Adding a volt meter with a switch, basic question

Just got a small LED volt meter, looks like they are designed for cars but can handle 4-30v and I want to use it to display the volts from my battery.

It has two wires pos and neg, attached it to my battery terminals to test it, it seems to work fine.

Don't want to have it powered up all the time so want to add a switch to it.

is it a good idea having something like this attached to the battery all the time?

As well as a switch I guess a fuse may be also a good idea?

May get another one to attach to the solar panel, could I just put that across the terminals on my charge controller for the solar input? or again is that not a good idea?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,626 admin
    Say your meter was this one:

    http://www.amazon.com/niceeshop-Digital-Display-Voltage-Voltmeter/dp/B00825NB24
    TYPE : LED;
    Power range : DC 4.2 V - 30 V ;
    Current consumption : 12mA ;
    Accuracy : 0.1V;

    Say you have a 200 AH battery bank and it has a 0.05% self discharge rate (average to low average for a flooded cell lead acid battery):
    • 200 AH * 0.0005 self discharge = 0.1 amps self discharge
    • 12 mA = 0.012 amps for meter power (may be less current as input voltage rises)
    The meter (according to specifications) would draw almost 1/10th the self discharge of a 200 AH flooded cell battery--So, I would not put a switch on it unless you wanted to (perhaps turn it off in winter if lock the cabin up). If the battery is small (i.e., 20 AH), then you probably want a switch to prevent discharging the battery just to run the meter display.

    Yes, you should put a fuse near the + battery/solar bus (1/2 - 1 amp or so, depending on wire gauge) just to protect against a short circuit in the wiring.

    On the solar panel--You might want to try the first meter you got... The charge controller will pulse on/off the array (i.e., jump around between 14.x volts battery charging and ~21 volts Voc)--See if this meter gives you "useful" readings in the afternoon as the battery transition into absorb and float. Its readings may jump around a bunch at that point.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    I have a several of those going by your description. One has been connected and illuminated continuously for about two years. Another has a push button momentary "on" contact switch. Yet another has an on-off switch, but only because it is in a travel trailer and the brightness of the display was a nightime distraction. Another in a truck, only comes on when the ignition is powered on. All running normally, no noticable discharge.


    I never bothered with a fuse in any of the voltmeter circuits. That may be technically wrong. But the skinny wires that come connected to the meters probably make good fuses when the balance of the wiring is 16 gauge and many times larger. :)
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    I never bothered with a fuse in any of the voltmeter circuits. That may be technically wrong. But the skinny wires that come connected to the meters probably make good fuses when the balance of the wiring is 16 gauge and many times larger. :)

    Some folks are experimenting with single cell battery monitoring (using a battery bank of 2 volt batteries). Their battery boxes are a rats nest of unfused skinny wires. If one of these wires melts, it might constitute a source of ignition inside a hydrogen filled battery box... How safe is that?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • raymateraymate Registered Users Posts: 15
    Thank you all for the replies most helpful, I will add a switch mainly as they are bright, and they are indeed the ones i got. Going to mount them into a bit of plywood, any tips on mounting them so I don't mess it up, Does anyone make a face plate that may take them I actually have 3 of them in different colours along with one that just shows temperature going to have the probe near the battery. I want to have them all lined up in a row on my panel where I have my fuse box and controller etc.
  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    I made rectangular openings in 5 mm plywood for some mountings. Another in a plastic wall plate, starting with a blank. My method was to drill large holes with a forstner bit in a drill press. Then I used a dremel tool with a cut off wheel to get a rough rectangular opening and finished with a file.


    Re: the fusing.... My mention of skinny wires being fuses was partly tongue in cheek. It's just that I have not seen other instrumentation being fused. Things like the trimetric. Outback FNDC. ???
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    I made rectangular openings in 5 mm plywood for some mountings. Another in a plastic wall plate, starting with a blank. My method was to drill large holes with a forstner bit in a drill press. Then I used a dremel tool with a cut off wheel to get a rough rectangular opening and finished with a file.


    Re: the fusing.... My mention of skinny wires being fuses was partly tongue in cheek. It's just that I have not seen other instrumentation being fused. Things like the trimetric. Outback FNDC. ???

    To some extent the small wires themselves act as fuses, limiting the current from the battery to far less than a hard short would produce. Just make sure there is nothing flammable around the wires.
    SMA SB 3000, old BP panels.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: the fusing.... My mention of skinny wires being fuses was partly tongue in cheek. It's just that I have not seen other instrumentation being fused. Things like the trimetric. Outback FNDC. ???

    The positive power wire to the trimetric is most certainly fused. The sense wires from the shunt do not need to be fused because both sides of the shunt are at ground potential.

    By the way, a current shunt would work just as well on the positive terminal of a battery as on the negative. The reason we put shunts on the negative side is so that we do not have to protect the wires with a fuse.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    vtmaps wrote: »

    The positive power wire to the trimetric is most certainly fused. The sense wires from the shunt do not need to be fused because both sides of the shunt are at ground potential.

    By the way, a current shunt would work just as well on the positive terminal of a battery as on the negative. The reason we put shunts on the negative side is so that we do not have to protect the wires with a fuse.

    --vtMaps
    And because, although it is technically feasible, the circuit in many CCs would not work properly if the two sense wires were near battery voltage rather than near ground.
    If they are just inputs to an op amp, then it should be OK either way.

