Shunts

westbranchwestbranch Solar Expert Posts: 5,180 ✭✭✭✭
found this statement on Wikipedia...

In order not to disrupt the circuit, the resistance of the shunt is normally very small. Shunts are rated by maximum current and voltage drop at that current, for example, a 500 A/50 mV shunt would have a maximum allowable current of 500 amps and at that current the voltage drop would be 50 millivolts. By convention, most shunts are designed to drop 50 mV when operating at their full rated current and most "ammeters" are actually designed as voltmeters that reach full-scale deflection at 50 mV.

Does this mean that one should try to use a shunt that is closely matched to the current available or a max ( eg 500 amps). Would using a hi rated shunt give erronious readings when a low current is passed through it?

Eric
 
KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,987 admin
    Re: Shunts

    You don't want to exceed the shunt's current capability... 500 amps max (any voltage) gives 50 mVolt drop:

    Pdiss=V*I=500a*0.05v=25 watts of heat (500amp shunt at full current)...
    Rshunt=R=V/I=0.05v/500a= 0.0001 ohms (1 x 10^-4 ohms)

    Problem is that it is pretty difficult to measure small voltages without error--50 mV @ 500a full scale is hard enough--but if the current is only 5 amps max, then full scale would be 0.5 mVolts--not very easy to measure accurately. The shunt is still accurate--but the meter reading it would not be very accurate...

    But, it also depends on the application... a 50mV drop in a 12 volt circuit is 0.05/12=0.4% voltage drop.

    If you were measuring current for a 48 VDC bus, you could have a shunt calibrated to 200 mVolts (4x the voltage) and be just fine (other than it would waste more heat--and the shunt would have to be larger to dissipate more heat--not usually an advantage if the 50mV meter is accurate enough).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,573 ✭✭✭✭✭
    Re: Shunts

    The idea is to use a "standard" shunt, that is rated for about 2X your normal current. Then you avoid trying to measure 4mV out of a 500A shunt. Pay good money for a shunt, make sure it is thermally stable, and use silver conductive paste on it's connectors ( to minimize losses ) (used in large buildings on transformer connections, hard to find)
    You can use a nice digital meter with it, avoid squinting at a needle.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • westbranchwestbranch Solar Expert Posts: 5,180 ✭✭✭✭
    Re: Shunts

    thanks Bill, where I was going is that there is a shunt needed for eg a TriMetric, and I do not see where the current for the system is a consideration in buying one. This , coupled with an Ebay search for shunts which generates a long list, got me thinkning about the accuracy of the 'device'. Guess it is all relative, but at 12 v a .1 v difference is significant...

    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.BB. Super Moderators, Administrators Posts: 29,987 admin
    Re: Shunts

    Well, what is significant is at 12 volts and 500 amps is that the shunt generates 50 watts of wasted heat, vs 25 watts for a 50 mV shunt (out of about 6,000 watts total)...

    So, in real life, the extra 50 mV probably does not make a huge difference on the shunt--and you could always use a 2x larger shunt at 100 mV and just simply use it at 1x current and 50 mV max... If the price is right (the 100 mV shunt would probably be bigger and, normally more costly, because of the extra heat that would need to be dissipated).

    -Bill

    PS: I should add the extra 50 mV drop probably would not affect the load--however, if you had a shunt in the charging circuit--the extra 50 mV would terminate your charging early (by at least 5% capacity)... Hence, why good meters have separate sense leads from the charge controller to accurately measure the voltage at the battery, and not based on the wiring and shunt losses).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 5,180 ✭✭✭✭
    Re: Shunts

    thanks Bill.

    Hmmmmm. Looking at the Bogart site and their installation drawing

    http://www.bogartengineering.com/accessories.htm

    it appears that they want the shunt to be placed in a common conductor from both the solar panel and the loads. My setup is that these are, currently, separate cables. Can be done with some more modificaitions to present wiring. I imagine adding a 'T' to the battery, off of the joining of the 'IN' from solar and 'OUT' to the fuse panel, with the shunt placed in the 'T'. Just thinking about loads while the CC is in operation, my lights, etc, would be getting up to 14.3 v and the battery the residual. Right?

    Next Question is: NAWS shows 2 shunts 500 amps - 50 mv and 100 a - 100 mv. I do not presently need the larger one but it appears to be a better choice as it will allow for future expansion...or should I go with the smaller one for the present as it 'wastes' less energy, in a small system, since it will be on 24/7.

