# 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>
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 donâ€™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
>> 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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>>
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>
> Thanks everyone for supporting this list with your contributions.  Each
> and every one is greatly appreciated.  If you want to support the list -
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>
>
>
> _______________________________________________
>
> 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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Thanks everyone for supporting this list with your contributions.  Each and
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