- 17 Jul 2009
- #1
L
Longshanks
New member
Might be the first time anyone has ever wanted to REDUCE the output from an alternator, but at 60A rating, our retro-fitted auto unit is actually too powerful for the small engine!! At 60-70% state-of-charge, the immediate output when I switch the battery in is in excess of 30A and I hear the engine slow right down. I'm worried what will happen if the service battery runs flat and the alternator tries to output 60A even for a 5-10 seconds...I suspect the engine will stall. Any ideas on how I could limit the alternator output in such a case (other than fitting a smaller unit)??
- 17 Jul 2009
- #2
seamaster30
New member
Longshanks said:
Might be the first time anyone has ever wanted to REDUCE the output from an alternator, but at 60A rating, our retro-fitted auto unit is actually too powerful for the small engine!! At 60-70% state-of-charge, the immediate output when I switch the battery in is in excess of 30A and I hear the engine slow right down. I'm worried what will happen if the service battery runs flat and the alternator tries to output 60A even for a 5-10 seconds...I suspect the engine will stall. Any ideas on how I could limit the alternator output in such a case (other than fitting a smaller unit)??
Can you fit a larger pulley wheel?
- 17 Jul 2009
- #3
exfinnsailor
New member
Reducing output
You could reduce it by fitting a larger pulley to the alternator . The engine is having trouble with the higher ratio so making the alternator pulley larger will in efect slow it down and thus produce less ..
- 17 Jul 2009
- #4
savageseadog
Well-known member
Alternator output is set by field excitation. That's what the regulator does, it increases the field current when the output voltage drops and the other way about. And there's the problem there is regulation of the charging voltage and indirectly the charging current but there is no current limiting. The regulator is a sealed unit and you will be unable to access the innards, you could go for an external regulator but they are all designed to maximise the output to charge the battery more quickly. You would in any case need to monitor the current in order to control it, which would require a shunt or hall effect probe. Only other way would be to reduce the initial alternator output voltage either manually or automatically from the normal 13.8-14.5V range. Again the lack of access to the innards makes this unviable.
Possible solution. You could put some resistance between the alternator and battery, perhaps 0.5 ohm (experimentation required) which would limit the maximum current but still allow full charging voltage. It would need to be air cooled and fireproof. Perhaps a length of thinner cable with a high temp insulation.
- 17 Jul 2009
- #5
G
gordmac
Well-known member
60 amp would be about one horsepower, a bit more depending on alternator efficiency. Seems strange that this would stall your engine, turn the tickover speed up a bit? There was a device available, sold for motorsport cars, that could manage the alternator output but I can't remember much about it.
At the end of the day if your engine is very low power then the only real solution is to change the alternator output to a lower value. Sometimes doing the job right is easier than trying several bodges!
- 17 Jul 2009
- #6
P
peteK
Well-known member
What is the engine and what was the original alternator output,most small engines slow down a bit when the alternator load comes on them. What size is the battery being charged? On my last boat I had two 85amp batteries being charged from a 6hp Petter via a 35 amp alternator.I switched them in seperately to reduce load on engine,after a few minutes Ide then connect together.
- 17 Jul 2009
- #7
K
KenMcCulloch
New member
On my 10hp Yanmar with a standard (35A I think) alternator you can hear the revs drop when switching from the engine battery to both for charging, quite noticeably if the service battery is a bit discharged. Just putting a few revs on before switching works for me, it's only for a few minutes as the current draw evidently reduces quite quickly..
- 17 Jul 2009
- #8
Anthony
Active member
Why not just up the revs a little before switching in the domestic bank it it ever does cause a problem. Has it ever actually stalled? Are you sure the engine is running ok, maybe its a symptom of an engine problem.
Ants
- 17 Jul 2009
- #9
K
KenMcCulloch
New member
Anthony said:
Why not just up the revs a little before switching in the domestic bank it it ever does cause a problem. Has it ever actually stalled? Are you sure the engine is running ok, maybe its a symptom of an engine problem.
Ants
...and I claim a clear Lakesailoring.
- 17 Jul 2009
- #10
L
Longshanks
New member
Thanks everyone. The pulley idea is an interesting one. No, the engine has never stalled, just slowed-down noticeably, and actually I usually switch the service battery in whilst we are motoring, so the the prop is sucking HP as well. Perhaps letting the engine deal with the initial alternator surge whilst moored is the answer (along with some higher rpm). Good thoughts anyway.
