Any reason I can't use 2 TM2020s on 1 shunt?

I would like to have 1 monitor close to equipment ( Charge controller, inverter and batteries) and the other in the house about 125 feet away.

Thanks for any help

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    I'm not absolutely certain on this, but I don't think so. The reason being that a shunt is a "known resistance" which carries "99%" of the load, allowing the meter to carry and read the other "1%" so to speak. If you add another meter in parallel, neither would give accurate readings, and the wiring lengths being different it wouldn't be a matter of "divide by two".

    But I could be wrong. It happens. A lot. :p
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    As Marc says--there is a lot of variations in equipment... But the "typical" current meter reading a shunt is a voltage meter... So, we are talking about very little current flow (should be around 1-10 MegOhm resistance on the "volt" meters, 0.0001 ohms for a 500 amp 50 mV shunt).

    For the Meter Function itself--there will be current draw (probably on the voltage sense lines) to run the device (I would guess). So, long wire has higher resistance, so the voltage function can be less accurate.

    Because of this, you should be able to parallel two or more battery meters on a single shunt without issues (the meters will not read identical values--but within a few percent of each other).

    The bigger issue is extending the wires 125 feet. I am not sure if you would pickup electrical noise or not and make the readings less accurate.

    The Xantrex Link Pro claims 30 meter / 100 feet for the shunt leads on 24 awg wire using twisted pair. It also has separate wires for DC meter power--will help keep the voltage drop errors to a minimum.

    Reading through the specs... I would give it a shot. Use twisted pair for everything. If you want, try CAT 5 network cabling. Just cut off 150' from a spool, wrap it in a figure eight (reduces electrical noise) and connect the meter to see if it functions accurately before you bury the cable.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    Likely, YES, it's OK to try. The meters measure the Voltage across the shunt. Volt meters are really high impedance. If it appears that you are under reading, perhaps there is a calib adjustment in the software, to get it back on track.
    At the end of the day, is there much difference if you
    used 200A and replaced 197A
    vs
    used 220A and replaced 217A
    the consumed and replaced absolute numbers may be off a bit, but the tracking will be the same
    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 ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    now this is interesting and i'd be curious as to the real life results. i'm sure he won't parallel 2 meters without knowing for sure and i believe that each meter should be on its own shunt in series as the impedance, even though high, will be cut in half with 2 meters being parallel and will affect the readings, but the question would be to what extent. maybe talk to the manufacturer of the meter and see what they say about it and please let us know what is said.
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    known resistance" which carries "99%" of the load, allowing the meter to carry and read the other "1%" so to speak That is not correct the meter would not even be carrying .00001 % If the load was 200a thee is no way the meter would be carying 2 amps...
    I can see no real technical reason that even 10 milllivolt meters acrosss the shunt no matter how long the different lengths of wires would make any diffference.. any small readind differences would be errors betewwe the different meters only
    I have a 2 meter setup almost as described one milivolt meter at the load shunt near the batteries and the other wone near my refrigerator and water pump about 20m (66ft) away.. and no problems.
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    Shunt resistance is what you buy. Either a 1 milliohm (100 amp) or 0.1 milliohm (500 amp). Shunts are made from strips of metal alloy that keeps its resistance over temp. All shunts are manufactured low in resistance and then ground on one side of the straps to calibrate it to exact specified resistance. You can see the grinding if you look for it.

    If it was any different then you would have to have a shunt specific to every manufacturer's monitor.

    Yes, it will work putting two monitors on one shunt.

    Monitors usually have a calibration adjustment, not for variation in shunt but for voltage measurement accuracy of the monitor.

    If you run a long run just make sure you use twisted pair to avoid interference pickup. Cat 5 or Cat 6 cable should be fine.
  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    RC...
    I wish my Trace TM500 meter had a voltage adjustment! You can set other parameters, but my actual voltage reads .6vdc higher than the battery actual voltage. The reading will even reach .8 higher if the generator has been running a long time (Nov, Dec) with lots of current passing through the shunt (I suppose).

    It would be nice to have something like a Vbatt calibration function available on my MX60, but for the T500 meter.

    Lack of accuracy makes me wonder if the amp and amphour readings are accurate or not. I have a top notch hydrometer which is my true reading instrument...the meter readings are just a rough indicator of my system status.

