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Topic: Anyone wire a generator starter through a shunt?  (Read 829 times) previous topic - next topic
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Anyone wire a generator starter through a shunt?
Hi All.

 I'm going to move a Trimetric battery  monitor from a Class B that we will most likely sell soon after recently purchasing a  2003 23.5 TK. The generator in the Class B has its starter ground wired directly to the frame of the van so the battery monitor never factored in the drain from the battery when the generator was started. So I only wired the ground for the converter through the shunt.  I was never too concerned about battery percentage charge accuracy being impacted by starting the generator since I didn't use the generator all that much.
 
So the Lazy Daze has a thick #2 cable supplying ground to the generator starter and the second smaller gauge cable feeds the converter. I was going to just route the converter ground through the shunt like I did in the Class B. Is it recommended to run the starter ground cable through the shunt as well?
Steve
2003TK
Re: Anyone wire a generator starter through a shunt?
Reply #1 –
The shunt should be the only connection from the 'lower battery' negative to ground, excepting the monitor wiring. As a result, ALL battery current passes through the shunt, and thus will register on the monitor.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #2 –
Quote from: Steve  - 
The shunt should be the only connection from the 'lower battery' negative to ground, excepting the monitor wiring. As a result, ALL battery current passes through the shunt, and thus will register on the monitor.

That's how all the shunts I have installed or seen have been installed .
Shunts are rated in the amount of amperage they can safely handle, the battery capacity meters come with a 500-amp shunt, more than adequate for our uses.

Larry
Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
Photo Collection: Lazy Daze
Re: Anyone wire a generator starter through a shunt?
Reply #3 –
Just a thought: if it were me, I'd leave the Trimetric meter in the rig I was getting rid of, and install a Victron BMV-712 in the Lazy Daze.

It's a better battery monitor in a number of ways, not the least of which is that it has wireless capability that lets you can take a comprehensive look at your system--WAY better than on the Trimetric's three-digit LED display--using your phone, tablet, or computer. Likewise, you can easily change any setting from your phone, etc., without having to work your way through a bunch of menus. This works so well that I don't even bother to install the BMV-712 monitor where I can see it--I do everything from my iPhone.

I've talked about this at length elsewhere, but in a nutshell: there's just no competition. Let your old rig's buyer keep the Trimetric, which is certainly better than nothing, and treat your new Lazy Daze to a modern battery monitor from Victron. :-)
Andy Baird
2021 Ford Ranger towing 2019 Airstream 19CB
Previously: 1985 LD Twin/King "Gertie"; 2003 LD Midbath "Skylark"
Re: Anyone wire a generator starter through a shunt?
Reply #4 –
Makes sense, I'll attach the generator starter at the shunt as well. I'll need to replace the 100 amp shunt with a 500 amp shunt. Originally I liked the lower value shunt so I could get higher resolution for small loads that ran in the Class B.

My plan is to locate the shunt in the battery box right on the battery. I saw a DIY project where a fellow pounded flat a couple inch section of copper water pipe and put a bend in it so one end connected to the battery terminal and the other end connected to the shunt that sat on the top of the battery giving a good structural foundation.

That Victron monitor looks pretty nice.
Steve
2003TK
Re: Anyone wire a generator starter through a shunt?
Reply #5 –
Steve-

A few pictures of a shunt installed in the battery box of a 1997 MB are here. You may be able to install the same way in your TK.

Mark H.
Former owner, 31-foot gas Class A
Former owner, 1997-8 mid-bath
Re: Anyone wire a generator starter through a shunt?
Reply #6 –
Quote from: 73gitane  - 
Makes sense, I'll attach the generator starter at the shunt as well. I'll need to replace the 100 amp shunt with a 500 amp shunt. Originally I liked the lower value shunt so I could get higher resolution for small loads that ran in the Class B.

My plan is to locate the shunt in the battery box right on the battery. I saw a DIY project where a fellow pounded flat a couple inch section of copper water pipe and put a bend in it so one end connected to the battery terminal and the other end connected to the shunt that sat on the top of the battery giving a good structural foundation.

That Victron monitor looks pretty nice.
If you have a single cable from the negative post of your coach batteries to the chassis, any and all batteries loads will return through that cable.  Just put the shunt in the middle of that negative battery cable and you should be good.  Any current drawn out of the batteries to start the generator has to return via the battery ground connection.

Art

Art and Barbara
Settled in Atterdag Village of Solvang
2015-2022 fulltime in a 2016 Tiffin Allegro Bus 37AP
2002-2015 2002 LD MB
Art's blog
Re: Anyone wire a generator starter through a shunt?
Reply #7 –
Quote from: 73gitane  - 
Makes sense, I'll attach the generator starter at the shunt as well. I'll need to replace the 100 amp shunt with a 500 amp shunt. Originally I liked the lower value shunt so I could get higher resolution for small loads that ran in the Class B.

My plan is to locate the shunt in the battery box right on the battery. I saw a DIY project where a fellow pounded flat a couple inch section of copper water pipe and put a bend in it so one end connected to the battery terminal and the other end connected to the shunt that sat on the top of the battery giving a good structural foundation.

