similar terms to describe the process by which alternators get going. They all said it was quite normal to have to rev the motor a bit to 'excite' (all three used this word) the alternator and then lower the revs. They may have been right. In the three years since then, we have started the engine, 'given it the gun' for just a moment to get the alternator going and then dropped the revs back to a sensible count and never had a moments'...oh, oh, I don't think I want to finish that sentence and tempt fate, but you get my drift...it seems it's a non-problem...Cheers!
Maybe that is the problem? I have to run the engines up to say 1500 revs to get the alternators to kick in. The rev counters are of course dead until charging starts. There is no other way of telling if a charge is flowing other than the voltmeters suddendly rising. Do you think indicator lamps would help?
Ever since we changed over from generators to alternators (certainly on cars) there seems to have been around the idea that these 'magic' thingies are able to supply all our needs under all circumstances.
One snow and windswept day I tried to help a Volvo owner whose car would not start even though he had been 'driving' for an hour. The battery was flat. But I've got an alternator he cried. Driving was not the right word, he had been in a traffic jam for an hour. As I pointed out to him he had his headlights on, heater going full, rear window heater on full, etc. This is power; you have to put power into a generator to take power out.
Yes, an alternator will generate pulses of current that will slightly charge a battery at tick over. This is how we were sold them. Just turn on a heated rear screen and listen to the revs drop a bit. Don't forget that at 60Amps this is equivalent to about one horse power and you are not going to get it at no r.p.m. Your engine, whatever rating, is producing just enough power at tick over to overcome its own frictional losses, there is very little left for other things.
This is a general plea. Not only do you have to sometimes kick them into generation but you can then NOT expect them to run your power steering and recharge the battery on tick over revs.
i.e. be carefull, watch your battery status all the time if you have ANY current drain. car, boat, aeroplane.
The fact that your tachometers don’t start until the alternator is giving output, suggests they are driven (electrically) from the alternator “stator” O/Put. This indeed implies that there is insufficient excitation current getting to the field coils. What is happening here is the residual magnetism of the rotating part eventually starts off the charging process, but only at the higher revs.
I’m surprised there are no warning lights? I guess your boat is neither that old, nor a one-off, so what do others like it, have in the way of dashboard layout?
You mention the voltmeters, do you mean they rise in volts, or indicate nothing until charging starts? The latter does not sound good, i.e. something in the wiring is missing from battery to the whole panel supply rail (a fuse?).
There is a vague possibility that there is some delay mechanism built in to the charging circuit, either by way of the oil pressure or a simple timer, which keeps the alternators from taking any load until the engines are running well. If you leave the engines ticking over, do they ever start charging, or do you always have to rev ‘em a bit?
Be interested to hear the outcome of this one - Hope you solve it!
You say Could it be anything to do with the fact that the batteries have been on shore power charge shortly before starting the engines, could be if shore power charger is left on while the engines are running or just turned off before. we have a similar effect with our consta volt and lucas ac9 s. Charge lights glow slightly till the fans drop the voltage or the engine is blipped. Is also more likely to happen when a split charge diode is installed. If the effect has only recently started check the diode if fitted, and if not check main diodes in alternator. Hope that helps David
You're absolutely right not to tempt fate. 20 yrs ago I was driving across France with a heavily pregnant wife and almost all our possessions. '2000 miles round trip,; sez I, 'this old car's done well.' With which it went phut.
Hi CRB Strangely there are no indicator lights at all on the dash! The voltmeters indicate a little over 12v until they suddenly rise to 13+v when the alternator kicks in. It's quite a long delay, could be minutes and the revs have to be blipped quite high. Could it be anything to do with the fact that the batteries have been on shore power charge shortly before starting the engines? The system is Taiwan "American" and so I have little to compare it with. Bernie
I’ve not heard that fully charged batteries make a difference. The Alternator should still “work” even if in float mode, so there ought to be some O/Put at all times, otherwise your rev counters would turn on & off at idling speed? Presumeably once it has got going, it stays thus, until you shut the engines down again?
Very interesting - No lights? What happens if a drive belt fails, or the oil pressure drops dangerously low?
Bernie, the situation you have is quite typical and I do not beleive you have a problem at all. OK some installation are such that the alternator starts charging at lower revs than yours, but so what. As long at you do not blip the throttle before you have full oil pressure, then just blip the throttle to start the alternator(s). If you had warning lights on the for the charge system you can change the wattage of the bulbs to chage the resistance and therfore change the kick-in voltage. As you do not have lights (not un-common) then the resistance is built-in somwhere. You may be able to find out where it is and change the resistance, as was suggested by a previous person, but I recommend that as your alternators charge OK once started that you leave well alone.
I had exactly the same problem on one engine of a twin setup. After many, many hours of head scratching and testing, it turned out to be a corroded contact in the engine loom connection. Might be worth a look