Re: [A/S] batteries (again) was boondocking in the snow

["Craig Eldridge" <craig_e@xxxxxxxxxxx>]

<I have always based my battery purchases on CCA's.
When you're in freezing weather, and you have a car
that doesn't want to start, you learn to buy big CCA's
versus wha the manufactur says your car needs.>

A battery's capacity for storing energy is rated in different ways depending 
on whether it is an automotive starting battery, a deep cycle/RV, or the 
"hybrid" marine (combination) type.

Starting batteries need a very large starting current for a very short time. 
  They have a large # of thin plates for maximum surface area.  These plates 
are composed of lead similar to a fine-hole "sponge".  If deep cycled, this 
"sponge" is can be quickly damaged and falls to the bottom of the battery.  
All it takes is 30 or so deep cycles and the battery is history.  These 
batteries are rated in CCA's (cold cranking amps) which is the discharge 
load in amps which a battery can sustain for 30 seconds at 0 degrees and not 
fall below 1.2 volts per cell in a 6 volt and 7.2 volts in a 12 volt 
battery.  CCA's are the "bursting" energy a car needs to start on a cold 
morning.  You are right when you say that you always bought the battery with 
a high # of CCA's for extra power.

Deep cycle batteries are designed to be discharged down as much as 80 % 
repeatedly and have much thicker solid lead plates.  These batteries are 
usually rated in Amp/Hours.  This rating is calculated lby multiplying the 
current (in amps) by time (in hours) the current is drawn.  A battery which 
can deliver 4 amps for 20 hours before being discharged would have an 80 
amp/hr rating.  They are designed for prolonged discharges at lower current 
and not for high current discharges.

Marine batteries may be the "hybrid" type I mentioned above and fall between 
the starting and deep cycle battery although some of the Marine type are 
true deep cycle.  This is what confuses some people and they buy the wrong 
battery.  These "hybrids" may have plates composed of lead sponge but it is 
a coarser/heavier type of plate.  Hybrid types should not be discharged more 
than 50%.  This "hybrid", dual or marine battery is a compromise between the 
car and deep cycle battery and can work as a starting battery in an auto but 
not as well as a regular auto battery.

You will also see batteries rated with a Reserve Capacity.  This is the 
number of minutes a new, fully charged battery at 80 degrees F. will sustain 
a discharge load of 25 amps to a cut-off voltage of 1.75 volts per cell on a 
6 volt battery and 10.5 on a 12 volt battery.  This rating measures more of 
a continuous load on the battery.  For our RV use, this rating is a little 
less useful since the common loads an RV uses puts a lot less than the 25 
amp load used to determine RC.  You can convert RC to amp hours by 
multiplying RC X .6 (RC of 120 X .6= 72 amp hours).  A draw of 1 amp for 72 
hours or 10 amps for 7.2 hours.  I have also see this conversion using .4167 
at the 25 amp rate rather than
the .6 mentioned above.  Why, I don't know?  Someone may be able to clarify 
this for me.

For RV use, the true deep cycle battery showing the designated Amp Hour 
rating is the most useful designation.  CCA's are not even a consideration.  
Motorhomes usually use an auto starting or "hybrid" battery and then deep 
cycle batteries for the electrical system.

Hope this helps.