My Atomic 4 starter draws 120 amps. That is 0.1 ohms resistance. My little U1 battery has – in theory – 250 MCA, so it should be able to crank the engine just fine at 32 degrees (MCA is at 32 degrees, CCA is at 0 degrees). In practice the start battery has gotten old enough so that if the engine has been sitting overnight in freezing weather, it cranks slow enough to be quite hard to start. If I parallel the big house battery in, the starter still draws 120 amps, but at 11.9 volts during cranking instead of 11.0. That is just enough extra power to make the difference.
Joe Coquina From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Rick Brass via CnC-List Sent: Tuesday, May 01, 2018 10:51 AM To: cnc-list@cnc-list.com; 'Edd Schillay' <e...@schillay.com> Cc: Rick Brass <rickbr...@earthlink.net>; 'Thomas Delaney' <tomcdela...@gmail.com> Subject: [EXTERNAL] Re: Stus-List Parallel Batteries -- Double the Ah, but Double the CCA too??? (Edd Schillay) 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:cnc-list-boun...@cnc-list.com] On Behalf Of Thomas Delaney via CnC-List Sent: Monday, April 30, 2018 6:27 PM To: cnc-list@cnc-list.com<mailto:cnc-list@cnc-list.com>; Edd Schillay <e...@schillay.com<mailto:e...@schillay.com>> Cc: Thomas Delaney <tomcdela...@gmail.com<mailto:tomcdela...@gmail.com>> 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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