Ammeter Wiring

Status
Not open for further replies.
Aug 16, 2009
1,000
Hunter 1986 H31 California Yacht Marina, Chula Vista, CA
I am mounting digital ammeters in the cabin using shunts for house and start banks. I thought I might also put an auto type 50 amp meter in the cockpit instrument panel for the start battery. I am concerned however that if the meter fails it might affect the alternator because the meter is wired between the starter and alternator. Are these meters ill advised? If not, is there a way of wiring so that if the meter fails the alternator is still properly wired?
 
Feb 26, 2004
22,760
Catalina 34 224 Maple Bay, BC, Canada
Do you have a battery monitor? If so, it reads NET output, and it rarely is necessary to monitor the start/reserve bank separately.

What you're suggesting, IMHO, is like going back to the Dark Ages.

The very LAST thing you ever want to do is run your AO up through any sized wire to your cockpit panel to measure AO current. If you do, however, then get a remotely shunted ammeter.

Ammeters & Shunts 101 http://c34.org/bbs/index.php/topic,6032.0.html

Not sure why you'd bother, seems like over doing measurements.

Perhaps I'm misunderstanding, been known to happen before...:doh:
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
I am mounting digital ammeters in the cabin using shunts for house and start banks.
Why the start bank and why just an ammeter? A battery monitor is a much better investment and will give you loads more information.

I thought I might also put an auto type 50 amp meter in the cockpit instrument panel for the start battery.
Please don't... These can lead to tremendous voltage drops and destroy your ability to charge properly. I remove every single one I can talk an owner into removing and that is about 98% of them. The only way to properly measure alternator output is to use an external shunt 500A/50mV with an isolated ground alternator on the large GA neg wire or to use one on the positive side but this may require a shunt shifter.. You can also use a hall effect "shunt" but these are pricey.


I am concerned however that if the meter fails it might affect the alternator because the meter is wired between the starter and alternator. Are these meters ill advised? If not, is there a way of wiring so that if the meter fails the alternator is still properly wired?
Yes if that shunt fails, and these things are TINY, the alt will likely be destroyed. They are a poor choice in a charging circuit in deep cycling applications. There is no way to wire it so the shunt works and the alt still works if the meter fails. Shunts are "pass through" meaning all current has to pass through the shunt for them to work.

Again a battery monitor, like the Victron BMV-600, is your best bet especially when you start adding up $$$ to features. They also come standard with a REAL shunt rated at 500A..

Why does the alt charge the start battery when the motor is running? Ideally the alt will be charing the house bank which is the bank that really needs it. You can then use BOTH or an ACR or Echo type battery to battery charger to top up the start battery.

There is very little need to every monitor a starting battery, starting a small diesel uses less than .5Ah, but house batteries do work best with a battery monitor.
 
Jul 25, 2007
320
-Irwin -Citation 40 Wilmington, NC
"starting a small diesel uses less than .5Ah, but house batteries do work best with a battery monitor."

Please explain how you arrived at this? No matter how I do the math I cannot see how this makes sense?
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
"starting a small diesel uses less than .5Ah, but house batteries do work best with a battery monitor."

Please explain how you arrived at this? No matter how I do the math I cannot see how this makes sense?
I measure this stuff almost daily using tools designed for this purpose eg: Midtronics EXP-1000HD, Fluke 376 etc..... The math is easy once you realize what the actual currents are and the average start durations, loaded to unloaded. This is a 44 HP diesel. Note the average cranking amps and the duration of start time at 0.75 seconds. This is fairly normal starting performance for the 30F day this motor was started. Many smaller motors draw significantly less than this, especially when it is warmer.. So an average of 286A for .76 seconds winds up at well under .5Ah. Half an amp hour would actually be a lot for many sailboat aux engines...


You can also watch it here:

And here is a Fluke capable of measuring in-rush DC currents set to capture the peak in-rush current on a Universal M-25. The absolute peak for this start was 198A. Average over the starting duration is about half the in-rush for a second or so....

This is why monitoring starter batteries, other than the occasional voltage check, is not entirely necessary... In terms of Ah consumption starting an engine uses very little battery capacity and which is replenished very easily.
 
Jul 25, 2007
320
-Irwin -Citation 40 Wilmington, NC
Ok maybe its just me but I still cannot make the math work on this one. I am bit short of time right now but could you let me know what formula you used to calculate this and I will mess with it later when I have some more time.

