Acceptable voltage drop when cranking?

It's been an interesting week of wiring aboard (new fuse block, pulling romex and replacing with boat cable, diagraming old wire, etc.) in preparation for a weekend on the hook. In this time I've had the battery charger off. Now that we are on the hook neither the generator or engine will start. Both cranked instantly yesterday. House banks are independent so no draw down. I'll figure out the generator when we get back; the priority is the engine (no danger, can always use my TowBoat subscription).

I'm wondering if I'm getting too much voltage drop, 12.8 down to 9.5 or so, at the starter when cranking. Is this too low or is such a drop normal? I'll try jumping the battery tomorrow, just thinking now... Thanks!

That is a big drop. Something isn't right.

If your battery is reading 12.8V at the same time your starter is reading 9.5V you either have undersized cable or a bad connection.

If you measured 12.8V at your battery and then cranked it over and measured 9.5V at the starter, it may be a bad battery. (bad cell or sulphated)
I'd say this is the more likely scenario.

Back in the day the starter folks were more honest and many starters were labeled 9.5 V , at least on DD engines.

As 9.5V is what many starters were built to operate at , the question is not the voltage its the starter speed.

IF the engine spins and starts as normal in modest weather , you are fine.

Having the V meter read the starter terminals is probably the best location.

You could notice a CHANGE in start voltage while cranking , a stuck starter being driven by the engine as well as the charge as the alt kicks in.

The V meter has to be wired somewhere , the starter is a good spot.

You should not see that large a drop.

Start by checking and cleaning all the connections. Then make sure the battery is fully charged and test it.

If all that is good move on to the starter.

If you engine starts really easily, you just might get a start from a weak battery immediately after running the charger for some time. Leave it to rest for an hour then watch the voltage at the battery while you hit the starter. My money is on bad connection(s) at this stage.

Thanks, all.

The foregoing advice is on the right track. Even a battery in top shape will see a voltage drop at its terminals while cranking, but not 3 volts like you report. So it could be the battery, or the wiring. To see which, measure the voltage at the battery terminals before cranking- should be about 12.5 and while cranking- should be no less than 11.0.


If it 11.0 or better then you have a problem with wiring. If it is less than 11.5 then depending on how much less it could be both the battery and the wiring.


David

You guys stating that 9.5v is too low when cranking- Go put a DVM on your starter and read volts while cranking in shutdown. 9.5 is about typical.

To Ski's point, we are now being towed in after a jump didn't work. Seems like it may not be a voltage issue. Even having TowBoat jumping directly to the starter was no good. I'm finished screwing with it today. Tomorrow I'll get the charger connected again, change all fuel filters, and start trying to troubleshoot.

I already have a marine electrician coming out Wednesday to help me prioritize what I need to do and point out things I haven't thought. I'll ask him to double check the electrical to the starter but it seems like it may be something else. Too weird that both the engine and the generator refused to start the same day. Makes me think it's not a coincidence. But they use different filters and different batteries... Switching racors did nothing. Thoughts? Thanks!

Ski in NC said:

You guys stating that 9.5v is too low when cranking- Go put a DVM on your starter and read volts while cranking in shutdown. 9.5 is about typical.

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Respectfully disagree. 10.5 volts should be minimum for a system with proper cabling, good batteries, at moderate temps. You would never start an engine in winter if it had that much drop in moderate temps.

Check your crank voltage both at batteries and at starter to narrow down where drop is. Check drop thru each cable, don't forget the switch contacts and negative cable.

What's in common between the two engines? Common ground cabling?

I have always heard 10V as an average low...so 9.5 isn't great but isn't out of the question.


I think most shoot for above 10...10.5 would be great but not something you need to change wires for if starts seem normal and nothing is getting hot...warm, but not smoking.

It can take 250 amps to turn over/start a diesel. Hitting the key/switch, will drop the voltage to ~9.5 voltage from ~12.5. Both should be looked at when trouble shooting. If you only draw 100-250 amps at 9.5 volts, you may have a battery or cabling issue. If the amps are way high, and I don't remember the number, you can have a starter problem.

For most mid sized diesels with adequate battery capacity, it is normal to drop to about 10.5 volts. Our Cummins QSB 5.9 with a 4D start battery drops to 11 volts.

"but it seems like it may be something else. Too weird that both the engine and the generator refused to start the same day.

AS someone noted , the ground connection is common to all.

It's a little confusing why you would want to change the fuel filters if you are having issues with it not wanting to crank over. Is the engine cranking over but not wanting to fire off, or is it just not cranking over?

If all of the wiring checks out, a starter going bad has the same symptoms. Its usually easy to pull the starter and have it checked at any auto electric shop.

Ski in NC said:

You guys stating that 9.5v is too low when cranking- Go put a DVM on your starter and read volts while cranking in shutdown. 9.5 is about typical.

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Well YMMV but I don't normally see less than 10 - 10.5 volts with a good battery. Once I start seeing less than that consistently I start looking for a problem.

To add one more curve ball... TowBoat got us back to our slip, we plugged in shore power, and I did nothing for two days over which time the generator batteries and house banks have been charging. I just reconnected the charger to the engine banks last night. Last night I went to crank the generator just to try it one last time before starting to troubleshoot everything and it started up instantly to my surprise. I let it run for a few minutes and went to shut it off...and it would not shut off. Eventually, with the help of the manual, I found the start/stop lever and shut it down that way at the engine. My head is starting to hurt from all this... Hopefully this weekend I can get it figured out. Any thoughts with the addition of the new info? Thanks!

Ignore the generator for the moment. Your start system is most important. You have to go through your entire system 'systematically.' Start with the batteries - charge them and turn off all power and let them rest for a day (essential for an accurate reading). Now do a voltage and/or specific gravity check. Is the result within specs? Check all the main cables to the battery and measure the cable runs - are the cables large enough (I had to add another positive and ground cable to my 6CTA as the battery bank was on one side and the starter on the other and the cable run was too long for the size of cable) and look for corrosion? Particularly check the ground connection to the engine block or any bus bars etc. If you haven't turned up any problems yet, remove the starter motor and the solenoid (Cummins have 2) and have it all tested.

My money is on the cabling, it's the low-hanging fruit here. However, as stated above, the voltage drop you have is not a problem if the engine easily starts. Remember Ohm's Law? My understanding is that resistance is a constant in a circuit so a drop of 24% voltage will cause an equal 24% rise in amperage. If you assume 200 amps or less (ignore inrush for now) to spin the starter without the engine there will be a rise of 48 amps with the load of the engine which is manageable.

Now I'm off to my boat to change the oil...

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

"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.

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

Thanks for posting that link.

caltexflanc said:

As voltage drops due to resistance, amps must increase to supply the same amount of power,

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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.

CMS said:

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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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.

caltexflanc said:

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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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...

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.

Ski in NC said:

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.

Click to expand...

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..

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!