How do I know which alternators are appropriate?

[Richard10002]

8 minutes ago, LadyG said:

Oh, well here's me thinking demand from batteries 'regulated' output fron alternators?

Correct in "the old days" when there were mostly lead acid batteries. Internal resistance increases substantially as the SOC increases.

 

Now that we have lithium, with minimal resistance, there is no regulation, and the alternator is free to run at full chat, with gay abandon.

[LadyG]

5 minutes ago, Richard10002 said:

Correct in "the old days" when there were mostly lead acid batteries. Internal resistance increases substantially as the SOC increases.

 

Now that we have lithium, with minimal resistance, there is no regulation, and the alternator is free to run at full chat, with gay abandon.

I don't have lithium, I have agm, and the alternator is rated at 70 ?amps output , the battery gauge shows it is drawing 30 ,or a bit more if I run the engine to charge the batteries, which only happens occasionally, but probably  more frequently now, due to winter.

Edited November 5, 2020 by LadyG

[Chewbacka]

9 hours ago, nicknorman said:

Briefly reading the spec sheet implies that the fuse is the shunt. Different rated fuses will have different resistance and hence drop a different voltage at the same current, or more to the point, drop the same voltage at a different current. Provided the fuses are designed appropriately there is no reason not to use it as a shunt as well - seems quite a clever idea.
 

As to the PWM bit presumably there is a bit more to it than a fast switch, it will have some capacitors etc to smooth the input current. Probably looks more like a Buck converter with the switch permanently on, until the alternator tries to exceed the specified current, when it starts modulating. This will allow the alternator voltage to be higher than the battery voltage thus reducing the demand from the alternator.

As the current through the fuse starts to approach the fuse rating the fuse will get warm, the fuse will have a positive temperature coefficient and the volt drop will rise.  So it is probably not that difficult to detect when at 80%.

[dor]

I had a thornycroft 4 cylinder Mitsubishi with a 30 or 35 amp alternator on a previous boat. I looked to fit a higher rated alternator but couldn’t find one that would fit without fouling the exhaust manifold/ header tank.

So check that what you replace it with will physically fit.