First, I completely agree with everyone else about amp hours and meters and the sort.
But.... Just looking at the voltage, the easiest way is to think of the available capacity as 1 volt from 11.7v to 12.7v. Each 0.1 (tenth) of a volt is roughly equal to 10% of usable capacity. *Long winded explanation:* This is actually pretty conservative since minimum voltage of a dead battery is 10.5v (0%) Full is 12.7v (100%). A difference of 2.2v battery service life (think warranty or years before replacement) decreases exponentially the further discharged you get. Normally a service life measurement is based on 50% discharge cycles. Imagine that the battery is rated for 200 cycles @ 50% for its entire life. That number might drop to 100 cycles @ 70%, and increase to 400 @ 20%. Because of all of this, *the typical recommendation is to minimize depth of discharge overall but to absolutely avoid discharging deeper than 50%*. Using the assumption of a linear relationship of 2.2v between 0% and 100% we can extrapolate that our 50% minimum to 100% is equal to 1.1v. Since its just easier to say one volt, and 12.7v is easy to associate with 11.7v. I come full circle to the 10% = 0.1v. Did I explain that well enough? Josh Muckley S/V Sea Hawk 1989 C&C 37+ Solomons, MD On Feb 12, 2018 3:02 PM, "David Knecht via CnC-List" <cnc-list@cnc-list.com> wrote: > This discussion raises an issue I have struggled with as I have started > cruising more: deciding how much battery power I have. I have 2 AGM > batteries, one house, one starting and a panel voltmeter for monitoring. > My batteries are now separated so I no longer have to worry about being > able to start the engine if I run the house too low. The fridge is the only > major power draw, so I usually am just conservative, running it only > periodically to make sure I don’t overdraw the battery. So what is the > most efficient way to figure out how much I can safely run the fridge? If > I just watch the voltage, how do I decide if I can leave the fridge on > overnight? Dave > > Aries > 1990 C&C 34+ > New London, CT > > > On Feb 12, 2018, at 2:33 PM, Josh Muckley via CnC-List < > cnc-list@cnc-list.com> wrote: > > Much of your problem is a matter of battery capacity as much as a matter > of charging capacity. I have ~450 Ah of capacity on one bank, a 90 Amp > alternator that never reaches full load, and 200 watts of solar. Even > without the solar I was able to comfortably keep the the fridge running and > the lights on when cruising for ~2 weeks. The half hour to hour of engine > operation to anchor or moore in the evening and the same in the morning was > always enough to keep the batteries charged. > > Keep in mind that the battery capacity should be at least 4x of the charge > capacity for flooded lead acid and at least 2x for AGM. So a 400 Ah or 200 > Ah respectively for a 100 amp alternator. > > Josh Muckley > S/V Sea Hawk > 1989 C&C 37+ > Solomons, MD > > > > On Mon, Feb 12, 2018, 12:55 PM Damian Greene via CnC-List < > cnc-list@cnc-list.com> wrote: > >> A question for your collective wisdom: >> >> I am scoping out upgrading my stock 55A alternator to a 100A Balmar, and >> related upgrades to the controls. I had a very productive discussion with >> Rod Collins at Compass Marine (mainesail), and we worked out the details. >> Unfortunately he's booked out through the spring, so this job will wait >> until next winter. >> >> So thinking then about keeping the batteries charged, and the fridge >> running on our long summer cruise - where we may go for weeks without >> access to shore power, I got wondering about using a portable generator to >> charge the batteries - as an alternative to many hours of running the >> diesel. There are a couple of Hondas that might do the trick 2000 Watt, >> weighing 47#, 1000 Watt weighing 29#. >> >> Have any of you tried this? What could (would) go wrong if I plugged this >> generator into my inverter, to charge the batteries? >> >> Regards, >> >> Damian >> >> 1986 Sabre 38 Freefall >> Previously 1984 C&C 34 Ghost >> Bass Harbor, Maine >> _______________________________________________ >> >> Thanks everyone for supporting this list with your contributions. Each >> and every one is greatly appreciated. If you want to support the list - >> use PayPal to send contribution -- https://www.paypal.me/stumurray >> >> _______________________________________________ > > Thanks everyone for supporting this list with your contributions. Each > and every one is greatly appreciated. If you want to support the list - > use PayPal to send contribution -- https://www.paypal.me/stumurray > > > > _______________________________________________ > > Thanks everyone for supporting this list with your contributions. Each > and every one is greatly appreciated. If you want to support the list - > use PayPal to send contribution -- https://www.paypal.me/stumurray > > >
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