1987 MK 1 Electrical System Upgrade - Feedback Requested

[mainesail]

Is this the correct way to take this measurement? Thanks for the help.

Jon W.

No..

** Set DVM to DC Volts and 12V Range
** Place one lead on the alternator B+ terminal (alligator lead required)
** Place the other lead on the House bank + terminal
** With no current flowing ( 0A ) the reading should be 0.00V DC
** Fire up alternator to full output (apply a heater or hair dryer via inverter if battery is full)
** What voltage is now displayed on the DVM screen?
** This voltage is your positive side voltage drop at XX or XXX amps

** Repeat for negative side and add positive and negative together as total voltage drop at XX or XXX amps.

You can also just measure across the back of the alt B+ and alt case or isolated ground then do the same at the battery end. This only tells you the total voltage drop not how much is occurring in pos or neg leg. For example if you read 14.21V between alt B+ & B- and then 13.26V between battery + & -, with both measurements taken at 82A, then you have a 0.95V total drop and you'll now want to check each leg anyway.

In order for voltage drop to occur you must be passing CURRENT through the wires.....

[J_Sail]

Not to put too fine a point on it but...

As MaineSail said, "for voltage drop to occur you must be passing CURRENT through the wires". The Voltage drop is the Current going thru the wiring multiplied by the Resistance of the wiring (V=Amps * Ohms). It's usually easiest to measure total drop (i.e. source voltage minus delivered voltage*), but as MaineSail pointed out, by carefully measuring the individual drops for the positive lead and the negative lead you can better identify the source of any problem.

Do keep in mind, though, that you should not try to measure drop from the alternator to your starting battery, as it is effectively isolated by the EchoCharge.

Jeremy

* In the end, you really only care that the necessary voltage is delivered to the load when needed, but minimizing the voltage drop is an important part of that (and something you invested time and money in), so measuring it upon initial commissioning to confirm installed quality is a great idea. 

[Jon W]

Alternator B+ is the positive output from the alternator correct? Is there value in measuring the starter, and if so how is that done?

Jon W.

[Stu Jackson]

Alternator B+ is the positive output from the alternator correct?

Is there value in measuring the starter, and if so how is that done?

1.  Yes.

2.  Maine Sail has measured this many times.  I can't find his specific topic on this right now, but here's a link with his results:  http://forums.sailboatowners.com/index.php?threads/ammeter-wiring.155237/&highlight=measuring%20starter%20current

[KWKloeber]

Jon

I'm confused - maybe not others.  What are you trying to do and your goal.   What do you mean 'measure' the starter.   Specificity would help answer your
Q?s.

[mark_53]

Is there value in measuring the starter, and if so how is that done?

Jon W.

It's an academic question since you have a starting battery with relatively short cables you should not have a problem starting that little engine.

[Jon W]

Hi Ken,
Throughout this thread there has been lots of discussion about voltage drop. I've captured each run in a spreadsheet and used the information to estimate voltage drop. I would like to see how the actual compares to the estimate. Example estimate says the alternator to house bank round trip voltage drop is 2.85% (w/o connections) or 5.35% (with connections). Is it?

Hi Mark,
In one sense it is an academic question, in another it is learning how to properly troubleshoot the system so think it's a worthwhile exercise. My reserve battery is not under the aft cabin, it is forward of the starboard water tank. No problem starting the engine with either reserve or house.

Jon W.

[Stu Jackson]

Hi Ken,
Throughout this thread there has been lots of discussion about voltage drop. I've captured each run in a spreadsheet and used the information to estimate voltage drop. I would like to see how the actual compares to the estimate. Example estimate says the alternator to house bank round trip voltage drop is 2.85% (w/o connections) or 5.35% (with connections). Is it?

Jon, of course voltage drop matters.  However, if your calculations and wire sizing were correct, unless you have poor connections, you should be just fine, because most sizing ends up using the "next biggest" size wire 'cuz if you're on the cusp you pick the biggest one.  In addition, alternator output, even with external regulation, drops off rapidly after the first few minutes of charging.  Finally, proper voltage sensing from the regulator, in my experience (documented in the Electrical Systems 101 topic) gets the voltage properly to the house bank, the current follows.

PS - if you add your name to your signature you can avoid having to type Jon W. with all your posts.   

[KWKloeber]

Hi Ken,
Throughout this thread there has been lots of discussion about voltage drop. I've captured each run in a spreadsheet and used the information to estimate voltage drop. I would like to see how the actual compares to the estimate. Example estimate says the alternator to house bank round trip voltage drop is 2.85% (w/o connections) or 5.35% (with connections). Is it?

Hi Mark,
In one sense it is an academic question, in another it is learning how to properly troubleshoot the system so think it's a worthwhile exercise. My reserve battery is not under the aft cabin, it is forward of the starboard water tank. No problem starting the engine with either reserve or house.

Jon W.

