Mike,
Except the moral of my story was your alternator's rating is based on the charge rate dropping fairly quickly to 50% of it rating or under 28 amps for any extended charging. Two 79 Ah batteries 50% down would most certainly push you alternator's case temperature to its limits. If that melts the brush holders then you're done after that. IMHO you definitely need to maintain the factory A-B switch approach so you can direct the alternator to a single battery at a time until they are mostly recovered, especially if you ever need to use the jumper pack. It might be possible to parallel them up using a truck style battery isolator in parallel with the A-B switch. The reason I say that is the Isolator is a pair of large rectifiers in a heat sink. Each one creates a .6V drop between the battery and the alternator output which slows the initial charge rate. It would then also function as a diode blow out preventer since the isolator would keep the alternator loaded even if a crew member turned the A-B switch through Off while the engine was running at speed. It still does feel as safe as the factory solution. Phil Agur <http://www.catalina27.org/public_pages/profile270.htm> s/v Wing Tip C270 LE #184 MMSI 366901790 From: [email protected] [mailto:[email protected]] On Behalf Of mkeller23173 Sent: Thursday, July 01, 2010 11:10 AM To: [email protected] Subject: [IC27A] Re: AGM Batteries Gotcha, we're on the same page, I misread. I think I've decided to wire the new batteries in parallel to increase the amp hours. I have one of those car jumper packs I will bring as a backup starting battery until I can install a real starting battery in the fall (probably one of those small PWC ones will work). Regarding the alternator, my off the cuff calculations are as follows. Two 79AH group 24 batteries = 158 amp hours total. Since we'd never drop below 50% of those amp hours, the max that would be needed to put back into the batteries is 79. We have a 55amp alternator on our Yanmar. Since AGMs can draw as much as 75% of the needed amp hours, that really only leaves us with a 4 amp larger draw than the alternator is capable of providing (59 vs 55). I figure 50% would even be excessive so I think we're safe. Time will tell if I'm right. Thank you everyone for the help and comments, much appreciated! Mike
