Waht current should I be thinking of if I get a new Alternator?

[Theo]

A supplementary to the other thread that I have just started.  I have a 10mm wide pulley on the crank shaft of my BMC 1.5  What is the maximum current that I should be thinking of when/if I get an new alternator?

 

N

 

 

[Boater Sam]

With a single "B" width  V belt you will not drive an alternator bigger than 100 Amps @12 Volts. Trying to drive more by over tightening the belt will wreck the water pump.

To get reasonable belt life, use a 90 Amp alternator.

 

For more power you would need to convert the crank pulley and the water pump pulley and use an alternator with a poly vee flat belt, tres expensive.

[Tony Brooks]

Agreed

[OldGoat]

As said above. My lovely twin row 'B' section V-belts sheared off my 10" pulleys and dumped the lot in the bilge....

[alan_fincher]

As above - 90A or 100A absolute maximum.

Make sure the pulley ratio is at least 3:1 if you want it to work well at lower revs.

Edited July 29, 2019 by alan_fincher

[dmr]

I am confused, I thought the common 13mm (half inch) belt was an "A" and that the "B" was a big jobbie at about 17mm, whilst the skinny little 10mm things are called a "Z". I have read that 100 amps is about the maximum for an "A", and my own personal experience confirms this, so 100 amps is really pushing it for a "Z".

 

..............Dave

[Theo]

Thanks all.  The 90A alternator has served me well for CC'ing.  We did nearly three years of that.  A replacement will be 90A when the time comes.

 

N

[Detling]

Unless you are driving big loads via the inverter you will find that the batteries start limiting the charge very quickly, I have a 90 Amp alternator which I have never seen above 80 Amps and that only for 30 seconds before dropping usually about charge at 60 and downwards as the batteries fill up (600Ahour AGM's ) usually above 60% when I turn engine on.

[Theo]

4 minutes ago, Detling said:

Unless you are driving big loads via the inverter you will find that the batteries start limiting the charge very quickly, I have a 90 Amp alternator which I have never seen above 80 Amps and that only for 30 seconds before dropping usually about charge at 60 and downwards as the batteries fill up (600Ahour AGM's ) usually above 60% when I turn engine on.

Like what he said.

 

 

[nicknorman]

1 hour ago, Detling said:

Unless you are driving big loads via the inverter you will find that the batteries start limiting the charge very quickly, I have a 90 Amp alternator which I have never seen above 80 Amps and that only for 30 seconds before dropping usually about charge at 60 and downwards as the batteries fill up (600Ahour AGM's ) usually above 60% when I turn engine on.

It's not quite as simple as that. The regulation curve of an alternator is pretty "soft", ie it is not like the current stays at maximum until the voltage reaches its regulated value of 14.4v or whatever, then with the voltage remaining at 14.4v the current starts to tail off - which would be "hard" regulation ie a graph of current and voltage would be two straight lines touching at an angle. Rather, the current falls below maximum output long before the voltage reaches the regulated value - the angle is replaced by a long curve.

 

So in practice a 90A alternator can only produce 60A at 14v, 50A at 14.1v, 40A at 14.2v, 30A at 14.3v etc (figures made up to illustrate the point). So really, it is not the battery that is limiting the current, it is the alternator.

 

Swap to a 180A alternator and everything is doubled - 120A at 14v, 100A at 14.1v, 80A at 14.2v, 60A at 14.3v etc (figures made up again).

 

Now it is more likely to be the battery that is limiting the current, at a much higher value than could be achieved with the 90A alternator. To put it another way, charging voltage will be higher for a given SoC, and hence charging rate will be faster, at least for the earlier part of the charge.

 

So for us, we start off with about 140A, it does drop fairly quickly but takes a long time to get down to 90A. And that is with 450AH of Trojans, which are much less good at accepting charge fast than 600AH AGMs.

 

In other words, you would charge your batteries much faster if you had a 180A alternator - for the first good chunk. Of course to finally get to 100% SoC takes a long time regardless.

 

BUT, as has been said, with a single V belt the max alternator size is around 90A.

Edited July 29, 2019 by nicknorman