I do not agree with the assumption that this starting time is typical nor does it take into account an engine that does not start within .75 seconds.

I am just trying to get my head around your conclusions on this one.
 
Feb 26, 2004
22,760
Catalina 34 224 Maple Bay, BC, Canada
Starting Amp Hour Math

Wayne, whether or not an engine actually starts in this amount of time is meaningless, for two reasons

1. If it doesn't it should, so something would have to need to be repaired

2. One can't assume a recalcitrant engine in doing the math. You could take it to absurd lengths and say the engine is broken and won't ever start so you'd run down the world's biggest battery to try to start it! :)

Here's the math, using 200A starting current and 30 seconds (which is an awfully long time to crank)

30 seconds = 0.5 minutes divided by 60 minutes per hour = 0.00833 hours times 200A = 1.6 amp hours

Still a drop in the ocean.
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
The math is pretty straight forward:

0.75 Seconds is approx 0.002 hours - 286A X 0.0002 = 0.06Ah's

1 second is approx 0.0003 hours - 286A X 0.0003 = 0.086 Ah's...

2 Seconds is approx 0.0005 hours - 286A X 0.0005 = 0.14 Ah's

3 Seconds is approx 0.0008 hours - 286A X 0.0008 = 0.23 Ah's

4 Seconds is approx 0.001 hours - 286A X 0.001 = 0.28 Ah's

5 Seconds is approx 0.0014 hours - 286A X 0.0014 = 0.40 Ah's




Keep in mind that the test at 286A was a 44 HP four cylinder diesel with an actual physical battery temp of 32F. Starting currents are highest with cold engines and cold batteries.

If an engine is physically cranking for more than 5 seconds, even when cold, there are issues that need to be addressed. Most engines I measure have a start duration of under 2 seconds. The range of 0.75 - 1.5 second being the most common loaded to unloaded duration.

Even if we gave that Universal M-25 shown in the video 3 full seconds to start, which it was not, and we rated it at full in-rush for the entire starting duration, which it does not do, it would look like this:

3 Seconds is approx 0.0008 hours - 198A X 0.0008 = 0.16 Ah's

5 Seconds is approx 0.0014 hours - 198A X 0.0014 = 0.28 Ah's

Of course in this example we are using absolute worst case scenarios, that are not true, and we are still at just 0.16Ah's for a 3 second start and 0.28Ah's for a 5 second start.. The reality is that the average cranking is far less than 198A and this motor starts in less time..

This is the entire test done on that particular engine.

Resting Bank Voltage, Tested CCA of The Bank, Rated CCA of Each Battery, Battery Case Temp


Cranking Current Over Duration Of Start (this represents 0.75 Seconds)


Voltage Performance During Starting Eposide


Average Voltage During Starting Episode, Average Current During Episode, Duration Of Start Loaded to Unloaded, Circuit Resistance
 
Jul 25, 2007
320
-Irwin -Citation 40 Wilmington, NC
The correct formula for amp hours is Ah=(3600s)(A)

http://en.wikipedia.org/wiki/Ampere-hour

Ah=3600((286)(.75))=772200, which makes sense if you are using 214.5 amps per second that would be 772,200 amps in an hour.

What you are saying is starting the engine only used .5 amp over an hour which is clearly not true as you know you used 286 amps to start the engine. You cannot somehow end up using less than that because you spread it out over an hour.

That makes sense to me I hope is does to you, or maybe I am not looking at this right?
 
Aug 16, 2009
1,000
Hunter 1986 H31 California Yacht Marina, Chula Vista, CA
Points well taken, Stu and Main. I'll throw that old ammeter back in the bin for some future sand rail project and check prices on a monitor. Stu, on the link you reference it shows 2 wiring diagrams, one with and one without a shunt. Do they use the same gauge? I would guess that the old ammeters with internal shunts cannot operate with an external shunt and the light gauge wiring normally associated with shunted ammeters.
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
The correct formula for amp hours is Ah=(3600s)(A)

http://en.wikipedia.org/wiki/Ampere-hour

Ah=3600((286)(.75))=772200, which makes sense if you are using 214.5 amps per second that would be 772,200 amps in an hour.

What you are saying is starting the engine only used .5 amp over an hour which is clearly not true as you know you used 286 amps to start the engine. You cannot somehow end up using less than that because you spread it out over an hour.

