Phil, I ended up emailing SAIL to get their take on this and I wanted to share Nigel Calder's response regarding wiring the batteries in parallel:
"If the alternator is well built, which it should be, it will handle the situation with no problem. Let's say you pull the batteries down to a 50% state of charge, at which point their charge acceptance rate is around 40% of capacity - i.e. 67 amps. Assuming you are running the engine fast enough to get your alternator to maximum output, it will go to its nominal 55 amps (less than 50 in practice, especially once it has warmed up). The voltage on your batteries will climb fairly rapidly. Once the voltage reaches the voltage regulator's set point (probably around 14.2 volts - it will probably get there within 20 minutes to half an hour) the alternator's output will start to taper off. You won't be driving it at full power for long enough to damage it. Nigel" --- In [email protected], "Phil Agur" <pja...@...> wrote: > > 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 >