- 17 Jul 2009
- #11
halcyon
Well-known member
savageseadog said:
Alternator output is set by field excitation. That's what the regulator does, it increases the field current when the output voltage drops and the other way about.
The alternator regulator contols voltage by cutting feed / or earth to the rotor , ( the bit that spins ) This causes the alternator output to drop to zero amps. With no charge current, the battery voltage falls, so the regulator cuts back in and charge commences. Thus voltage is controlled by turning on or off the output amps. So when regulating, the alternator output may be zero for a lot of time, you only get a charge pulse as it turn on, then back off. The voltage you see below the regulation voltage is set by the battery, not the alternator.
The 30 amp charge seen by the OP, is controlled / limited by the battery, not by the alternator.
It is quite possible that with a flat battery the charge current will only be 30 amp, or a little over. The 60 amp is only a nominal maximum output for the alternator.
It would be better to run the battery a little lower and try it, and then a bit more, see if you have a problem.
Easier than rebuilding the alternator.
Brian
- 17 Jul 2009
- #12
savageseadog
Well-known member
Sorry, what you're saying is simply wrong apart from the armature being the field winding, which it is. I can assure you that the output voltage is regulated by the regulator that normally sits in the alternator. Current flows into the battery as a result of the alternator output voltage being higher than the battery voltage.
- 19 Jul 2009
- #13
L
Longshanks
New member
halcyon said:
It is quite possible that with a flat battery the charge current will only be 30 amp, or a little over. The 60 amp is only a nominal maximum output for the alternator.
It would be better to run the battery a little lower and try it, and then a bit more, see if you have a problem.
Easier than rebuilding the alternator.
Brian
Yes, I'm going to try that next week. I do need to find out what's going to happen if I flatten the battery during a passage and then use the engine to recharge it.
- 19 Jul 2009
- #14
A
Anonymous
Guest
Longshanks said:
Yes, I'm going to try that next week. I do need to find out what's going to happen if I flatten the battery during a passage and then use the engine to recharge it.
Savageseadog has explained how it works (rather well, actually). Normally with smaller boats you have smaller batteries. The terminal volts on small batteries tends to rise quite quickly with modest current. So, suppose you have an 80Ah battery the initial current might be 60A but the terminal volts will rise quite quickly to 13.5V and the current will have fallen a long way. If you had 800Ah then that 60A might carry on for several hours. So normally these things are not a big problem as small boat goes with small engine and small battery.
If you are having problems you might have a dodgy battery. If a cell goes short circuit it will draw a high current. You can check in two ways -- use an hydrometer in each cell and the shorted cell will show as undercharged. Or use a load tester; someone in your vicinity will have one to lend you or take the battery into a battery specialist such as a Lucas centre.
Incidentally, you could do worse than call a specialist to check it all out for you. These days, electricity is vital for boat systems.
- 19 Jul 2009
- #15
Danny
New member
I don't think you need to worry about stalling. As the revs drop the maximum current that the alternator can deliver also drops. At tickover, or when starting the engine, the power consumed by the alternator (and the current delivered) should be quite small, even if the battery charge is low. I suspect you notice a significant drop in revs because you switch in the service battery when under way. If you switched it in on tickover I think you'd find the effect will be much smaller.
- 19 Jul 2009
- #16
TrueBlue
Well-known member
Methinks you do protest too much. Not unreasonably you are concerned about the engine appear to be labouring by the drop in revs: but I think, perhaps you are putting the wrong construction on it, and that's because you're not feeling any direct benefit. When you engage the gearbox to move the boat, you increase the throttle to give you the desired level of motion; the same in a car, bung it into gear and press the pedal to the metal.
With the alternator, you change the switch setting and the engine falters a bit. Shock, Horror, because you can't see anything happening.
Fiddling about with alternator pulley - although being the easiest and cheapest solution, isn't a good idea as it will negate the advantage of the larger alternator and you'll have to run the engine faster to charge the batteries when you need it.
Anything else will cost more and introduce more complication.
Do what I do - just advance the throttle lever a tad - and don't worry It's doing what it's supposed to.
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