That makes sense to me I hope is does to you, or maybe I am not looking at this right?
You are simply not understanding that we use the fractions of an hour to calculate the amp hours.. In order to consume 286Ah's you would need to burn that load for 1 hour. We burned an average of 286A for 0.75 seconds hence we consumed just 0.0002 of what an hour at that load would be..

1 amp for 1 hour = 1 amp hour

1 amp for 1 second = 0.0003 Ah's

100A for 1 hour = 100 amp hours

100A for 1 second = 0.03 Ah's

286A for 1 hour = 286 amp hours

286A for 0.75 seconds = 0.057 Ah's
 
Jan 22, 2008
8,050
Beneteau 323 Annapolis MD
It's like batting averages. If a batter has not been at bat 100 times, you project the actual number of times he HAS been at bat, as though he had done 100. In this case, you can meter-read and reverse the amp hours down to the actual time used. That's how i read it.
 
Jul 25, 2007
320
-Irwin -Citation 40 Wilmington, NC
What you are saying is if you use 214.5 amps and then spread that out over an hour you will use .5 amps per second not .5 Ah. I see what you are doing dividing the amps by an hour but it is a bit of a misleading number when done like that and does not really reflect correct power usage as I understand it. Amp hours, as I see it and as you stated it, is amps used in an hour, so if you started the engine and used no more power you still would have used 286 amps in that hour not .5 surely that makes sense. Oh well I guess we all interpret things differently.
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
What you are saying is if you use 214.5 amps and then spread that out over an hour you will use .5 amps per second not .5 Ah. I see what you are doing dividing the amps by an hour but it is a bit of a misleading number when done like that and does not really reflect correct power usage as I understand it. Amp hours, as I see it and as you stated it, is amps used in an hour, so if you started the engine and used no more power you still would have used 286 amps in that hour not .5 surely that makes sense. Oh well I guess we all interpret things differently.
Wayne,


214.5A burned for a 1 second duration equals 0.058 Ah's consumed from the battery.

214.5 amps X 0.00027 (fractions of an hour) = 0.0579 amp-hours


You used 286A, yes, but for how long is what determines how many AMP HOURS were removed. We don't generally reference removing amps from the battery we reference amp hours consumed over time. Amps are what are used to determine Ah's consumed but you must divide the fractions of an hour used to arrive at Ah's.. The only time a load in AMPS would = the AMP HOURS is if you ran at that amperage for a full hour. Any faction of that hour needs to be calculated for the time, at load.

1 amp for 1 hour = 1 amp hour

1A for any fraction of an hour is not 1 amp hour. It is still a 1A load but not 1 Ah if used for any less than 1 hour. If you draw 1A for less than an hour it is governed by the time or fractions of an hour the load was on for.

Amp hours are simply a reflection of the current (amps) used and for how much of an hour.

The .5Ah I referenced is a simple average of what most sailboat aux engines will not draw more than in Ah's to start the engine. Ah's are not amps they are amps X fractions of an hour to arrive at the Ah's...
 
Jul 25, 2007
320
-Irwin -Citation 40 Wilmington, NC
"We don't reference removing amps per-SE from the battery we remove amp hours" this is not correct amp hours are not a unit of power, amps and watts are but not amp hours. We remove X amp over Y time but we do not remove Z Ah. you go on to say this yourself ""Ah's are not amps they are amps X time to arrive at Ah's..." Correct but because that time is short is does not decrease the amps used.

Amp hours is a unit of measurement used to determine the useful life of a battery, IE a 10 Ah battery can provide 10 amps for one hour but that is not a unit of power. Amps is the unit of power and the time is whatever time used.

"1A for any fraction of an hour is not 1 amp hour" not completely correct but if you only use 1 amp in an hour it is 1 Ah as you have said. If you use more amps then the Ah used will change based on the total amps used in that hour.

I think the confusion here is you are using amp hour as a unit of energy which it is not. You do not remove amp hours from a battery you remove amps over a period of time which then becomes amp hours. You say this yourself "Amp hours are simply a reflection of the current (amps) used and for how much of an hour."

You said "1 amp for 1 hour = 1 amp hour" so with that 10 amps for an hour = 10 Ah and 100 amps for an hour = 100 amp hours so 286 amps in an hour = 286 Ah.
 
Jan 22, 2008
8,050
Beneteau 323 Annapolis MD
"We don't reference removing amps per-SE from the battery we remove amp hours" this is not correct amp hours are not a unit of power, amps and watts are but not amp hours. We remove X amp over Y time but we do not remove Z Ah. you go on to say this yourself ""Ah's are not amps they are amps X time to arrive at Ah's..." Correct but because that time is short is does not decrease the amps used.

