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Old 07-15-2013
btrayfors btrayfors is offline
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Re: m25 alternator question

Originally Posted by Maine Sail View Post

That would be very misleading at 50% DOD for that alt if he was looking for 14.4V at 50% DOD.. It will be in bulk and having a tough enough time just getting to 13.6V let alone 14.4V... It should not hit absorption voltage until the bank is around 80% SOC but with voltage drop typical of these factory wiring systems it may not come up to absorption at the batteries until the high 90's when the current really starts to drop off....................
Yes, I agree it could be misleading. I should not have been so cryptic. Thanks for catching that.

At 50% discharge one would expect to see the alternator putting out near maximum current while it's still cool and IF the voltage output is high enough. Later on, nearing 70-80% you would expect to see a maximum of absorption voltage....14.4VDC or greater for flooded batteries.

However, there is an important interaction between charging capacity, battery absorption capacity, and the time to reach absorption voltage.

Take, for example, these results from real world laboratory testing by Concorde/Lifeline, using a new AGM 4D battery discharged to 50% SOC and charged with three different size chargers. The tests continued for 4 hours and voltage and amperage readings were taken each minute.

1. 400A capacity charger: Voltage reached 14.4VDC in under 2 minutes while the charger was delivering 265 amps.....@ about 53% SOC.

2. 105A capacity charger: Voltage reached 14.4VDC in 24 minutes while delivering 105A.....@ about 70% SOC.

3. 52A capacity charger: Voltage reached 14.4VDC in 66 minutes while delivering 52A......@ about 77% SOC.

The 14.4VDC charging voltage was continued until the test ended at 240 minutes.

These three charging capacities were chosen to represent:

1. virtually unlimited charging capability (400A with a 210AH battery bank);

2. a 50% rate of battery rated capacity (105A with a 210AH battery bank); and

3. a 25% rate of battery rated capacity (52A with a 210AH battery bank).

Point is, absorption voltage maximum varies directly with the charging capacity. Most boats don't have more than 10-25% of rated AH charging capacity.


Last edited by btrayfors; 07-15-2013 at 06:17 PM.
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