Real Life Alternator Output?

Tim Good

Tim Good

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- This morning my 4 x Trojans were 50% discharged
- I have 60amp Alternator.
- I’m motoring along at reasonable revs at 6kts
- My battery monitor stated showing 40amps but after about half an hour it’s dropped to about 30amps.

Does this seem normal or a little on the low side?
 
Perfectly normal. The regulator is doing its job making sure the voltage on terminals doesn't exceed 14.5 V (or whatever voltage it is set for). As the batteries are getting closer to full charge, current has to be reduced to achieve the correct voltage drop across the terminals.
 
My 2 X t105 will start to tail off about 75% or so from a 20a mains charger when it gets to absorption, with absorption at 14.9v so not that different, that's in the warm as well - what sort of temperature are yours seeing this? They accept less when cooler.
 
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And if you think the alternator is failing try adding some extra big loads... It will likely up its output to support them...
 
As said, northcave, all normal. Our 4 x T105 behave in the same way, as as every system I've had before that. Since the effect becomes even more pronounced after about 85% charged, this is precisely why it takes so long to reach 100%. (But it's important that lead-acid batteries get to 100% fairly regularly.)

The alternator will produce more if the system's subject to a large load, such as the windlass lifting your gear. The battery monitor will only show the net current to/from the batteries, rather than what the alternator is actually producing. But you could expect to see a brief drop to discharge, followed by compensating extra output from the alternator as it responds to the continued drain. (Depending on its rating, if the windlass is working hard your 60A alternator might well not be able to keep the charge net-positive.)

P.S. ShinyShoe said the same rather more succinctly whilst I was typing.
 
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Measure the volts.
The alternator will attempt to hold its regualted voltage, but cable resistance will mean the battery voltage may be either slightly less or significantly less.
If you want to charge fast, you need to increase the volts.
 
Ok thanks all. Seems like it should be working as it should. Equally importantly it doesn’t seem to interfere with the solar controller by forcing it into float.
 
If you want a higher charging rate, then look at a Sterling Alternator to Battery charger. It tricks the alternator in to running closer to its rated output for longer.
 
Martin&Rene said:
If you want a higher charging rate, then look at a Sterling Alternator to Battery charger. It tricks the alternator in to running closer to its rated output for longer.
Click to expand...

If the alternator is rated at around 14.5v, which most modern ones are, a Sterling A-to-B charger won't make a great difference. However, what the A-to-B charger can do, unlike most "smart regulators", is drop the voltage to a float setting.
 
northcave said:
- This morning my 4 x Trojans were 50% discharged
- I have 60amp Alternator.
- I’m motoring along at reasonable revs at 6kts
- My battery monitor stated showing 40amps but after about half an hour it’s dropped to about 30amps.

Does this seem normal or a little on the low side?
Click to expand...

An alternator is/was rated in watts at a nominal 14.0 volt.

So decrease voltage and amperage goes up, increase voltage amperage comes down.

Voltage is generated by the load, battery charge or load ( lights etc ), the regulator on the alternator only limits max voltage.

Increasing regulation voltage will increase amps into battery (FLA battery needs around 15.4 volt to fully charge ) it will not increase actual alternator output, this is always what the load will take. Remember the alternator was designed to supply, lights, heating, etc to a car, not as a power station to charge boat battery banks.

Amps out can vary due to internal alternator temp, battery temp, state of battery charge, what other domestic equipment is turned on, even type of domestic load can affect output voltage and thus battery charge current, can be cable size/resistance, every terminal and connection can add resistance and thus reduce battery voltage.

So what you see on the ammeter can be not related to a printed alternator nominal amperage output.

Brian
 
What size type of Trojans - wet, agm or gel.
How old are they
Have you done an equilization charge
What regulator are you using
What load is there overnight - Ahr
But what little you gave in the op - thats what my sterling regulator does
Fair winds
 
The reason I suggested measuring the volts is partly that any alternator rated as low as 60A is likely quite an old design/spec, so may be
a) regulated at a lower voltage than a more modern car alternator and
b) not capable of delivering a high current into a higher voltage at low rpm.

Measure the volts at alternator and battery, then you will have some information to work with.
See if they go up with a few more RPM.

I improved the charging on one of my boats by getting a Mondeo alternator from a breaker's yard.
 
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