alternator output qsm-11

[Tom Gaddis]

Had alternator (delco 22si) stop putting out volts so removed and had it rebuilt. Installed it today (added new belt) and now its putting out 14.50 volts at idle. Drops to 14.35 at 1800rpm. I thought this alternator was regulated at 14.2? Is this enough voltage to damage batteries? Not a big deal and run it for awhile or pull it off again and take back to alternator shop? Engine is 535hp QSM-11 with 3,000 hrs.

thanks,

tom

Topic

[Rob Schepis]

Did you check with the battery manufacturer?  I did some more reading on this and found on Optima’s website that alternator charging max voltage is 15.0 volts…

[Tom Gaddis]

Since I had bought a new 22si as a spare i decided to put it on today. 14.07 at alternator and batteries and  13.75 at helm gauge in idle, cold. Identical to  Gonna go back to alternator shop and ask to revisit the rebuilt unit and adjust voltage regulator back to 14 volts and it will now be the spare. Thanks all.

tom

[Rob Schepis]

Even if it drops down to 14.3 you’re still on the high side.  The batteries are likely in the warm environment of the engine room?  Here’s some reference numbers from Deka.  You could call the battery manufacturer for specifics and use what they tell you as the basis for what you go back to the alternator shop for.

 

 

[Tom Gaddis]

Retested today. At  strbd. alternator terminal 14.81 volts at idle, cold. At strbd battery post 14.80 while running at idle cold. This was with minimal load on 12v system.

Two 4d battery for starting each engine. Have a seperated 4d house battery. Port bank is 14.07 at battery post idle, cold. Before starting engine with minimal load the battery banks were 12.7v strbd and 12.8v port.

Alternator shop says 14.8v no problem. I understand when warm the output will drop up to 0.5v but is 14.3 enough to damage an expensive 4d agm battery pair?

thanks,

tom

[Rob Schepis]

Yes, measure at the battery terminals so you know what is really being seen.  On a new to me boat years back I spend an hour or so with a DVM checking all DC points to get an understanding of the DC system as a whole.  Here’s a chart I made up at that time which addresses what Tony was getting at re: the different test points and I took it a step further as to how the draw from different DC eqpt effected the voltage drops.

[Tom Gaddis]

was measured at the alternator terminals and was not warmed up. The other engines alternator puts out 13.75 at idle cold. Is it worth measuring at the battery terminal when cold or just run it and then measure?

thanks,

tom

[Tony Athens]

Just about every alternator of this type has a regulator that has a voltage output vs temp.. They all charge higher when cold and by the time the case is about 150F-200F, they drop about , 0.3 – 0.5VDC..

Also, the shop that rebuilt it may have had a choice as to the BASE Voltage the regulator was set for.. 13.9, 14.0 , 14.3 etc….

And last, where are you measuring the voltage?  That is key to what is really going on–At the alternator output terminal, at the battery terminals on on a bus bar in the house. To me all that matters is Battery Terminal Voltage, at least initially.

Tony

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