I think I should add a comment for the benefit of the non-engineering types on
the list:
Yes, the ABILITY of the parallel batteries to deliver cranking amps does double.
But the actual current flow will not. The current draw (cranking amps) is a
function of the voltage of the battery and the size of the engine being
cranked, and that is a relative constant.
The parallel batteries are still 12 volts (well, maybe 12.6) and Ohm’s Law
still applies. E=I*R where E=volts, R=resistance, and I=current flow. When you
parallel the batteries the voltage is the same. The “resistance” of the starter
is the same regardless of how many batteries. So the current draw remains the
same.
A note regarding the “resistance” of the starter: Someone would probably
comment that the windings of the starter are a dead short, so there is
negligible “resistance” if you measure across the starter windings with a OHM
meter. The “resistance” we’re actually talking about here is inductance, or the
resistance generated by the magnetic field in the starter windings as the
starter tries to produce torque to spin the starter against the compression of
the engine. It only appears when the starter solenoid is closed while starting
the engine.
The specs for the Yanmar 3YM30 shows a 1.4kw starter that draws 90 cranking
amps at 2700 RPM when you start with a 12V battery. The current draw would go
up if battery state of charge were less, or if the ambient temperature were
lower (hence the Cold Cranking Amp rating for the battery – which is the output
at 0 degrees F. Marine cranking amps are at 32F IIRC). If you crank the 3YM30
with a battery having an initial voltage of 11v, the starter will draw just
over 100 amps. If the engine is frozen up (or the battery is about 5 or 6
volts) the current draw to produce the 1.4KW of cranking power is between 200A
and 300A.
I wasn’t able to find an installation manual for the 3YM30, but I’d expect it
to tell you to size and fuse the battery cables for about 200 amps of current
draw – even though normal starting current is just 90 amps. Just for
comparison, on my 36HP, 4-cylinder Universal M35B the installation manual says
to size the cables and fuse for a max of 175 amps.
OK. Everyone (except us nerds) are bored now. So I’ll fall out of technical
trainer mode.
But, Edd, the essential message is the CAPACITY to deliver current doubles, but
the actual current flow will not because it is dependent on what is connected
to the batteries.
Rick Brass
Washington, NC
From: CnC-List [mailto:[email protected]] On Behalf Of Thomas
Delaney via CnC-List
Sent: Monday, April 30, 2018 6:27 PM
To: [email protected]; Edd Schillay <[email protected]>
Cc: Thomas Delaney <[email protected]>
Subject: Re: Stus-List Parallel Batteries -- Double the Ah, but Double the CCA
too??? (Edd Schillay)
Hi Edd,
CCA, AH, and reserve capacity double if you put the batteries in parallel. Here
is the documentation from East Penn I used to set up my MPPT to work with my
AGMs, it has lots of useful battery information:
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