That Victron monitor looks pretty nice.

Just to be clear, ALL grounds should connect to chassis ground - the generator, converter, etc, etc. The lower side of the shunt should also go to ground, and the upper side should be the ONLY connection at the negative battery terminal. Do not think of any component ground connections as going to the shunt. They should just go to ground. I do not recommend mounting the shunt on top of the battery, nor connected to anything with a rigid cable. It is a high-precision item that should be mounted close to the battery, but protected from close contact with battery fumes, battery movement, and connected with a long enough cable to allow sliding the batteries in and out.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #8 –
Quote from: 73gitane  - 
Makes sense, I'll attach the generator starter at the shunt as well. I'll need to replace the 100 amp shunt with a 500 amp shunt. Originally I liked the lower value shunt so I could get higher resolution for small loads that ran in the Class B.

The AH to start a generator is so small, it's not worth knowing. Here's an example calculation: the starting current for the Onan 4000 is less than 50 amps, but let's use 50 amps; crank for 10 seconds, and you've used (50A x 10sec)/3600 = 0.13 AH.
2005 Jayco 24SS
Re: Anyone wire a generator starter through a shunt?
Reply #9 –
Nope, 100 amp shunt will work fine.
Harry 2006RB
Re: Anyone wire a generator starter through a shunt?
Reply #10 –
Quote from: hbn7hj  - 
Nope, 100 amp shunt will work fine.

Actually, the reason for the 500A shunt is not to carry a higher current load. As the shunt carries current, it heats up, and the metal's resistivity changes, which affects the calibration accuracy. The higher the current rating above the expected load, the less resistivity change, so better accuracy.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #11 –
Don’t need accuracy and high currents are short lived. I like the 100 amp shunt ‘cause it gives 10 ma resolution when you are dealing with LED lights and such. Each to his own.

I’m dealing with 95 amp charging rates and it works fine. I think the microwave off the inverter draws 120 amps or so. That works, too, though I prefer the generator.

The generator ground line probably goes to the chassis. Don’t think you want to go to the trouble to separate it out.
Harry 2006RB
Re: Anyone wire a generator starter through a shunt?
Reply #12 –
Hmm, good stuff to consider. I wont be running an inverter so the starter motor (50 amps) probably represents the highest current draw (albeit momentarily) through the shunt that will occur. I've read that the inrush current could be 4x the 50 amps so that also persuaded me to get the larger shunt which is on its way. I suppose another advantage of the larger shunt is that it has larger terminals on it to better accept the cables currently attached to the battery.

I agree with Eric. I never calculated what I was missing when I didn't include the generator starter with my installation in the class b I had but I assumed it wasn't much and didn't worry about it.

I'm going to end up with a large and small shunt so I'll eventually settle on one of them. I do like the idea of running just the #6 converter ground cable back through the battery box into the coach where it will connect to the smaller shunt and then in turn back through the battery box and connected to the battery. Then the shunt and the little wires that connect to the battery will not be subject to any gassing from the batteries.
Steve
2003TK
Re: Anyone wire a generator starter through a shunt?
Reply #13 –
Quote from: 73gitane  - 
I'm going to end up with a large and small shunt so I'll eventually settle on one of them. I do like the idea of running just the #6 converter ground cable back through the battery box into the coach where it will connect to the smaller shunt and then in turn back through the battery box and connected to the battery. Then the shunt and the little wires that connect to the battery will not be subject to any gassing from the batteries.

Are you saying that the only ground coming from the battery is 6 gauge? That seems unlikely. The ground cable on our battery is 2 gauge. This cable was cut, terminated at both ends, and connected to the shunt. The thin converter cable was reconnected to the grounded side of the shunt, along with any other cables that had been previously connected to the battery negative terminal. The result is a single 2-gauge cable that is the ONLY connection to the battery negative at one end, and the ONLY connection to the high side of the shunt at the other. The batteries are charged not only by the converter, but also the engine alternator, so reducing the gauge of the battery negative circuit is unwise.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #14 –
Presently there are two cables connected to the ground terminal on the battery. The #6 cable goes to the converter and the #2 cable goes to the generator starter.

I was just focused on the converter cable since I’m leaning towards only connecting that one through the shunt. I would then use the 100 amp shunt to get the resolution I like since I only run small loads. I would then leave the #2 starter cable connected directly to the battery.
Steve
2003TK
Re: Anyone wire a generator starter through a shunt?
Reply #15 –
There is no reason to expect less resolution from the 500A shunt than the 100A one. The key is what the mV/A specification may be - which should match the input requirement of the battery monitor. In addition, Any charge or discharge that bypasses the converter will not be recorded by the battery monitor, not used in the integration, and thus will nullify the primary function of the battery monitor. You will be able to monitor the current used by a few items, like your lights, the furnace, etc, but battery state of charge will be unknown. And, there is no reason to do this. You will gain nothing over correctly wiring the system.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #16 –
“There is no reason to expect less resolution from the 500A shunt than the 100A one.“

Huh? The Trimetric works that way. The 500 amp shuts reads out in 100ma increments. The 100 amp shunt reads out in 10ma increments. There is no way to change it. Other brand monitors may be different.
Harry 2006RB
Re: Anyone wire a generator starter through a shunt?
Reply #17 –
Yes, it  does come down to this... I want the 10mA resolution the 100 amp shunt provides.

