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Thread: Best Vmp for use with SolarBoost 2000E

  1. #1

    Default Best Vmp for use with SolarBoost 2000E

    Hi all,

    I am new to solar and am about to begin installation of a system on my small vintage 1976 Boler trailer. I have purchased two AE-80HE (high efficiency) panels from AEE Solar and a BlueSky SolarBoost 2000E MPPT charge controller. This will be used to maintain a 12V/240 amp/hour battery bank consisting of two 6-volt golfcart batteries in series.

    I purchased the panels based on a combination of their smaller physical size and the fact that AEE advertises they can produce a Vmp of 17.6 volts.

    This Vmp of 17.6 seems important to me as the BlueSky 2000E literature states:

    ["The effect PV panel design has on performance is that panels with a maximum power voltage (Vmp) of 17 volts or greater will tend to produce more boost, whereas PV panels with Vmp less than 17 volts will tend to produce less boost"

    "To obtain good charge performance a rule of thumb is to not attach a PV module of less than 16.5V to charge a 12V battery. PV modules with higher Vmp in the range of 17.5V-18.5V will provide improved MPPT and battery charge performance."


    The specs for the panel were listed as follows in AEE literature:

    Imp 4.55
    Vmp 17.6
    Isc 4.89
    Voc 21.4

    However: the specs listed on the back of the actual panels are

    Imp 4.8
    Vmp 16.7
    Isc 5.43
    Voc 21.1

    I have asked AEE head office in the USA and they are looking into the discrepancy. (It may be a typo error or I may have received the wrong panel design)


    I have also received the following from an AEE rep here in Canada

    "This being said, he (kevinhenrycalgary) is incorrect about more power being delivered through MPPT with a higher Vmp.

    It is the amps that charge the battery, so higher current will yield higher power delivered to the batteries.

    In fact, with MPPT, the only thing that matters is the power output of the module.

    So, the new specs yield 80.16 watts, while the old specs yield 80.08 watts.



    My question is this:

    What is the proper train of thought in regards to Vmp and MPPT in conjunction with the SolarBoost 2000E controller?

    My system is small and my power consumption tends to be minimal so good MPPT performance may never be an issue. However, for optimal performance, should I hold out for the panels with the higher Vmp?

    Thank you for your help

    K. Johnston
    Calgary, Alberta, Canada

  2. #2
    Join Date
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    Quetico, Ontario
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    Default Re: Best Vmp for use with SolarBoost 2000E

    My hunch is, and Niel and Bill will correct me, but I don't think it is going to make much difference with the system you describe. Since the Vmp of both panels is the same, the controller will handle that voltage and convert it based on the battery voltage. I think that BlueSky is suggesting that you would get maximum MPPT advantage with a bit higher Vmp but the marginal difference in your small system would almost be too small to measure.

    Tony

    PS. The panel specs you list convert to almost identical wattage output. The one has higher voltage/lower amperage, the other the reverse. In the net/net, the higher the voltage the more "headroom" the controller has to use to convert MPPT efficiency.
    Last edited by icarus; January 31st, 2009 at 8:07 PST.
    Please note, being a moderator does not add any weight to my opinions 300 watts Siemens/BP panels,plus a Sun 90,, making ~400. ~30 amps into Rogue MPT-3024, 450 ah of Trojan T-105, Morningstar ts300 inverter, a Tri-Metric meter.a collection of antique generators, plus 2 Honda eu-1000i's (also a BS2512 IX controller) and assorted other stuff!

  3. #3

    Default Re: Best Vmp for use with SolarBoost 2000E

    Quote Originally Posted by kevinhenrycalgary View Post
    "The effect PV panel design has on performance is that panels with a maximum power voltage (Vmp) of 17 volts or greater will tend to produce more boost, whereas PV panels with Vmp less than 17 volts will tend to produce less boost"

    "This being said, he (kevinhenrycalgary) is incorrect about more power being delivered through MPPT with a higher Vmp.
    Actually both of those statements are true.

    If your panel voltage is over 17 volts or so, you will get a bigger boost from that panel as opposed to say a 16.5 volt panel.

    However, that does NOT mean you will get any more power to your batteries if both panels are rated at the same wattage.

    What it means is that you lose more if you do NOT have an MPPT controller with the high voltage panel.
    Northern Arizona Wind & Sun Forum & Website Administrator

  4. #4
    Join Date
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    Default Re: Best Vmp for use with SolarBoost 2000E

    Tony is correct. And your AEE contact is correct--but possibly wrong too...

    The Solar Charge converter will simply maximize the solar panel output following the formula Pmax=Vmp*Imp and the battery output will be Pbatt=Vbatt*Ibatt (less some 5-10% or so power loss)--so absolute voltage and current do matter.

