Unloaded alternator

[Graham_Wright]

We are told that an alternator not connected to a load will output a high voltage which will probably destroy the diodes.

So;- what is an acceptable load? Must it be a "voltage" load (e.g. a battery) or will a resistive load do the trick. One convincing expert at the EC boat show had fitted zener diodes in the past (presumably with a resistive current limiter) but found that they tended to blow (evidence of doing the job for which they were intended?).

What duration o/c can the alternator accept before damage is caused? If using a switched output (e.g. between different batteries) is the 1 msec or so interruption dangerous?

Expert opinions very welcome.

 

[VicS]

[ QUOTE ]
We are told that an alternator not connected to a load will output a high voltage which will probably destroy the diodes

[/ QUOTE ] Then you are told wrongly.

You can safely run an alternator with no load BUT what you must not do is disconnect it from a load while it is running. That causes a voltage surge which can blow the diodes.

 

[Graham_Wright]

[ QUOTE ]
You can safely run an alternator with no load BUT what you must not do is disconnect it from a load while it is running. That causes a voltage surge which can blow the diodes.

[/ QUOTE ]

Like when you switch the output from one battery to another? (assuming not a make-before-break switch).

 

[VicS]

[ QUOTE ]
Like when you switch the output from one battery to another?

[/ QUOTE ] Yes, if the engine is running and you want to switch from one battery (bank) to another you must switch the new one on before you switch the old one off. If you have a 1,2, both, off type switch you switch via the both position.

(If the engine is not running its not important but most people would switch via the Off position to avoid paralleling the batteries unnecessarily but that could set off low volts alarms on bits of electronic gear and make stuff reboot so you might still want to switch the new battery on first.)