    Ralph
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    If you want accurate voltage readings just buy a very good multimeter.. like a FLUKE,,
    I have a top notch hydrometer which is my true reading instrument.. they dont give always the corect reading no matter how expensive.. many variables like temperature,, the quality of the battery acid where it was designed to be used..
  • GreenerPower
    GreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    Should work. Inside those battery monitors, the voltage across the shunt is amplified to a good range for an A/D converter. This low-noise opamp would take micro-amps from the shunt. This is similar to hooking 2 DVMs on the same shunt.
    They just need to calibrate to see the same voltage (or current on the reading).
    GP
    edit to add: at 125-ft, it's a stretch, using twisted pair cat5 type might work. It's more of noise issue than voltage drop (noise floor not to affect mV reading)
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?
    john p wrote: »
    If you want accurate voltage readings just buy a very good multimeter.. like a FLUKE,,
    I have a top notch hydrometer which is my true reading instrument.. they dont give always the corect reading no matter how expensive.. many variables like temperature,, the quality of the battery acid where it was designed to be used..

    I smell a Wumpus! (or something)

    I've got to disagree on several items.

    Fluke meters are as prone to drift, and need calibration, as any other meter. They might be good out of the box, but after that, all bets are off.

    Temperature Compensated Hydrometers are THE most accurate state of charge indicator. Or you can use a chart with a plain hydrometer .

    "Quality of the battery acid" Well, I suppose if you have contaminated it, the quality goes down. There is not really any easy way to tell, or correct, a bad fill from the major mfg's. If you get batteries from Bill's Battery Box down the street, maybe he had one too many the night before and borked up a batch of batteries.

    I'm trying to figure out how to make a homemade calib standard for my meter.
    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 ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    This has been quite interesting.

    I explained the principal behind the operation of shunt & meter and point out the distinct possibility of inaccurate readings if two meters are paralleled, especially with one on very long wires.

    Some have questioned this in a manner that implies the meters are inaccurate anyway, in which case why use them? And we have heard from people who say this isn't so, because some meters can be re-calibrated to compensate. How well that will work against parallel meters is yet to be seen.

    Some have said hydrometers are inaccurate, so why use them? You will see in the battery FAQS that with new batteries you should take a "base" reading of SG and write it down as all subsequent readings actually relate to that, and not some magic permanent number chart carved in stone somewhere.

    In the end, no matter what method you use to keep track of your batteries' health your system is relative to itself. Like any timepiece is only accurate when compared to another.

    It should be interesting to see what results the OP gets should he decided to go ahead with this experiment. But I think the calibration adjustments will only affect the baseline and not result in parallel accuracy over the range of readings.
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    It not important if the Fluke multimeter drifts, it still be very accurate for your purpose,(.05%)thats about 60mv), and its a one time reading you need., But what my suggestion is go buy a few digital volt meters connect them up to a short cable next to the battery (all together) ok now get a good multimeter and calibrate them so they all read the same. then conect them at various places you want to monitor.. then any variation be caused by the length of wires but it very unlikely to be much as they are very high impedance and draw microamps.
    As for specific gravity readings,, hydrometers no matter how fancy are not the "holy grail".. You not believe?/ go to 2 or three battery shops then test at random a few batteries from different manufacturesr and see the different readings... then you will believe..
    If you just go check with different suppliers you will find you can buy containers of battery acid with different specific gravities....
    Hydrometers certainly do have their uses but the readings are not absolute. only relative.. And they do show up very quickly a bad cell in a given battery.

    this is not a bad suggestion and worth doing..You will see in the battery FAQS that with new batteries you should take a "base" reading of SG and write it down as all subsequent readings actually relate to that, and not some magic permanent number chart carved in stone somewhere.
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Any reason I can't use 2 TM2020s on 1 shunt?

    The TM2020's has a calibration adjustment.

    I called Bogart Engineering and got answers to input impedance and rms averaging.

    The voltage reading sense has an input impedance of 150K ohms.

    The current reading sense has a very high input impedance, >1 meg ohm.

    As to the question of what would be the difference in reading for a true sinewave inverter versus a modified sinewave inverter, they are using R-C averaging. The cutoff freq on the R-C filtering is less then 1.5 Hz. Their argument is for battery current you want average current reading, not rms current reading.

    Their first stage of filtering, directly on the current shunt inputs, are 650 ohms on each wire leg followed by a shunt 100 uF cap. This goes into a very high Z, low D.C. offset drift op amp. I am not crazy about the two 50 uF electrolytic caps on the input filter because of their potential leakage increase over time, which against the two 650 ohm series resistors would change the calibration.

    I guess their opinion on battery current averaging has some validity against battery A-H capacity tracking but you will not get a true battery power output if you multiply their battery voltage reading by their current reading. There will be a difference in MSW versus true sinewave inverter in this power error.

    The average current reading from the TM2020 will be 10% low for rms current on a sinewave inverter and 22% low for rms current on a MSW inverter.

    Battery voltage reading is fine (after calibration) because there is relatively small A.C. ripple riding on battery D.C. voltage.

    Anyway, putting two monitors on one shunt is fine.