Amp hours is a unit of measurement used to determine the useful life of a battery, IE a 10 Ah battery can provide 10 amps for one hour but that is not a unit of power. Amps is the unit of power and the time is whatever time used.

"1A for any fraction of an hour is not 1 amp hour" not completely correct but if you only use 1 amp in an hour it is 1 Ah as you have said. If you use more amps then the Ah used will change based on the total amps used in that hour.

I think the confusion here is you are using amp hour as a unit of energy which it is not. You do not remove amp hours from a battery you remove amps over a period of time which then becomes amp hours. You say this yourself "Amp hours are simply a reflection of the current (amps) used and for how much of an hour."

You said "1 amp for 1 hour = 1 amp hour" so with that 10 amps for an hour = 10 Ah and 100 amps for an hour = 100 amp hours so 286 amps in an hour = 286 Ah.
You are only using half of the correct info. IF, If, if, you kept the current flowing for one hour, you would be at least patially right. BUT, you're only using it for 1/5 second(1/3600 of an hour) in Maine's example. Thus, you use only that portion of one hours amphours, which is a measurement of the STORAGE capacty of the battery, and is doled out only for that time you use juice from it. Otherwise, you'd do well to consult a book on the subject. There is a tee shirt that says, "We can explain it to you, but we can't make you understand it".
 
Jul 25, 2007
320
-Irwin -Citation 40 Wilmington, NC
lol ok well i give up i will say if you have a 100 Ah battery and you use 100 amps it will be dead. the battery does not care if you use that in 10 seconds or an hour you have still used 100 amps. Yes if you use it fast the battery can come back some but in the end 100 amps is 100 amps you cannot change that by using it faster I just do not see how that does not make sense. So I say if anyone can show me some documentation that says i am wrong I am more then happy to review it and I will humbly apologize. but I did go to school for this stuff so I do not think I need the T shirt but maybe it is still needed.

And because I am not a complete ass I will go off when I have some time and do some more research on this but for now I stand by what i have said.

And by all means please prove me wrong with some outside reliable source (Not an option facts only)
 
Jul 23, 2009
857
Beneteau 31 Oceanis Grand Lake, Oklahoma
Ah does't not = amps

I think your are confusing the units of measure. An amp is not an amp hour, they are related but not the same.

An 100ah battery would be depleted in an hour if you had a 100a load on it. The same 100ah battery would be depleted if you had a 50a load on it for 2 hours.

Also important. Don't confuse AH raitings and CCA for batteries. They are not even close to same thing.
 
Sep 25, 2008
615
Morgan 415 Out Island Rogersville, AL
lol ok well i give up i will say if you have a 100 Ah battery and you use 100 amps it will be dead. the battery does not care if you use that in 10 seconds or an hour you have still used 100 amps. Yes if you use it fast the battery can come back some but in the end 100 amps is 100 amps you cannot change that by using it faster I just do not see how that does not make sense. So I say if anyone can show me some documentation that says i am wrong I am more then happy to review it and I will humbly apologize. but I did go to school for this stuff so I do not think I need the T shirt but maybe it is still needed.

And because I am not a complete ass I will go off when I have some time and do some more research on this but for now I stand by what i have said.

And by all means please prove me wrong with some outside reliable source (Not an option facts only)
If you take a 100 AH battery and connect a 00 AWG wire across the posts, I'll wager that the wire will be carrying a heck of a lot more current than a 100 amps, that is until the wire or battery post melts.

Golly, this thread is a true golden nugget.
 
Feb 26, 2004
22,760
Catalina 34 224 Maple Bay, BC, Canada
Amps and amp hours

And because I am not a complete ass I will go off when I have some time and do some more research on this but for now I stand by what i have said.

And by all means please prove me wrong with some outside reliable source (Not an option facts only)

Here's an example:

http://www.westmarine.com/webapp/wc...e=Sizing-Your-House-Battery-Bank#.UgKSAW3pz-I

You might want to check the referenced energy budget worksheet.

An energy budget is developed by determining the LOAD in AMPS times the NUMBER OF HOURS OF USE. In the case of use of less than hour, the number of amp hours cannot be more than the amps.

Wayne, this is simple math.

100A load for 30 minutes is 50 amp hours, not 100 ah.
 
Status
Not open for further replies.