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Old 07-20-2016, 04:13 PM   #21
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A little birdie engineer chirped this to me before...

".......reviewing Ohms law ... if the resistance stays the same and voltage drops, the current drops. .......... the magic starter motor will not somehow produce the same amount of power as it would at the higher voltage. The reality is that when the voltage drops the power drops because current drops. That constant resistance impedes current flow, it doesn't increase it.

For example, if you have a 4HP starter (3kW) powered by 12V it might draw 250A spinning at a good clip. The resistance in the circuit would be around .05 or so. If you drop the voltage to 9V with the same resistance, the current drops to about 180A and the power drops to just over 1.6kW.

Starter current and power is relative to voltage and speed, the manufacturer publishes a graph of the current, power, speed, and voltage in the starter circuit. "

Hopefully that clears things up for some
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Old 07-21-2016, 05:44 AM   #22
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"For example, if you have a 4HP starter (3kW) powered by 12V it might draw 250A spinning at a good clip. The resistance in the circuit would be around .05 or so. If you drop the voltage to 9V with the same resistance, the current drops to about 180A and the power drops to just over 1.6kW."

This is true but combined with battery performance is probably a saving Grace.

Some folks attempt to prime the engine by cranking , others need multiple long (30second) cranking to warm the cylinders in cold weather starting.

Either way the 1.6KW of energy will heat the starter wiring less than 3KW would.

Start batts have loads of very thin plates which get covered with bubbles that reduces the batt output.

Working in tandem these probably help starters from melting.

Conversely with the engine on and charging and a big battery bank, it may be why some DC thrusters are so time limited.
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Old 07-21-2016, 07:34 AM   #23
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Maybe there is a language barrier here, not uncommon on the interweb.

Power is measured in watts. As voltage drops due to resistance, amps must increase to supply the same amount of power, in this case to turn the starter motor on and up to speed needed to crank the engne . The starter puts up a lot of resistance to get going, thus voltage drops. Once the starter and the engine are spinning, resistance drops. The easiest way to witness this is to put a clamp meter on the starter cable to see the change in amps during the starting process, and then measure the voltage at the starter before during and after the starting process. 9.5 volts isn't unusual, but is usually the minimum.
You will see this same phenomena to a lesser degree on pumps, AC and refrigeration compressors, etc.

Here is a handy explanation and trouble shooting guide I had in the bookmark archives. I'd also recommend a reading of Nigel Calder's Boat Owners Mechanical and Electrical Manual.

Marine Starting Systems
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Old 07-21-2016, 08:28 AM   #24
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Thanks for posting that link.
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Old 07-21-2016, 08:35 AM   #25
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As voltage drops due to resistance, amps must increase to supply the same amount of power,
For an inverter, a constant power device, yes, as battery voltage drops the current drawn from the batteries goes up.

For a starter motor, as battery voltage drops, the max current potential that can be developed by the stater motor goes down.
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Old 07-21-2016, 12:26 PM   #26
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For an inverter, a constant power device, yes, as battery voltage drops the current drawn from the batteries goes up.

For a starter motor, as battery voltage drops, the max current potential that can be developed by the stater motor goes down.
Do the test I described or those in the article and see let us know what you see. Or look at the ampmeter on your electrical panel when a compressor starts up.
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Old 07-21-2016, 01:51 PM   #27
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Do the test I described or those in the article and see let us know what you see. Or look at the ampmeter on your electrical panel when a compressor starts up.
I do & have to measure this stuff with regularity. Low voltage at starter = lower max current. Even for in-rush a lower voltage at the starter will result in a lower peak in-rush current...
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Old 07-21-2016, 02:00 PM   #28
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Lower voltage will mean lower peak in-rush, yep. But as cranking speed slows, the engine comes closer to stall on compression stroke and flywheel inertia is consumed. Takes more torque from the starter to continue rotation. And there is that concept on dc motors called "self generation" or similar term, might not have it right. So amps likely go up when cranking an engine and volts go down..

Can't model this simply by applying Ohm's law. Lots more going on.
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Old 07-21-2016, 04:17 PM   #29
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Lower voltage will mean lower peak in-rush, yep. But as cranking speed slows, the engine comes closer to stall on compression stroke and flywheel inertia is consumed. Takes more torque from the starter to continue rotation. And there is that concept on dc motors called "self generation" or similar term, might not have it right. So amps likely go up when cranking an engine and volts go down..