    However, Solar panel voltage must be in a range the charge controller can use... Obviously, too high, the controller is ruined. Too low, the charge controller cannot pass solar panel energy to the battery.

    Basically, the solar panel voltage must be greater than Vbatt-charging+controller drop.

    Vbatt-charging would be ~14.5 volts during normal charging, and controller voltage drop is around 1-2 volts (plus, you will have some voltage drop in the wiring too).

    Also, for flooded cell lead acid batteries, you will need to charge them at a higher voltage every few weeks or so--called "equalization"... That voltage can be around 15-15.5 volts.

    So, just from a conservative point of few, the solar charge controller (which can only drop voltage, not increase it):

    Vmp >= 15.5v + 2v = 17.5 volts to equalize
    Vmp >= 14.5v + 2v = 16.5 volts for normal charging

    And then you get into the ugly details.

    Batteries require higher voltage to charge if they are cold, and lower voltages if they are warm (~77F is "spec. temp.").

    And, solar panel Vmp (and Voc--open circuit) vary with temperature. When the panels are cold, they output higher votlages, and when the panels are hot, they output lower voltage.

    So, if you have cold batteries, on a hot, sunny, windless day, your Vmp of the panels may not be able to output enough voltage to poroperly charge/equalize your batteries.

    If the Vmp is correct for your panels, then the Vmp=16.7volts is getting very close to putting you in the gray zone where you have the "energy" of the panel, but not enough voltage to push current into the batteries. Remember solar panels get hot under full sun (black object under glass). And even on a 80F day, it is possible for your 16.7v panel to drop down to ~14 volts Vmp if mounted flat to the roof (poor cooling) on a windless day.

    So, your questions to ask:

    • What is the Vdrop requirement of the BlueSky controller
    • What is Vmp for your panel
    • What is the Voltage Temp Coeff - Vtoc (something like -0.0824 V/C)
    • What are the Vbatt-charge voltages for your battery bank
    • What are the temperatures of your batteries
    • What are the temperature of your solar installation
    • Will there be airflow around the panels (i.e., tilted up / raised up from roof for better ventilation)

    -Bill

    By the way, Vmp is a "fuzzy voltage"--the panel will output higher voltage, but at less current--so the system may still charge--it just won't do it with Imp*Vmp--V will be a bit higher, and Imp will be much lower--so P=V*I into the battery bank will be lower too.

  5. #5

    Default Re: Best Vmp for use with SolarBoost 2000E

    Thanks for the replies guys. You are fast.

    I didn't think it would make much difference in the big scheme of things as my battery will probably be 90% full most of the time.

    Last years power consumption averaged out to about 10% of battery capacity per day with conservative usage. I would go 3 days and then crank up the generator and recharge via a Xantrex Truecharge 20 amp charger. I have since changed all interior lighting from 1156 bulbs to LED's. so consumption may drop depending on usage this year.

    I should not be lacking for power with the combination of the modules and the charger/generator.

    As I need something to do this weekend I may begin the installation before hearing back from AEE.

    On another note, what is the proper procedure for wiring the PV system in with my existing Xantrex charger and Xantrex XBM battery monitor?


    Would it be charge controller positive to battery post positive with an inline fuse at the battery and charge controller negative to the charge side of the XBM's shunt or can they even be hooked together?

    Thanks again
    K. Johnston
    Calgary, Alberta

  6. #6

    Default Re: Best Vmp for use with SolarBoost 2000E

    Thanks Bill,

    You've caused me to change my mind again. I was originally thinking along those lines from the information I had gathered with my limited grasp of the subject.

    I will now wait to hear from AEE and will "rough-in" the wiring this weekend.

    AEE has a warehouse here in town so it may just be a matter of exchanging the panels if the others are in stock.

    K. Johnston
    Calgary,Alberta

  7. #7

    Default Re: Best Vmp for use with SolarBoost 2000E

    Kevin,
    One thing to note is that the whole "boost" from your Charge Controller tends to be inversely related to how full your batteries are. In other words, if they are up around 90% full (SOC), you wont see much if any boost at all. Where the real boost comes from is the controllers ability to convert "excess" voltage into higher current output in amps, in other words when the batteries are lower and the panels are producing a higher voltage (keeping them cooler will help in this regard).
    As far as wiring with your monitor, that shouldn't be a problem... I'd recommend getting the wiring diagram from Xantrex though (I haven't used that monitor, but with my Trimetric, it's just complicated enough that you really want to have a visual aid in order to not confuse where each wire goes).
    Good Luck

  8. #8
    Join Date
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    Default Re: Best Vmp for use with SolarBoost 2000E

    You are welcome "K".