Jon,

Got it.  You want to verify your assumptions (some were 'laws' like V=IR but still may want to prove them to yourself, and verify your work (very rewarding.)  So you know what each voltage drop *should* be between various connection points based on your spreadsheet -- but only under a specific amperage (current) flow.   BUT, you used the law V drop = I current x R resistance -- and that doesn't change.  Ever.  So if you measure resistance, you are essentially proving out your spreadsheet assumptions without having to actually pass thru the cable, the exact current that you had assumed in the spreadsheet.   I think what I would have done (time/situation allowing?) would have been to measure/record R for each completed (or at least major) cable to satisfy myself that my crimping work was up to snuff.  That would give you a baseline dataset, and if a leg now had higher R, then you would look to something in the bolted (or whatever type) connection.  There's not much you can do with those, except use a dielectric (preventing corrosion) and torque them down (and of course use the rules for layering multiple lugs on a stud.)

Tho as you say it's academic, what the starter draws is interesting to search out, to say the least.  You'd use the clamp-on to measure peak starting current.  Also, I've always wanted to measure the pull-in and hold-in current for the solenoid switch -- I don't believe the numbers a rebuilder gave to me.

You have two critical paths -- voltage to the batteries while charging AND voltage at the starter lug when cranking.  I'd measure and verify those both as a start.  Voltage at those points being what you did all the work for in the first place.  As RC said, you need to look at voltage drop between the sources and two end points, or the isolated voltage at those end points to verify that it meets what your goals were.

confusing?

Ken

[mark_53]

Hi Mark,
In one sense it is an academic question, in another it is learning how to properly troubleshoot the system so think it's a worthwhile exercise. My reserve battery is not under the aft cabin, it is forward of the starboard water tank. No problem starting the engine with either reserve or house.
Jon W.

I applaud your quest for knowledge but it seems like if you know the wire size, length, and expected load, voltage drop is a matter of looking at published values that have been around for years. Are you trying to prove them wrong or just verifying your spreadsheet. If the latter, easier to double check your spreadsheet but I understand if your from Missouri.

[J_Sail]

If you are inclined to spend the time, I think measuring a couple key voltage drops to verify that the connections are robust is a good idea and it could be a fun academic exercise to verify that the assumptions were all correct. As stated by others, the two that really matter are the drop from the battery to the starter while cranking the engine and the drop from the alternator to the battery while supplying high charging current (or simulated equiv load by attaching some sort of load to the battery). Neither is trivial to measure due to possibly needing more than two-hands at once, long meter leads, or other complexities, but it might be fun and enlightening if you are interested. Conversely it might be an unnecessary chore if it has no appeal. If you want to proceed, then MaineSail's approach is best. I can provide addtl details if desired.


By the way, contrary to what one might think, measuring the resistance with a meter and then calculating voltage drops is not exactly the same as verifying the drop under actual loads, due to critical non-linear behavior of bad connections. I've had two cases of failed 125A winch/windlass circuit breakers, where internally damaged/degraded contacts measured fine with the tiny currents used by meters, but would instantly convert to a high resistance when the motor drew full current. The physics are complex, but the results speak for themselves. So while I agree that measuring wiring resistance can be useful, the real test is voltage drop under real-life load.

[mainesail]

Do anyone know of any published guides for voltage drop across switches, corrosion, fuses, fuse blocks, lug to wire by crimp type, rusty engine blocks, terminals, lug to busbar etc. etc..??

Knowing the installed voltage drop is not a bad thing and quite often can highlight issues before they become a problem. Installed voltage drop simply doesn't line up with the "wire-drop" calculators because they only account for the wire-drop not all the other items that also have resistance.

That said if you want to measure starter VD then you need fast tools, or tools specifically designed for this that can measure circuit resistance during cranking. Most run of the mill DC clamp meters and volt meters don't have the resolution nor speed to do this. You can close the seacock and pull the stop lever and set the meter to capture peak low volts, if it has it, but it will only be a rough guide because starter motor current is not stable.

[Jon W]

Not trying to prove anyone or anything wrong. Just curious how close the estimates are to the actuals. I also think it's valuable to know how to troubleshoot on a "good" system so I know what to do if something goes wrong in the future. I'm still a novice in all of this.

[mainesail]

Not trying to prove anyone or anything wrong. Just curious how close the estimates are to the actuals. I also think it's valuable to know how to troubleshoot on a "good" system so I know what to do if something goes wrong in the future. I'm still a novice in all of this.

Jon,

You're on a good path.. As I mentioned the as installed voltage drop simply won't line up with the voltage drop calculators because they don't and can't take into account each and every termination, fuse, switch busbar etc. etc...

[mark_53]

Jon,

You're on a good path.. As I mentioned the as installed voltage drop simply won't line up with the voltage drop calculators because they don't and can't take into account each and every termination, fuse, switch busbar etc. etc...

Good point about the other items that effect VD. I stand corrected in that regard. but, if it requires specialized expensive tools that won't line up with his estimates, and there is no problem with the starter actually starting the engine, to me that would be a wild goose chase.