In the case being discussed the batteries are a power source for the starter, and not a load. The output voltage of the two batteries in parallel will be the same as the output of one battery. (Presuming, of course that the two batteries have the same state of charge. If one of two batteries in parallel is a “better” battery than the other, the lower state of charge battery will be drawing power from the higher state of charge battery and the output voltage from the pair will be something less than that of the “better” battery. But that whole discussion is another topic.)
What you say is correct for the case when the parallel batteries are being charged and become a load. Then the internal resistance of the pair of batteries will be perceived by the charging source as being half that of each individual battery. And since the charger reacts to internal resistance (which goes up as the battery state of charge increases) in determining charge voltage and current flow from the charger, having a smaller than expected internal resistance can result in the charger delivering too much current to the battery bank. Which is one of the reasons that you need a charger that can deliver amperage at least 10% of the size of your battery bank in AH. A 10 amp smart charger (and similarly an old style ferro-resonant charger) trying to recharge a 300 AH house bank with 3 batteries in parallel can get “stuck” in bulk charge mode because the internal resistance of the battery bank is very low, and charger continuously delivers maximum voltage and current to the battery bank, creating excess heat and boiling off the water in the electrolyte. From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Marek Dziedzic via CnC-List Sent: Tuesday, May 1, 2018 3:26 PM To: cnc-list@cnc-list.com Cc: Marek Dziedzic <dziedzi...@hotmail.com> Subject: Re: Stus-List Parallel Batteries -- Double the Ah, but Double the CCA too??? (Edd Schillay) Rick, you should include in your calculations the internal resistance of the batteries and the fact that it would be half (if connected in parallel) (;-) Marek From: Rick Brass via CnC-List Sent: Tuesday, May 1, 2018 10:51 To: cnc-list@cnc-list.com <mailto:cnc-list@cnc-list.com> ; 'Edd Schillay' Cc: Rick Brass ; 'Thomas Delaney' Subject: 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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