    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.BB. Super Moderators, Administrators Posts: 29,987 admin
    Re: Shunts

    Bascially, you place the shunt in the "wire" you want to measure the current in...

    You can measure current from your panels, to your loads, and in/out of your battery. They all have their uses.

    A reason for placing a common shunt on a battery is that you can connect it with a "smart battery monitor"... With a battery monitor, you now have the ability to accurately measure the "state of battery charge" directly. Batteries are pretty much electron storage devices. Charging, you move the electrons one direction (to 100% capacity) and discharging you move them the other (towards 0% capacity).

    The advantage is that a battery is fairly close to 100% efficient if you are counting electrons (amps*time). So you can have a pretty simple totalizing meter (amps*time=Amp*Hours) and simply add up all of the charging current and subtract all of the discharging current = what is left/available in the battery.

    Check out Wind-Sun's battery monitor page or other vendors for various brands.

    The problem with measuring voltage to determine charge state, is it is dependent on temperature, battery "Mode" (charging, no-current, and discharging). So, just reading a voltage does not tell you much with out knowing several other variables (temperature and current flow).

    The other "accurate" way to measure state of charge is with a hydrometer--but that has its own issues (temperature, mixing of electrolyte, contamination, loss of fluid over time, sealed batteries, etc.).

    Placing the shunt in-line with the battery (and, ideally, connecting it to a battery monitor), and the other end connecting loads, charging sources, etc. would be best for monitoring the state/health of the battery. You can place other shunts in load circuits and charging circuits if you want to monitor those more closely.

    Obviously, keeping the voltage drop low is the best (reducing losses). However, you also have accuracy to worry about. You want to worry about the meter reading the shunt... Look at the sensitivity of the meter measuring the current.

    Just making a number up... Say the meter reads 0-100mV in 1mV increments... So, from the spec., we cannot feed it more than 100mV or it will "peg" the meter. Accuracy wise, a 500 amp 100mV shunt would read 5 amps as its minimum current. And a 50mV shunt would read 10 amps as its minimum current. So, you can see in this case a 100mV shunt would have twice the resolution of a 50mV shunt in this system.

    And, if you start with a small system--you have the ability to only measure 0, 10, 20.. amps or 0, 5, 10, 20... amps. A too large of shunt would not allow your battery monitor to accurately totalize the storage capacity of the battery--you could have a 9-19 amp load and never see it (for example).

    A 100 amp/100mV shunt would allow your system to accurately read down to 1 amp (in my example above) ... Much more accurate (5-10x more accurate) for a "smaller" system.

    So, calculate the maximum current you want to measure (so that full current reads ~50-100mV--depending on meter and shunt) and read the requirements of the meter you are going to connect to the shunt.

    So, I would recommend not to over size the shunt for future expansion (unless it is going to happen shortly).

    And for a small system, the amount of power wasted by either 500 amp shunt is probably pretty trivial (assuming you are not anywhere near 500 amps)... I would tend to "error" more towards accuracy (100mV shunt, lower current rating)--in most cases.

    -Bill

    PS: I should add that it is "easy" to measure small voltages (such as a few amps going through a 500 amp shunt), but the errors become larger and larger relative to the "signal" (dirty contacts, alternative current sources like dirty/moist insulators, galvanic corrosion, thermocouple effect, radio noise, etc.).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Robin GudgelRobin Gudgel Registered Users, Solar Expert Posts: 58 ✭✭✭✭
    Re: Shunts

    The 500 amp/50mV shunt has been the Re industry standard for as long as there has been an RE industry. I designed DC breaker boxes at Trace/Xantrex, OutBack and MidNite. They all use the 500 amp shunt. OutBack and MidNite include them as standard on most products. There is no big problem going over 500 amps on the shunt either, you just lose accuracy, but then how often are you over 500 amps? Not a big deal.
    You also should not be too worried about the connections as battery status monitors use a 4 wire system. The sense wires have such small amounts of current running through them, that it is not an issue. You do not use the high current connections for the sense wires. Keep the wires as short as practical, 20 feet or les and you will still have an accurate system. I believe the Trimetric can use a 100 amp shunt, but the mounting is not standard. Mounting holes are provided on the MidNite MNDC boxes. The shunts are installed on all E-Panels as well as on the OutBack DC boxes. Can't remember about the old Trace/Xantrex DC250. I'm not sure what we did there, but if mounting holes are provided, they would have been for the 500 amp shunt.
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