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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. Most diesels in decent
shape start in about 1 second, maybe two. what is required is a lot of Amps for
a few seconds. Starting batteries are made to do that. They are cheap,
service free, and if used only for starting last 3 years or so. Also, if you look
closely at the 4 and 8 D batteries, they are usually listed as "Dual Purpose"
batteries. This is my understanding: Start batteries have a whole bunch of thin
plates so they can deliver high amps for 30 seconds. Deep cycle batteries have
thick plates and are created to deliver 20 amps or so continuously for a long
time, and to continuously cycled to 50% or so, then recharged. "Dual purpose"
batteries are a compromise between starting and deep cycle. While I am sure
different manufacturers products have differences, the batteries made by Penn
for West Marine clearly state that the 4 and 8 D's are dual purpose. On my
trawler, I start the main engine and the gen set with starting batteries (each has
it's own for redundancy) my house bank is made up of 6 each 6 volt batteries
wired in series/parallel. All the batteries seem to last 4 to 5 years. I have
them all tied together with combiners so all of them get charged from any
source (shore power or gen set thru the inverter charger, or from the single 120 A
engine alternator. I never have had any issues with battery failure or
starting problems. Also to keep it simple, I keep a set of jumper cables aboard in
case of a battery failure rather than a bunch of switches and wiring that never
get used.
Brian
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