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From: Robin (no email)
Date: Thu Mar 15 2007 - 13:49:30 EDT
"I am totally unclear as to why it is so common for people to use 4 and 8D
batteries for starting, especially for starting gensets..."
Your explanation of deep cycle and start battery differences is right on.
There are a number of answers. Perhaps the simplest is "Just wait until you
are in a remote location and a puny 100 amp hour battery hat you need has
failed." But of course big batteries die as well. Extra power means you just
might get that last start you need as another boat drifts down on you in a
sudden squall!!!
Another might be: if you are tethered in a marina 99% of the time, the
smallest of batteries will probably serve you just fine. Last week in 12
degree weather in NY, a live board friend called to tell me they lost marina
power...first time in seven years. I told him my diesel heater was run from
three 8D's and would probable be ok for three days without shorepower..maybe
longer. And they'd recharge automatically when shorepower was restored. I went
back to watching the "Pussycat Dolls" tryouts. No worry, mon!!!!
Yet another answer is that some bigger engines need bigger batteries. My
8V71TI's and 6-92's and 8-92's (all 2 cycle Detroit diesels) require about
1500 to 1800 amps for a proper strong starter rotation. These engines all use
a pair of 8D's. For even high amp capacity, AGM batteries rather than wet
cells do even better.
Another answer is "multiple use". For example I use three 8D's in my stb
engine bank. These three not only start my stb engine (really fast rotation!)
but also power my inverter and sometimes refrigeration and or freezer. So it's
a bigger bank than necessary, but put to multiple use.
Another answer is that bigger house banks last longer than smaller ones
(although I am beginning to wonder about that claim) and are more efficient
due to Peukert's Law. They last longer in deep cycle applications because they
discharge less. Deep discharges kill batteries fast. And they are more
efficient when lower currents are drawn for deep cycle purposes (Peukerts
law).
Yet another reason is for charging efficiency. Whereas a big battery bank may
have to be recharged only daily or alternate days, or perhaps every third day,
a smaller bank may require more frequent charging. If you are running a genny
daily, then a smaller bank can be entirely appropriate; if you like to have
"quiet days" a larger battery bank can save genny wear and tear. And you can
use a much larger charger if you have the available AC power. Why run a genny
if you can use a battery??
An alternative answer might be "You don't need large banks all over your
boat...just carry jumper cables. I do both.
So, as usual ,there is no single answer that is correct for all cruising
situations. How you equip your boat depends on how you use it.
Rob Brueckner
Hatteras YF
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