choosing alternator size

[sequena1]

I'm beginning the, for me, the rather scary job of wiring my reconditioned Mercedes OM 636. My first step is to replace the old (very old) alternator. I understand that it is pointless having an alternator which is too big so, since I'm thinking of having a starter(and anchor winch) battery of about 100 ampH and a domestic of about 120 amph, would a 55 A, or a 75 A alternator be best. Or any other thoughts? My thanks in advance!

 

[pvb]

For the negligible difference in price, you might as well fit a 75A alternator, but you really ought to increase the size of your domestic battery bank to get the best out of your boat.

 

[damo]

PM'd you

 

[boatmike]

I would agree, but either way fit a smart controller or you won't get anywhere near the output you expect from it.

 

[oldsaltoz]

Good advise Boatmike.

Avagoodweekend......

 

[roly_voya]

Agree - about 75A unless you intend to fit an inverter of 1200w or more, in which case go up to 100A but everything else has to be equally increased in size. As said above the controller is more important than the alternator!

 

[lw395]

With an anchor windlass, go for an alternator with a good low rpm output, as you will normally be running engine at fast idle not flat out when weighing anchor. Your 12V windlass will be much happier at 13V than 11V. better to let the alternator do most of the work than flattening the battery a lot. You mostly need the battery for the starting current and breaking out the anchor, when reeling in the bulk of the chain, the alternator could be doing the work.
Some Bosch alternators are rated for quite high low rpm outputs. They use two numbers eg 70/110A. Have a look around a Land Rover forum about running winches.
Obviously you need to ensure all cabling is big enough.
Will your smart regulator current limit the alternator?
What prevents the windlass from drawing a lot more than the rating of the alternator?
Can you fit two?
Hope that helps!

 

[wagenaar]

Be aware that increasing the size of the alternator increases also the tension in the V-belt that drives the alternator. This can not go on to infinity and after a certain output a double V-belt is required. I don't know anymore where the take-over point is, but seem to remember something like 120 Amps.

 

[lw395]

Yes the vee belt is an issue.
Also getting enough rpm on the alternator.
I think the 3gm has too small an engine pulley!
If I were starting from scratch, a polyvee belt system might be considered.

 

[pvb]

I reckon about 90A is the biggest which will happily run on a single V-belt.

 

[oldsaltoz]

<<<Will your smart regulator current limit the alternator?>>>
No, it will lift the output not reduce it.

<<<What prevents the windlass from drawing a lot more than the rating of the alternator?>>
The battery.

<<<Can you fit two?>>>
Not required for this application.

Hope this helps

Avagoodweekend......