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Old 17-11-2014, 11:20   #16
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Re: A Question for Electrical Geniuses

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Originally Posted by cal40john View Post
He said no one cable carries all of the current. If there is one set of batteries in one location and another in a different location each with a wire on the negative post leading to loads, and another wire connecting the negative posts together then there is not the wire that you postulate to connect to.
By definition, a "battery bank" is a group of batteries wired together so as to operate as a single higher capacity battery. This means a single positive connection and a single negative connection. If this is not the case, he does not have a "battery bank".

Without knowing exactly how his boat is wired and from his post, all that can be done is guess what he actually has and suggest solutions based on that guess.
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Old 17-11-2014, 11:31   #17
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Re: A Question for Electrical Geniuses

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Originally Posted by Terra Nova View Post
Why didn't you simply replace the bearings?
I bumped up the alternators from 55A to 80A. They were the biggest I could go to without spending thousands on new pulleys and harmonic balancers. The wire could explain why I never saw more than 55a. In any case my plan is to have the bearings in the 55s replaced and keep at least one as a spare.
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Old 17-11-2014, 12:41   #18
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Re: A Question for Electrical Geniuses

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Originally Posted by Dockhead View Post
I would like to measure DC current into and out of my batteries. But my battery bank is divided into two parts (4x Trojans in each) in two different locations. So there is no single cable which carries all the current.

So I need some kind of totalizer -- some instrument which can add the current from shunts or inductive sensors on two different cables and display the total.

Does such a thing exist?
Dockhead,

way back to your original question, if none of the other answers about single-bank/common conductor work for you...

The standard current shunt that gets used in most cases is a 50mV at full scale output, and then the meters that get attached to them match the shunt. Since this is a voltage signal you can connect the signals from two shunts in series and route them to a single meter. You will lose some resolution but can measure total current.

Example: Say you want to be able to measure 100A out of each bank, for a total of 200A. Put a 200A shunt at each bank, and wire the signals in series to a 200A meter. 100A at bank A should put out 25mV and 100A at bank B should also put out 25mV, resulting in 50mV total at the meter. The loss of resolution comes in because you have a 200A shunt in a line where you only expect a maximum of 100A, so you'll never drive the shunt past 50% (and you want to set it up that way so you won't over drive the meter).

If you're using some kind of monitor rather than an analog meter you may be able to set the range for the input signal to something other than 50mV, in which case you might be able to use appropriately sized shunts. If all you want to measure is the current, then you can use two 50mV shunts and wire them (in series) into a configurable industrial thermocouple display. Lots of these out there for 24VDC systems, you would just set the range to 100mV and then the scaled value to match whatever shunts you are using.

Another (small) caveat, since these are mV signals it's easy to lose them with noise and distance, so the more wire you have to run to make this happen the more likely you are to degrade the signal. If you do have to run the signals for a significant (?) distance you you may be better off using a thermocouple wire that closely matches the signal level, as that will help reduce signal changes with distance.
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Old 17-11-2014, 13:42   #19
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Re: A Question for Electrical Geniuses

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Originally Posted by Dsanduril View Post
Dockhead,

way back to your original question, if none of the other answers about single-bank/common conductor work for you...

The standard current shunt that gets used in most cases is a 50mV at full scale output, and then the meters that get attached to them match the shunt. Since this is a voltage signal you can connect the signals from two shunts in series and route them to a single meter. You will lose some resolution but can measure total current.

Example: Say you want to be able to measure 100A out of each bank, for a total of 200A. Put a 200A shunt at each bank, and wire the signals in series to a 200A meter. 100A at bank A should put out 25mV and 100A at bank B should also put out 25mV, resulting in 50mV total at the meter. The loss of resolution comes in because you have a 200A shunt in a line where you only expect a maximum of 100A, so you'll never drive the shunt past 50% (and you want to set it up that way so you won't over drive the meter).

If you're using some kind of monitor rather than an analog meter you may be able to set the range for the input signal to something other than 50mV, in which case you might be able to use appropriately sized shunts. If all you want to measure is the current, then you can use two 50mV shunts and wire them (in series) into a configurable industrial thermocouple display. Lots of these out there for 24VDC systems, you would just set the range to 100mV and then the scaled value to match whatever shunts you are using.

Another (small) caveat, since these are mV signals it's easy to lose them with noise and distance, so the more wire you have to run to make this happen the more likely you are to degrade the signal. If you do have to run the signals for a significant (?) distance you you may be better off using a thermocouple wire that closely matches the signal level, as that will help reduce signal changes with distance.
That sounds like a bad idea to me.

