Question on "alternator max output" - my M3-20 has a 51 amp alternator - is that 51 Amps per hour, 51 Amps constant or a potential for 51 Amps depending on battery condition, engine speed etc?
I wired to the house battery with a 60 amp maxi fuse as a service cutoff in the engine compartment, should that be fused at 75 amps or replaced with a switch and add a terminal fuse at the buss bar to the house bank?
The 51A is the alternators SAE J56 "rating" and when cold it can produce more than 51A but when hot considerably less. Alternator
ratings are based on either ISO test standards or SAE test protocols. For SAE J56 the "rating" is at 6000RPM and 13.5V in an ambient room temp of 73F. In the real world, on a boat in a hot engine room, you will rarely see the full output, except when cold. This is especially true with an automotive based unit such as the 51A alt found on many Universal engines.
Expect about 25-35A, as installed, as your max bulk output for a 51A alt.. It's still best to size your fuse at 150% of "rated" so you don't inadvertently cause a nuisance trip, which could blow your alternators diodes..
In terms of units there really is no term "
amps per hour". The unit of current is called
amps or
amperes. If your alternator could run at 51 amps continuously for 1 hour straight, and it can't, you would have delivered 51
ampere hours to the battery, but not 51
amps per hour or
amps/hour.
I would still
know what you are saying but an EE might look at you like you have eight holes in your head if you said that.
Amps or
Ampere are a measure of electrical force at a
single point in time.
An Ah or ampere hour is a measure of energy use over time.
1 amp for 1 hour = 1 Ah
"My alternator is putting out 51 amps." Is correct..
If you ran if for one hour it would have delivered 51 ampere hours but what if you only ran it for 10 minutes? If it was delivering 51
amps but for only a 10 minute duration it would be:
51
amps X 0.17 hours = 8.67
Ampere hours
"My alternator is putting out 51 Amps per hour." Is incorrect because
amps is an instantaneous measurement..
A battery for example is a storage device and as such contains XXX number of
ampere hours of energy or
Ah or
amp hours of energy capacity. Of course that stored Ah capacity is only true at 77F and at the 20 hour discharge rate.
A battery for example is not a
"100 amp battery" it stores 100
amp hours, at 77F, at the 20 hour discharge rate. It would be referred to as a
100Ah battery.
What about a starter motor that draws 286A, seems like battery killing discharge amperage right?? Starter motors only run for very short duration's so the
ampere hours consumed are negligible.
Cranking Amps Converted to Ampere Hours:
0.75 Seconds is approx 0.0002 hours - 286A X 0.0002 = 0.06Ah's
1 second is approx 0.0003 hours - 286A X 0.0003 = 0.086 Ah's...
2 Seconds is approx 0.0005 hours - 286A X 0.0005 = 0.14 Ah's
3 Seconds is approx 0.0008 hours - 286A X 0.0008 = 0.23 Ah's
4 Seconds is approx 0.001 hours - 286A X 0.001 = 0.28 Ah's
5 Seconds is approx 0.0014 hours - 286A X 0.0014 = 0.40 Ah's