Alternators AMP's
04-26-2004, 12:48 PM
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Alternators AMP's
What size alternator (amp's) should I have to run an MSD system and an electric fuel pump. Also how many amp's should the alternator be putting out at 800, 2500 and 4000 rpm's.
Last edited by seahawk; 04-26-2004 at 12:52 PM.
04-26-2004, 08:04 PM
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MSD pulls around 8 to 16 amps, rising with RPM.
Fuel pumps vary in current draw, but figure 10 amps for most.
Stock alts put out 75% of rated current by 2500 rpm, if pulleyed at factory spec.
Baldors and other alts put out 75% at as low as 1800 rpm.
Your trim pumps will be your biggest amp eaters, not the ignition and fuel pump.
Fuel pumps vary in current draw, but figure 10 amps for most.
Stock alts put out 75% of rated current by 2500 rpm, if pulleyed at factory spec.
Baldors and other alts put out 75% at as low as 1800 rpm.
Your trim pumps will be your biggest amp eaters, not the ignition and fuel pump.
04-26-2004, 09:03 PM
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Generally most alternators put out between 60 and 135 amps... it depends on how much power you need. So... if you are running during the daytime... not constantly running the trim pumps up and down... you can easily get by with a 60 amp alternator... I think most marine alternators start at 85... If you are planning on running lots of lighting in the cabin at night... and have a bank of batteries to charge while running... go for as much amperage as you can fit into the mounts.
04-27-2004, 11:18 AM
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I am getting two diff. reading from my amp meter at dash at idle (port = 0 to 3 & stbd = 15) and the stbd motor will not start as easy at end of day or when batteries have been used the most. the batteries are new and the stbd side always seems to be weaker. where am I loosing the amps???
04-27-2004, 11:24 AM
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If you truly have an ammeter... it shows the amount of current flowing through the circuit... if the starboard ammeter is showing more flow... it's going somewhere.... I don't know your wiring or how the alternator is wired so I'm only guessing... you say that the starboard motor cranks slower... and yet it is getting more power through the ammeter... maybe you are using the starboard charging system more... what I mean is you have more accessories on that system...
04-27-2004, 11:52 AM
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They are ammeters. Yes all of the accessories are on the stbd side but when all is off it is still the same. Do you think the amps read higher because the power is not getting back to the battery.
04-27-2004, 12:10 PM
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The ammeter shows the amount of load passing through it... if there are more accessories loaded on the starboard charging system... it's going to read higher... that is normal..The problem may be that there are too many accessories on the one side... that is why the charging system is having a hard time keeping up with the load... again... I don't know how your boat is wired... so I'm just guessing... If the power wasn't getting back to the battery.. you wouldn't see as high a load reading on the meter... but... it all depends on how your boat is wired... there could be more accessories attached to the starboard charging system that aren't tied to the ammeter... the ammeter only reads the current flow on what is wired through it... it may be the power to the batteries doesn't even flow through the meter... Usually to get an accurate reading on what the charging system is doing, ALL power is wired through the ammeter.... if the charging circuit bypasses the ammeter... the ammeter hardly moves at all... I've converted a lot of old classic cars from generators to alternators... but the original wiring isn't able to handle the extra current without melting the wires... so I have to bypass the ammeter and run an extra wire to the battery... that in essence bypasses the ammeter.. then the ammeter only moves measuring the current that is going to the accessories that are wired through it... still following me? So... the charging system can be putting out 60 amps... but the ammeter only shows about 5 amps movement... This may be why your port ammeter isn't moving... and your starboard ammeter is... again... who knows what has happened to your wiring system... I've found boats that have had both charging systems cross wired... then the voltmeters act alike... even when the battery switches are on separate batteries. It can all seem confusing if you don't understand how the boat and the two charging systems have been wired... or more importantly.... mis-wired. Most boat manufacturers install voltmeters rather than ammeters... because for an ammeter to work properly.. everything has to be wired through them.. with a voltmeter.. it measures the amount of power in the battery. So it only needs one wire going to it.
Last edited by Reed Jensen; 04-27-2004 at 03:20 PM.
04-27-2004, 01:19 PM
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The alternator has an orange wire that gos to the ammeter at dash from there a red wire gos to the starter then from the starter to the battery. Are the charging systems crossed when the batteries are on both???
