Easy alternator question

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Oscarpop

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Am I correct in assuming that the higher the revs of an engine, the greater the rate at which the alternator charges the batteries?

Thus the maximum rating for an alternator only happens at full revs?

Thanks
 
The Hitachi Yanmar alternator, for which I have data, reaches max. output amps around 4000 rpm. Don't know the pulley ratio, but this probably equates to ~2000 engine rpm or a bit more.
Probably typical.
 
cliff said:
No.
Click to expand...

That's a bit short :)

It's yes and no.

A alternators voltage goes up at increasing revs, limited by the voltage regulator.

So the alternator must run at sufficient RPM to reach the voltage that your batteries require to accept a given charge (in Amps).
So, yes your alternator need to run at a minimum RPM to reach it's designed Amps, but this is often already reached at an engine RPM of 1500. After this the Voltage Regulator will adjust the alternator to prevent it from a melt down.

Cheers.
 
aluijten said:
That's a bit short :)

It's yes and no.
Click to expand...
Read the OPs question - the answer is NO - his assumption that "the higher the revs of an engine, the greater the rate at which the alternator charges the batteries" is incorrect. My system charges quite happily at around 1200rpm - way below max - mind you the alternator is doing around 4k rpm and producing full charging current - increasing the revs does not increase the charging rate.
 
cliff said:
Read the OPs question - the answer is NO - his assumption that "the higher the revs of an engine, the greater the rate at which the alternator charges the batteries" is incorrect. My system charges quite happily at around 1200rpm - way below max - mind you the alternator is doing around 4k rpm and producing full charging current - increasing the revs does not increase the charging rate.
Click to expand...

I did read the OP questions (2!) The answer to the first is yes (to an extent) the second is no. Hence my response.

Many systems need a bit more then 1200 RPM. Mine needs at least 1500.

Cheers.
 
The more modern the alternator, the less it is true!
Many modern car alternators will deliver nearly the same at tickover as they will at the redline.

In any case, the alternator will limit the voltage, so higher revs will not put more charge into the battery once the regulated voltage is achieved.

What the OP suggests is very true of permanent magnet motorbike alternators though, one of my bikes struggles to break even at low rpm with the lights on!
 
The problem with high alternator output at low revs is it can overload the engine and stall it if the battery is flat, especially a small engine. It won't load it when the battery is well charged which results in a difficult tickover setting.
 
savageseadog said:
The problem with high alternator output at low revs is it can overload the engine and stall it if the battery is flat, especially a small engine.
Click to expand...

+1

I have a Smartbank, which couples in the house battery a second or two after the engine starts. If the house battery is low you can really hear the engine falter as the load comes on, though only for a moment before it picks up again. I could imagine a very flat battery stalling a cold or weak engine.

Pete
 
Easy alternator question
Click to expand...

Weren't expecting straightforward answers were you?

1st answer: Only at low engine revs. Up to perhaps a fast idle

2nd answer: No. The regulator is controlling it from around fast idle upwards
 
savageseadog said:
The problem with high alternator output at low revs is it can overload the engine and stall it if the battery is flat, especially a small engine. It won't load it when the battery is well charged which results in a difficult tickover setting.
Click to expand...

There is some truth in that, you can get around it by using fast idle when the battery is really low. Since the alternator operates at a roughly constant power regardless of RPM, the torque to turn it goes down at a few more revs.
My high output alternator worked very well at low rpm, but did give fan belts a hard time!

To actually stall a diesel engine takes quite a current from the alternator, the belt will usually slip first.
 
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