At some point in the system, the entire current will be present, no matter which bank or both banks are switched in to feed the main DC power distribution panel. This could be a common ground. It could be the board side of the battery switch. But at such a point a single shunt can be installed, and a meter wired in parallel to it, which will then read the total system current.

The shunt does not produce a voltage. Rather, it is a low resistance resistor. The resistance reduces the voltage on the negative end in relation to the positive end.

Now, if you reduce the voltage by 50mv with the bank A shunt, connect the negative end of it to the positive end of the bank B shunt, first of all you are raising the voltage back to the full bank voltage. Second, you are then only measuring the voltage drop across shunt B. What that gives you, then, is just the bank B current.
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Old 17-11-2014, 14:51   #20
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Re: A Question for Electrical Geniuses

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Originally Posted by GrowleyMonster View Post
That sounds like a bad idea to me.
You're right, and I take it all back (since it's too late to delete the post).

For some reason my brain was thinking like a signal generation device, when in fact the shunt is referenced to the battery (either + or - depending on where it is installed). I could make it work for you with a couple of signal isolators, but now it's getting too complicated for a boat in the real world.
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Old 17-11-2014, 17:39   #21
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Re: A Question for Electrical Geniuses

This is easy to do... the proper way. You really have a two bank system and you'll need to and want to measure/ monitor them separately. Fortunately two bank battery monitors are readily available in a single small unit with switches to see bank 1 and bank 2 information.

1) Find a place to mount your new two bank battery monitor and do the deed.

2) As it's instructions will direct you... connect one of the supplied shunts between the negative battery terminal and ship's ground of one of your battery banks. Usually the ship's ground is the engine block (just follow where the heavy black negative battery wire goes)

3) Then do the same for your other battery bank.

There are a several other small gauge connections to make to the shunts and other points as per directions, but your basically done with the heavy gauge wire stuff... and on the home stretch of being done.

Just make sure all of the negative leads from loads/ connections that might have been connected to the battery negative terminal are connected to the ship 'a ground side of the shunt(s ). You don't have to touch/ change anything on the + terminals.

This won't (and shouldn't) give you a single total of both banks in one reading. You have two separate banks that can charge/ discharge at different rates and should be monitored/ managed separately.

I found/ still find my amp-hr battery monitor is one of the most importation meters on my boat... right up there with engine oil and temp meters.

Good luck


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Old 17-11-2014, 17:50   #22
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Re: A Question for Electrical Geniuses

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Originally Posted by colemj View Post
There isn't a single cable coming out of the charging source? How do these banks supply distributed power? Is there a common point at the panel? These banks should be tied together with a common ground.

Mark
There are two cables coming out of the charging source. The grounds are separated by 10 meters.
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Old 17-11-2014, 17:52   #23
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Re: A Question for Electrical Geniuses

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Originally Posted by GrowleyMonster View Post
Yeah just install the shunt in series with the battery switch. A meter there will measure total current, whether you are on bank A, B, or both.

You can also meter each bank seperately. Simply add the currents to get total current.
I have two separate battery switches. Big expensive remote control solenoids per the original build.

Yes -- just installing to meters and totalizing in my head did occur to me . Certainly one way to skin the cat. Is there no other?
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Old 17-11-2014, 17:54   #24
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Re: A Question for Electrical Geniuses

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Originally Posted by sparrowhawk1 View Post
If you have a battery switch for both banks negatives must be connected. Just put the shunt in the middle of the negative cable. most shunts are designed to put on the negative cable
Indeed, but as the Russians say, "If!".

I have two different, separate battery switches! Hence the problem!
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Old 17-11-2014, 17:55   #25
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Re: A Question for Electrical Geniuses

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Originally Posted by Dsanduril View Post
Dockhead,

way back to your original question, if none of the other answers about single-bank/common conductor work for you...

The standard current shunt that gets used in most cases is a 50mV at full scale output, and then the meters that get attached to them match the shunt. Since this is a voltage signal you can connect the signals from two shunts in series and route them to a single meter. You will lose some resolution but can measure total current.

Example: Say you want to be able to measure 100A out of each bank, for a total of 200A. Put a 200A shunt at each bank, and wire the signals in series to a 200A meter. 100A at bank A should put out 25mV and 100A at bank B should also put out 25mV, resulting in 50mV total at the meter. The loss of resolution comes in because you have a 200A shunt in a line where you only expect a maximum of 100A, so you'll never drive the shunt past 50% (and you want to set it up that way so you won't over drive the meter).

