Genset cranks up at 14.5v but not at 13v

[Tornadosailing]

Originally Posted by a64pilot

The starter was pulling down battery voltage,

All I want to know is HOW did the starter pull the battery voltage down. Because if you believe the starter pulled the battery voltage down, we are in complete agreement.

I'm just saying that it's mathematically impossible for high resistance to cause high current, which would pull the battery voltage down. It is however very possible for excess current to cause battery cables to heat up, and to pull the battery voltage down.

[Quadrille in JB]

Thanks

Originally Posted by masonc

Use a meter across the connections and switches to see where the voltage drop is happening. Check from the battery terminal, the lead, to the crimp, there should be no voltage. Continue down the cabling, check ever point where the cabling is terminated.
Check the negative also. Check any battery switches the same way, it should all be zero when cranking. When you find a voltage drop, bingo, you have your problem. Sometimes you find multiple problems, you have to work through all of them.

This is the most sensible post on this topic that I’ve seen. Lots of misinformation regarding low and high resistance.
Simply take your voltmeter, set on DC volts, and test across each connection in the circuit, including the start solenoid, while attempting to crank the engine.
If your voltmeter reads more than ~1 volt, disassemble and clean the connection.
Low resistance is what the start circuit wants to see.

[masonc]

Originally Posted by Tornadosailing

All I want to know is HOW did the starter pull the battery voltage down. Because if you believe the starter pulled the battery voltage down, we are in complete agreement.

I'm just saying that it's mathematically impossible for high resistance to cause high current, which would pull the battery voltage down. It is however very possible for excess current to cause battery cables to heat up, and to pull the battery voltage down.

If there is a high resistance connection, a high demand will result in a low voltage across the load.
If you short the source, you get zero volts across the load.
The reason we use large cables is they have low resistance and do not cause significant voltage drop.
Does that explain it?

[masonc]

Originally Posted by Tornadosailing

it's mathematically impossible for high resistance to cause high current, which would pull the battery voltage down. It is however very possible for excess current to cause battery cables to heat up, and to pull the battery voltage down.

It all depends on where you measure. High resistance connections cause low voltage ACROSS THE LOAD.

[Tornadosailing]

Originally Posted by masonc

It all depends on where you measure. High resistance connections cause low voltage ACROSS THE LOAD.

I'm fully aware of how important voltage drops are in a starter system, or any system for that matter. What I've been trying to get someone to do is explain why a high resistance connection pulled the battery voltage from 13 to 10.5 when attempting to start as several people here have suggested.

The short answer is that it can't. Therefore, assuming that the OP gave us good info, the answer must be that the starter is pulling too many amps and not doing it's job. Too many amps through the starter = battery voltage sag, and since the voltage must be consumed somewhere, the cables get hot. I would bet that the starter was warm as well.

All of that being said, as I've said before, the OP should absolutely check for any connection or resistance problems. That's just smart troubleshooting. Low voltage has lead to the demise of more than one starter.

[Tornadosailing]

Originally Posted by masonc

.

BTW, I was an avionics comm/nav tech in the Marine Corps. Since then I've been an ASE Certified Master Auto Technician for the last 30 years. I have 9 technicians working for me, most of them also Master Techs, and when they have an electrical problem they can't solve, they come see me. I usually troubleshoot their problem with a description of what's not working, what tests they've done, and a wiring diagram while sitting at my desk. Not saying this because I'm the smartest guy in the world. I learn stuff every day, and I'm always willing to be proven wrong. Just saying I've been around a few DC electrical problems in my day.

But, I would also be surprised if the OP gave us a 100% accurate picture of what's going on. Not saying he did anything right or wrong, just that he probably doesn't know the difference.

[a64pilot]

Originally Posted by Tornadosailing

I'm fully aware of how important voltage drops are in a starter system, or any system for that matter. What I've been trying to get someone to do is explain why a high resistance connection pulled the battery voltage from 13 to 10.5 when attempting to start as several people here have suggested.

So you finally concede that a high resistance connection will generate heat?
If so, then you have your answer, it takes a tremendous amount of power to generate heat by resistance, remember the Olympic cyclist trying to run a toaster?
So you have this high resistance connection that is using a great amount of the batteries power to generate heat, that may not leave enough current left to operate the starter.
That explains first how the battery is pulled down to 10.5 V, which is entirely normal, and also explains why there is zero or near zero voltage at the starter.

Doesn’t mean absolutely that is the problem, but heating of the wires needs to be investigated, as that points out a problem, bad starter or not, your starter wiring or any other wiring for that matter should never get hot, if any wiring ever gets hot, you need to determine why, and fix it, before it starts a fire.
No wires should ever get hot.

[john61ct]

That's a bit overly general.

Cranking an engine is for such a brief time, sure should not make healthy connections get hot.

But of course in a e.g. propulsion or winching context at high C-rates, especially with LI chemistries, temperature may rise at various points along the way, maybe 20 or even 40°.

There are upper limits spec'd for that reason, and increases in resistance causing higher than normal rises do indeed need to be monitored and investigated to stay within safe limits.