Can't model this simply by applying Ohm's law. Lots more going on.
The start cycle is not a flat amperage consumption like many assume it is. It goes up, down, up, down. Still a lower voltage at the starter motor will result in less available cranking current.

As can be seen the current does exactly as you describe up and down with compression.




The nice thing about this tool is it tells me the average current during cranking, showing 286A average, and then the average voltage during cranking shown here as 12.04V. Keep in mind these are the averages during the the cranking duration (0.77seconds) loaded to unloaded. The peak in-rush is much higher, over 640A, and the peak low voltage is much lower.

A starter will not draw more current with a lower voltage than it will with a higher voltage, nor will a bilge pump pump more water with a low battery voltage. This is one reason the bilge pump makers rate them at 13.6V and not 12V. They rate them at 13.6V to boost their performance numbers. Course this does not help you when you're bucket bailing in an emergency and are complaining that your 2000GPH bilge pump is barely pumping 1/3 of its rating..
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Old 07-21-2016, 05:07 PM   #30
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Great discussion! The main engine started up instantly after the batteries had charged for 18 hours. I will load test the batteries and check for voltage drop as well. It's hard for me to understand how the isolated generator batteries (2 group 27s) and isolated engine start batteries (2 8Ds) could have lost enough charge over 1 day to not start the engine or generator. The joys of boat ownership. Thank you!
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Old 07-21-2016, 05:15 PM   #31
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I do & have to measure this stuff with regularity. Low voltage at starter = lower max current. Even for in-rush a lower voltage at the starter will result in a lower peak in-rush current...
I think you may be confusing the non-load voltage with the load voltage (both initial start up and past that).
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Old 07-21-2016, 05:26 PM   #32
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QUOTE A starter will not draw more current with a lower voltage than it will with a higher voltage QUOTE

A starter WILL draw more amps at a lower voltage. Sometimes much more. This is the double whamy of winter starting... decreased battery voltage and thickened oil.
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Old 07-21-2016, 05:43 PM   #33
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I think you may be confusing the non-load voltage with the load voltage (both initial start up and past that).
Just pulled out my notes from a job a few weeks ago with a complaint of slow starting and "Not enough oomph" says the owner...

House Battery Bank Start:

Bank Resting Voltage 12.74V
Peak in-rush - Fluke 376: 378A
Peak Low Voltage Measured at Starter Motor - Fluke 289: 11.492V



Start Battery Bank Start:


Bank Resting Voltage 12.71V
Peak In-Rush Fluke 376: 264A
Peak Low Voltage Measured at Starter Motor - Fluke 289: 8.234V

Same starter motor but different batteries able to deliver different voltages at the starter. A higher voltage at the starter results in higher available cranking current.

This is also why when you do nothing but replace battery cables from say a 4GA factory installation, on some 70's & 80's boats, to 1/0 or 2/0 the owner always tends to think they got a new starter motor when nothing but the battery wiring has changed. The new wire is now able to deliver a higher voltage to the starter with less voltage drop, stacked on top of battery voltage sag, thus better cranking with more "oomph"..

Perhaps tomorrow if I have time I will demonstrate this on my test bench in a video clip...
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Old 07-21-2016, 05:54 PM   #34
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Since you did a rewire,i would review your work. Some where there is your problem.
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Old 07-22-2016, 05:40 AM   #35
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"For a starter motor, as battery voltage drops, the max current potential that can be developed by the stater motor goes down." YES

" A starter will not draw more current with a lower voltage than it will with a higher voltage" YES

"A starter WILL draw more amps at a lower voltage. Sometimes much more. This is the double whamy of winter starting... decreased battery voltage and thickened oil. "
This is true weather its AC or DC if the motor is "universal wound" and has brushes." NO

The joy of the brushed motor is feeding it lower voltage simply makes it weaker, and does not cause it to draw more amps .

This is the reason starters have brushes , and are not wounld like air conditoners , where when the voltage drops the watts remain the same by drawing more amps , till the white smoke escapes.

At tag sales I am always on the lookout for items with brushed motors.

Things like chain saws that may be on a 400 ft extension cord don't mind a bit.

Even circular saws can be found that work without overheating on a cruddy dock, a poor inverter or a too small noisemaker.
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