If you're using some kind of monitor rather than an analog meter you may be able to set the range for the input signal to something other than 50mV, in which case you might be able to use appropriately sized shunts. If all you want to measure is the current, then you can use two 50mV shunts and wire them (in series) into a configurable industrial thermocouple display. Lots of these out there for 24VDC systems, you would just set the range to 100mV and then the scaled value to match whatever shunts you are using.

Another (small) caveat, since these are mV signals it's easy to lose them with noise and distance, so the more wire you have to run to make this happen the more likely you are to degrade the signal. If you do have to run the signals for a significant (?) distance you you may be better off using a thermocouple wire that closely matches the signal level, as that will help reduce signal changes with distance.
Yes!! Genius!! That's just the insight I was hoping for!
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Old 17-11-2014, 18:11   #26
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Re: A Question for Electrical Geniuses

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Yes!! Genius!! That's just the insight I was hoping for!
Sorry to burst your bubble then, hope you read my post after Growley pointed out the error of my thinking.

After reading your posts, I could design something that works as I described. But, you need a couple of signal isolators between the shunt signals, and that adds to $//. Here in the States they run about $100-$150, don't know how that would sit with your bank account since you would need two, and you'd have to provide power to them. IIRC you have a 24V system that means pretty much the entire world of industrial control hardware is at your command.

If you want to explore that road, the 50 mV shunt signal is basically the same as a thermocouple signal, and there's all kinds of industrial TC equipment out there that is IP 67 and conformal coated and would survive just fine on a boat. For your reference:

http://www.coulton.com/res/tc_tc_the...isolator_c.pdf

Analog Millivolt Signal Isolators | Canopus Instruments

http://www.mtl-inst.com/images/uploa...00/MTL5081.pdf
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Old 17-11-2014, 19:00   #27
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Re: A Question for Electrical Geniuses

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Originally Posted by Dsanduril View Post
Sorry to burst your bubble then, hope you read my post after Growley pointed out the error of my thinking.

After reading your posts, I could design something that works as I described. But, you need a couple of signal isolators between the shunt signals, and that adds to $//. Here in the States they run about $100-$150, don't know how that would sit with your bank account since you would need two, and you'd have to provide power to them. IIRC you have a 24V system that means pretty much the entire world of industrial control hardware is at your command.

If you want to explore that road, the 50 mV shunt signal is basically the same as a thermocouple signal, and there's all kinds of industrial TC equipment out there that is IP 67 and conformal coated and would survive just fine on a boat. For your reference:

http://www.coulton.com/res/tc_tc_the...isolator_c.pdf

Analog Millivolt Signal Isolators | Canopus Instruments

http://www.mtl-inst.com/images/uploa...00/MTL5081.pdf
That kind of cost is ok, and the idea is good.
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Old 18-11-2014, 00:22   #28
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Re: A Question for Electrical Geniuses

the dual shunt in series should work. interesting concept. both would go on the neg cables of each "bank" since it sounds like he has 2. and would work with nothing else needed. I think some battery monitors you can enter the shunt size. which make things even easier to fake it.


however since he has 2 banks I'd rather have 2 battery montitors. and actually see current in and out of each bank.


one bank could be at 40% and one at 80% and your single meter would say 60%. saying you are still ok. when in fact one bank is at 40% and should have a charger on it asap.


or could be an issue with a charger wire. pluged into shore could have -20a on one bank and + 60 one 2nd. meter says +40a. everything seems ok. when it's not cause charger fuse is blown on bank 1 but you'd never know with single meter




if he posted an actualal diagram of his bank, switch, and wiring layout we'd know exactly what he had.
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Old 18-11-2014, 00:35   #29
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Re: A Question for Electrical Geniuses

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Originally Posted by W3GAC View Post
This is easy to do... the proper way. You really have a two bank system and you'll need to and want to measure/ monitor them separately. Fortunately two bank battery monitors are readily available in a single small unit with switches to see bank 1 and bank 2 information.
who makes a dual battery monitor? I haven't seen one for years.

the old link2000s did, but even that wouldn't help him as the shunts were side by side. if he could get shunts side by side then he could just use a single one. that is his problem, they are far apart.
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Old 18-11-2014, 01:13   #30
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Re: A Question for Electrical Geniuses

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Originally Posted by smac999 View Post
if he could get shunts side by side then he could just use a single one. that is his problem, they are far apart.
They make this thing called copper cable for just such an occasion
Re the original question, get two BMV-700's from Victron, learn how to read and interpret the data they spit out every second, write a routine for the computer that displays the data and optionally does calculations like adding Ah figures together. Probably too much hassle though
You would be better off with an independent meter for each battery bank (which is how I handled my setup) - gives you more and better information.
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