[Tornadosailing]

Originally Posted by a64pilot

So you finally concede that a high resistance connection will generate heat?
If so, then you have your answer, it takes a tremendous amount of power to generate heat by resistance, remember the Olympic cyclist trying to run a toaster?
So you have this high resistance connection that is using a great amount of the batteries power to generate heat, that may not leave enough current left to operate the starter.
That explains first how the battery is pulled down to 10.5 V, which is entirely normal, and also explains why there is zero or near zero voltage at the starter.

Doesn’t mean absolutely that is the problem, but heating of the wires needs to be investigated, as that points out a problem, bad starter or not, your starter wiring or any other wiring for that matter should never get hot, if any wiring ever gets hot, you need to determine why, and fix it, before it starts a fire.
No wires should ever get hot.

I never said that a high resistance connection wouldn't generate heat. What i said was that a high resistance anything reduces current flow. Reduced current availability would cause the starter not to run. However, and this has been my point all along, the reduced current available in the circuit cannot pull down the battery voltage. Excess current caused by a bad starter can, and excess current can overload wiring causing them to get warm.

After re-reading your last post, I wonder if you have a misconception about current flow in a circuit. You said that the resistance was using up the power of the battery and not leaving enough current to operate the starter. Do you understand that 100% of the current flows through 100% of the circuit? Any current going through the high resistance wire would also go through the starter. So if the wiring or a connection is pulling 150 amps, those 150 amps must also pass through the starter to get back to the battery.

I think you may have current and voltage mixed up in this regard. Voltage is consumed at each point of resistance or load. A high resistance wire would reduce the voltage available at the starter. But it would also reduce the amount of current flowing through the entire circuit. It's certainly likely that the voltage drop across the bad connection would result in heat generation, but it would never result in excess current that could pull down the battery voltage. Battery voltage sag is strictly cause by two things, bad batteries and high current draw.

[Tornadosailing]

Originally Posted by john61ct

Cranking an engine is for such a brief time, sure should not make healthy connections get hot.

I know you wouldn't be surprised to learn that some guys think if you just hold the key to start for a little while longer, something good will happen. I've seen guys melt the ends off a brand new battery cable.

Not saying the OP did this. Just saying it happens all the time in my world.

[a64pilot]

You can play with the words any way you like, but the fact is the heat comes from somewhere, the battery, there is a limited amount of current, or power that a battery can supply with an acceptable voltage drop.
If a significant amount of that power is used to generate heat, then then is less left to do other work, like starting an engine.

If a battery can supply say 200 amps, and it takes 150 amps to start an engine, life is good, the battery can start the engine, but if 100 amps is used to generate heat, then there is only 100 amps left, which is not enough to start the engine.

Of course the high resistance draws down the battery, where else could the heat come from?

[john61ct]

Yes I think the disagreement above came from semantics and misunderstanding, and from a functional perspective has now been resolved.

[donradcliffe]

I don't have any merit badges or certifications, but I have experience.

If the battery is pulled down to 10-11 volts, the starter is pulling pretty good current for a small genset motor. That means it is probably not the solenoid. The fact that there is almost no voltage across the starter cables means that you are losing 10 volts in the connections from the battery to the starter.

The first test is to measure voltage from the positive post on the battery to the positive post on the starter while you crank. If the voltage is less than 1 volt, you have a very good cable. If 2 volts or more, its part of the problem.

Repeat the first test, but measure from the negative post or case on the starter to the negative post on the battery. This will tell you whether the problem is on the hot side or the ground side.

If you have a high loss on either side, start working back from the starter motor to find the connection or wire which has gone bad.

My last diagnosis and fix this month was one of the YC boats with a 100 hp outboard. 11 volts across the battery, 5 across the starter. Voltage drop on the hot side was 5 volts, 1.5 on the ground. Problem was a hidden splice in the hot cable in a very inconvenient location. Fix was to abandon the cable and run a new one.

If you think you might have a bad starter, put the clamp on ammeter on either hot or ground cable and measure the current. A genset starter should not pull than about 200
cranking amps.

Another quick wiring check is to run a jumper cable from the battery to the starter solenoid. Try jumping the positive side first, then the ground. If that solves the problem, then you know which wires are bad.

If all you are getting is a click and the battery stays at 12v, its likely the solenoid or the solenoid wiring. The first thing to try is to jumper the little terminal on the solenoid (small red wire) to the battery terminal (big red wire from battery/switch). If that starts the engine, you need to work on the key circuit (some have small solenoids near the starter solenoid).

[smac999]

a bad shorted starter is possible, but then it still would not start with the shore power in. it would do the same thing and not start. he says it starts fine with shore power plugged in.

I'd bet that it's a cable / connection issue. and his shore charger or acr or whatever he has is connected downstream of the wire / connection issue. so when he starts while plugged in. the cranking current is coming from a charge source instead of from the gen battery.

he needs a clamp meter to know for sure. this is a 10 min job for a knowledgeable person with correct tools.

[jimbunyard]

Originally Posted by smac999

...This is a 10 min job for a knowledgeable person with correct tools.

Yep..