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Old 06-21-2008, 04:21 AM   #21
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RE: 125 amp alternator

Unfortuniatly the alt industry gets away with rating their products cold,

after it works for even 5 min the hot rating is about 25% less than the cold rating.

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Old 06-21-2008, 08:43 AM   #22
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RE: 125 amp alternator

If I recall right he only got 90 amps even when cold, but it sounds like he has some good ventilation since it stayed at 90 for an hour.

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Old 06-24-2008, 10:59 AM   #23
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RE: 125 amp alternator

Hi, I have a 165amp Delco Si either a 21 or a 27 that I ran on my Man with a 3/8 inch single belt/balmar ars4 regulator--high reading 65 amps on the link monitor. Now the same unit is on the Deere with a 8 row serpintine belt and it see's 90 amps on the monitor. Better grip
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Old 06-24-2008, 05:40 PM   #24
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RE: 125 amp alternator

I dont know anything about the RPMs on a Deere, but you might want to check the ratio of the main crank pulley to the alternator pulley, it would be a shame to let a 165 amp alternator only run 90 amps!! If it was grip related and the old setup was slipping you would probably hear some squealing or at least see some black dust from belt wear.
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Old 06-24-2008, 08:55 PM   #25
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RE: 125 amp alternator

You need a few things to get full output.

Obviously the alternator has to be 100% - all good diodes, brushes, etc.

It has to turn fast enough. 5000 RPM is a good target. You can figure that out by your cruise engine RPM multiplied by the ratio between the crank and alternator pulleys. A 3 or 4 to one ratio is going to be necessary for the tractor engine derivatives (Lehman, Deere, Perks, etc). But you have to be certian not to overspeed the alternator at engine redline.

The belt has to make it turn fast enough. Sometimes they'll make dust if they're slipping, sometimes they'll just glaze over and look just fine. If you have an external fan on the alternator and you can grab the fan and manage to get the alternator to turn, it's going to slip. Above 100 amps, dual belts are a good call if you're using V belts (I don't know what the power transmission capabilities of the flat serpentine belts are).

Using a handheld optical tach isn't a bad idea - measure the alternator speed at full load after it's run for a few minutes.

Then there's the regulator, the field voltage, and the wire between the alternator and the battery. To get full output, the regulator ideally should sense voltage at the battery, not at the alternator (like most internal regulators will). The field voltage is the voltage between the field termal and the alternator case - so if there is any resistance on the alternator ground path, that reduces the field voltage. For example, .01 ohm of resistance (which is very, very little) causes a half volt drop at 50 amps ... so the alternator case is actually at .5 volts, and the field voltage is reduced accordingly. With the alternator running at full output (or as full as you can get it), measure the voltage between the alternator case and the battery ground. Any drop there will result in a drop of output current. A solid ground wire between the alternator case and the main ground buss isn't a bad idea even if there's a solid ground to the engine block.

Finally, there's the wire between the alternator output and the main bus. A remote sensing regulator can compensate for voltage drop here, but only to a point. If you have drop, you're burning up a portion of your power output from the alternator and just turning it into heat. At 100 amps or more, #4 wire with properly installed terminals at each end carrying the alternator output is not overkill.

Temperature of the alternator is important, as FF points out. The magnetic characteristics of the rotor (which is just a spinning electomagnet) get worse as it gets hotter, and it gets hotter the higher the output. Plus the life of the diode assembly is inversely proportional to the temperature. If it's possible to duct some cool engine space intake air (without salt spray) to the alternator, it'll be just that much happier.

Finally (I've rambled too long, I know) -- conversion to a higher amp alternator is more than increasing the diode capacity. The rotor and stator magnetics have to be bigger in order to create enough magnetic flux to induce the greator current. The stator windings have to be big enough to carry the current without melting. Only then does the capacity of the diodes come into play.
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Old 06-25-2008, 04:17 AM   #26
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RE: 125 amp alternator

" Sometimes they'll make dust if they're slipping, sometimes they'll just glaze over and look just fine."

Most alts are lost from belt slip as the HEAT of the small pulley is directed back to the front bearing ( which may be under high load to stop the belt slippage) and the front bearing is lost , or the extra heat wipes out something else.

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Old 09-28-2008, 08:50 AM   #27
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RE: 125 amp alternator

I just joined this forum this morning and should sit in the background for a while, but...................

Increased battery charging capacity is not cheap or easy and must be designed with your boat's requirements*in mind.*I seem to need 200*amps per day on my GB-42 because**I am frequently an anchored out liveaboard.* Therefore I need to quickly charge my house bank to run my DC reefer and freezer not to mention my Sat TV and other goodies.* Last winter I left RI and headed down to the Bahamas and 7 months later returned home.**

My house bank of four 6 volt deep cycle batteries is primarily charged by a Balmar 200 amp large frame alternator which is double belted off the back end of my 110volt 8KW genset.* This alternator charges through a Xantrex smart regulator monitored by an XBM monitor.* The maximum hot charge I get is 140 amps which is darn good in my mind.* On my trip I ran the generator about 2 hours a day.* At the same time, I heated my water and charged my flashlights, I-POD and whatnot.**I also have a 1KW invertor to power my TV and microwave.

Just an idea.*

Howard*
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Old 09-28-2008, 12:12 PM   #28
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RE: 125 amp alternator

The problem is unless a bank is HUGE , the recharge at 70% and up gets really slow.

I suggest you read Pro Boat Builder June /July 08 the Nigel Calder article on gen sets.

At the end there is a good look at more modern batteries that would perhaps with great expense allow you to reduce charge times greatly.

Batteries (wet ) haven't improved in 110 years , lets hope the vast sums being diverted into this research and Mfg facilities will help.

" I need to quickly charge my house bank to run my DC reefer and freezer"

Our solution was an RV propane reefer , mounted properly , so months of no powerhose and anchor out live aboard are possible in silence.

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