One thing that should not be overlooked is that your big alternator could 
hardly ever be used at full power. Your batteries won’t absorb even half of 
that in the last 20% of the charging capacity. Since you have solar, you can 
use it for long (and slow) final charging. Also, your alternator may quite 
easily overheat if used at full power over longer time and without good cooling.

I might be stating the obvious, but if you don’t have the time or capacity to 
fully charge your batteries, you should really consider that what you have 
available is roughly 30% of your (current or real (vs. the nominal)) battery 
capacity (between 50% and 80% of SOC).

Another obvious thing is to make sure that for a large capacity alternator you 
have an external regulator with proper temperature sending and that your v-belt 
can drive it (serpentine or double).

Just a few thoughts

Marek
1994 C270 “legato”
Ottawa

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From: Josh Muckley via CnC-List<mailto:cnc-list@cnc-list.com>
Sent: Monday, February 12, 2018 17:44
To: C&C List<mailto:cnc-list@cnc-list.com>
Cc: Josh Muckley<mailto:muckl...@gmail.com>
Subject: Re: Stus-List Battery power

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<mailto: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

pastedGraphic.tiff<cid:4073BE72-4704-4EA7-8EBA-B73B833F502B>

On Feb 12, 2018, at 2:33 PM, Josh Muckley via CnC-List 
<cnc-list@cnc-list.com<mailto: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<mailto: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
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