    SMA SB 3000, old BP panels.
  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    OK, I'm wrong. The trometric wiring diagram does show a fused + lead. I don't own one and my memory is faulty. :) So I have some inline tubular fuse holders someplace in a box in the shop. It would be a good idea to add one in the meter + line. ..... great thing about discussions here is the learning experience. Thanks
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • JohannJohann Solar Expert Posts: 244 ✭✭✭
    I wonder.......
    A switch always has some ohms across the contacts of a switch and it does not matter if the switch is closed or open.
    In a case like that, would the volt meter read a different voltage if a switch would be used instead of just wiring the meter straight to the power source.
  • BB.BB. Super Moderators, Administrators Posts: 30,626 admin
    For a 12 mAmp load current (mostly used to light the LEDs), it will be close enough (I the one I linked too--The volt meter is not adjustable and a few people complained that it was not very accurate).

    You can treat this just like a standard cable voltage drop problem...

    http://www.calculator.net/voltage-drop-calculator.html

    100 feet (one way run) of 22 AWG wiring and 0.012 amps:

    Voltage drop: 0.039
    Voltage drop percentage: 0.33%
    Voltage at the end: 11.961

    Hardly affect the digit to the right of the decimal point.

    Of course, if you want a more accurate reading--Then use a LCD meter or one that draws its operating power different from the measurement leads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    I also have one of those (same model running 24/7). Yes it is bright, kinda of works as a night light! The accuracy of mine out of the box is about 0.1V off. One thing, the first one ran for 2 years unitl a particularly bad lightnening strike, whereupon it died. I suggest (given the hassle of maching the panel hole for it), you buy a couple to keep one as a spare. Or put a zener/varister/MOV across it. Fusing: a half amp PTC is, what, a few cents. Easy.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Johann wrote: »
    I wonder.......
    A switch always has some ohms across the contacts of a switch and it does not matter if the switch is closed or open.
    In a case like that, would the volt meter read a different voltage if a switch would be used instead of just wiring the meter straight to the power source.

    The ohmic resistance of the switch contacts will be exceedingly small compared to the ohmic resistance of the voltmeter. You might be able to measure a small number of millivolts across the switch, but more likely microvolts.
    The voltmeter will definitely read lower when the switch is open. :blush:
    SMA SB 3000, old BP panels.
  • raymateraymate Registered Users Posts: 15
    Added in the little volt meters, I put one on the solar panel feed, battery and load out from charge controller. Can someone explain why they all say the same when I'm using the system. I was expecting different values. I have each section with a breaker so if If disconnect the solar panel for example I then see I guess the panel voltage with out any load. If I flip the switch to make the panel live then the voltage becomes the same as the rest of the system. I assume thats correct

    Was thinking the load output from my CC would just give 12v it does not, it all seems to work fine but it can be 11,12,13 and 14v

    The little meters are cool and give me a good idea of what is going on, but like I say not understanding why they all say the same. But if I isolate them I get different readings.
  • BB.BB. Super Moderators, Administrators Posts: 30,626 admin
    If you have a PWM type solar charge controller (and not a MPPT type)--The Vpanel input, Vbatt output, and charge controller will be about the same (within a 1/10th of volt or so) of each other if:
    • All is working well (battery in the range of 10.5 to 14.8 volts or so)
    • There is light on the solar array.
    • Charge controller thinks the battery bank needs charging (PWM controller "turns on" in bulk mode 100% of the time).
    Voltage:
    • 11 volts on the battery bank is pretty low--Generally, we suggest that you avoid going below ~11.5 volts under load
    • 12 volts on the battery bank with no load is ~50% state of charge
    • 13 volts is the battery is not being discharged (~12.7 to 13.6 volts is float charging)
    • 14 volts is the battery starting to get a good charge on it
    • 14.5 to 14.8 volts is what the charge controller will hold at the battery for ~2-4 hours or so to fully charge the battery bank. Vpanel should slowly rise from 14.5 to ~21 volts as the charging current is reduced.
    • Vpanel should be pretty close to zero volts at night.
    • Load Control should be ~Vbatt voltage unless Load Control turns off (battery near dead, over current from LC terminals, etc.).
    At this point, I would worry that you are not getting enough charging current to your battery bank--The battery bank is probably not getting fully charged... And at 11 volts either you have heavy loads (for the size of the battery bank) or are significantly discharging below 50% state of charge.

    If flooded cell batteries, I would get a good hydrometer. Also, assuming the charge controller and everything else is working OK, I would turn off your loads and/or get an extra battery charger on the battery bank and get it fully charged (check specific gravity of all cells, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 30,626 admin
    What is the sunlight/current charging current/loads in this pic?

    I want to make sure your batteries are getting properly charged.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • raymateraymate Registered Users Posts: 15
    BB. wrote: »
    What is the sunlight/current charging current/loads in this pic?

    I want to make sure your batteries are getting properly charged.

    -Bill

    In that picture it was shaded (and cloudy today), charge current was not registering on ProStar (too low), no load (apart from a few MAh for the LED temp display unit.

    Just did it again, with sun (moved around, but cloudy right now)

    With a load of 1.2 Amps, Charge current 0.4 Amps (this load was running for about 15 mins, LED lights)

    On the displays I see

    Panel 12.4 v
    Battery 12.5 v
    Load 12.4 v


    update, battery now seems charged and is gone in a optimization mode from the CC, no load at this time. and now

    On the displays I see

    Panel 15.3 v
    Battery 14.1 v
    Load 14.1 v


    Another update as the sun went down I watched the solar panel V and it did indeed went down and down and then was not providing enough power to keep the display running and just cut off, so that does seem to work fine.
Sign In or Register to comment.