The trade offs seem acceptable for my particular application. I run light loads that I don't see ever heating up the 100 amp shunt. And, I only start up the generator when I have to which is not often on my typical boondocking adventures.

I did receive the 500 amp  shunt today.  It will probably go into my parts box:^) Thought I would include a picture showing both for those that are interested.
Steve
2003TK
Re: Anyone wire a generator starter through a shunt?
Reply #18 –
I think you are missing the issue. Wired up your way, the monitor will not see the 100 A-hr charge you put back in the batteries on a 4 hr drive; it will not see the 5 A-hr you used listening to the radio in the evening; it will not see the current consumed by anything plugged into one of the dash 12V outlets; it will not see the current draw used by anything that does not have its negative wire run back to the converter. The primary function of a battery monitor is to give an accurate report of the state of charge of the batteries, which it does by using its microprocessor to integrate ALL the current flowing in and out of the batteries over time. Wired up your way, the state of charge reading on the battery monitor will be meaningless.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #19 –
Quote from: hbn7hj  - 
“There is no reason to expect less resolution from the 500A shunt than the 100A one.“
Huh? The Trimetric works that way. The 500 amp shuts reads out in 100ma increments. The 100 amp shunt reads out in 10ma increments. There is no way to change it. Other brand monitors may be different.

The Victron monitor reads to two decimal points and comes with a 500-amp shunt.

Since a shunt is just a resistor, a 100 or 500-amp shunt would electrically "look" the same to the monitor, as Steve pointed out, the difference is the 500-amp has a better heat sink.  I would be interested in knowing how the Trimetric monitor "knows" the difference between a 100 and 500-amp shunt.
If it is easier to not run the generator's ground through the shunt, it doesn't really matter much in power actually used and would have a minimal effect on the monitor's readout.

Larry
Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
Photo Collection: Lazy Daze
Re: Anyone wire a generator starter through a shunt?
Reply #20 –
“I would be interested in knowing how the Trimetric monitor "knows" the difference between a 100 and 500-amp shunt.“

You program it to know. If you don’t it doesn’t display the correct current value.
Harry 2006RB
Re: Anyone wire a generator starter through a shunt?
Reply #21 –
Note that at the core of an ammeter shunt is a high precision, very low value, resistor that drops a very small voltage as current  flows through it.  Shunts are specified as to both their maximum current capability and the millivolts registered at that rated current.   So you can get shunts rated at 100A/100mV and 500A/50mV.  If your battery monitor needs a specific shunt, make sure you have the correct one.  If your monitor is configurable, make sure you know the specifications of the shunt you plan to use.

Art
Art and Barbara
Settled in Atterdag Village of Solvang
2015-2022 fulltime in a 2016 Tiffin Allegro Bus 37AP
2002-2015 2002 LD MB
Art's blog
Re: Anyone wire a generator starter through a shunt?
Reply #22 –
I now believe the OP is operating under a misconception that the 2-gauge cable at the battery negative terminal is from the generator starter. It is not. This cable connects the battery negative to the frame of the motorhome. It is the return path for everything that is grounded to the chassis frame, including the alternator, the radio, dash 12V accessory sockets, etc. If it remains directly connected to the battery post, none of the above will register on the battery monitor INCLUDING - but not exclusive to - the generator starter. For its part, the generator starter is grounded to the generator by its case bolts and the generator frame is connected to the motorhome chassis frame by a 2-gauge cable.

Steve
2004 FL
2013 Honda Fit
Re: Anyone wire a generator starter through a shunt?
Reply #23 –
You were right Steve! Thanks for setting me straight. I was assuming the #2 was only going to the generator starter.
I was thinking about this last night and realized I wasn't taking the radio into account. It didn't dawn on me I was totally forgetting about the connection to the alternator as well. Then there's the other accessories you mentioned. Sorry for the duh moment and thanks for sticking with me on this thread.

You can program the Trimetric monitor for either the 100 amp or 500 amp but you can only get the higher resolution with the 100 amp shunt (at least with the Trimetric monitor)  Oh well... in the end the 500 amp shunt is going to let me know when the battery is fully charged or if it's getting alarmingly low. If I ever have the need to track down a small parasitic load under 100mA I can always bring out my small portable  meter that has a current clamp.

Thanks all, I really appreciate this forum.
Steve
2003TK
Re: Anyone wire a generator starter through a shunt?
Reply #24 –
Just an opinion but if you only have the two six volt batteries don’t bother with the 500 amp shunt. Run all ground current through the 100 amp shunt. It works just fine. You will appreciate the 10 ma resolution.
Harry 2006RB