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Old 09-07-2019, 09:32 AM
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ndutton ndutton is offline
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Anecdotal alternator performance

I have a single wire Delco 10Si alternator, 105A (same family as MMI's 120A high output catalog offering) and a battery complement of 4 - Group 31's paralleled into two house banks and 1 - G24 isolated engine starter. My monitoring array includes a digital ammeter that reads alternator output via a Gauss loop sensor on the output wire (as opposed to the more common inline shunt).

Leaving Catalina following an overnighter with refrigeration, water pressure pump and lighting loads running off of the combined house banks I started the engine on the engine starter, dropped the mooring and headed for home. After about a half hour with only the engine starter battery switched to the alternator I switched over to the combined house banks and off of the engine starter. Engine RPM was roughly 1800 when I switched, noticeably lower with the charging load applied.

Upon switching to the partially depleted house banks, the initial alternator output was 75A but pretty quickly dropped to 60A and held there slowly reducing as the battery state of charge came up.

Not too shabby for an alternator mounted on an A4. The belt tension applied by the tensioner was spot on with no squeal (standard single 3/8" V belt). It had never been tested to that level of alternator load.
1977 Catalina 30
San Pedro, California
prior boats 1987 Westsail 32, 1970 Catalina 22
Had my hands in a few others

Last edited by ndutton; 09-07-2019 at 10:48 AM.
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