    Regarding wiring everything together... Generally, think of the battery as the "center" of the system, and everything else is "hung" off of it.

    So the "+" point and the "-" common connection points should have a striaght, large wire (or copper bus bar), short and well protected against metal to metal contacts (short circuit possibilities) to the battery as possible. If there is voltage drop anywhere, it costs you power and makes it more difficult to fully and quickly charge the battery.

    You may have a large fuse from the battery to the "+" bus bar/common connection point (for safety). And, in any case, you should have 1 fuse or circuit breaker for each branch that goes off of your common "+" point. This is to protect your wiring from melting if there is a short anywhere else in the positive run from the fuse to the AC charger, Solar Charger, interior lights, fan, radio, etc...

    There should be no fuses in the negative path.

    The battery can supply amazing amounts of current and you need the fuses/breakers to prevent cooking the "smaller" wires that go throughout your trailer.

    Typically, the best wiring for your chargers is to use the recommended wiring practices to the common "+" and "-" connections for the battery.

    I would not recommend, for example, using the same wiring from the AC Charger and the Solar Charger to the battery--the higher current from both chargers running will increase the voltage drop along the wiring--raising the "apparent" battery voltage (Vbatt+Vwire-drop) and fool the controllers into thinking the battery is charged higher than it really is.

    Also, if your controllers support it, get a remote battery temperature sensor. This can get you another 5-10% more charge in your battery...

    It may not sound like much, but typically (for example). You charge your batter to 90% (controller error), discharge another 10% per day in normal usage, and in 1.5 days you are at 75% state of charge--if you discharge below this point for more than a few hours (usually less than one day), the sulfates will begin to harden in the flooded cell lead acid battery and cause its early life failure (or you have to run the generator every day or so to bring the battery back up).

    If you charge the battery to 100%, then you now have 2.5 days of useful power before you need to run the genset (for bad weather, parked under trees).

    Undercharging is probably the #1 killing of batteries (#2 is probably overcharging). We all go through a set or two of batteries before we get the hang of it.

    Read through our host's website/store (NAWS)... They have posted many little FAQs around in the major sections (batteries, inverters, charge controllers, etc.) that do a very good job of answering many of these questions in detail.

    -Bill

    And to add to what "hillbilly" said... Your charge controller will only output Pmax when the battery is ~less than 90-80% full). Once the battery is over 80-90% full, the controller cuts back on the charging current. Another reason that having an accurate battery temperature sensor helps ensure a quick charge and good use of your $$$ you paid for your $olar panel$ and batterie$.
    Last edited by BB.; January 31st, 2009 at 9:47 PST.

  9. #9

    Default Re: Best Vmp for use with SolarBoost 2000E

    Hi Bill,

    My present AC charge system is set up as follows. Xantrex TrueCharge 20 with temperature probe installed and equalization capabilities. Monitoring is performed with a Xantrex XBM Monitor. (Presently the batteries/charger are in my basement and I plug in every week to keep them charged and equalize when required.)

    The trailer wiring is configured as follows:

    I have 6 gauge stranded wire running from AC charger positive to battery positive with inline fuse close to battery.

    I have 6 gauge stranded running from AC charger negative to the charge side of the XBM shunt.

    I have 1/0 stranded welding cable running from the battery side of shunt to the battery negative post.

    My plan is to run 10 gauge stranded wire from each PV panel through a common entrance on top of the trailer. This run should be less than 5 feet.

    The two positives will then be combined in a power distribution block, inside the trailer, and a short (12 inch)10 gauge lead will be fed to positive "PV in" on the controller.

    The negatives leads will be attached on a separate distribution block in a similar fashion.

    The output cables from the controller will be short 10 gauge leads (12 inches) connected to a 15 foot run of 4 gauge stranded welding cable.

    One 4 gauge cable goes to battery positive with inline fuse at the battery and one 4 gauge cable to the charge side of the shunt.

    I opted for the larger 4 gauge as I wanted the option to remove the panels from the trailer and place them in the sun if I am parked in the shade. This may never happen but, the option will be there. (If I do find the need to remove the panels I will fashion the proper sized cables for the total length of the run)

    I also did order the temperature probe for the SolarBoost 2000E and it will be attached to battery negative post.

    If I understand correctly, would this setup conform to your statement?:

    I would not recommend, for example, using the same wiring from the AC Charger and the Solar Charger to the battery--the higher current from both chargers running will increase the voltage drop along the wiring--raising the "apparent" battery voltage (Vbatt+Vwire-drop) and fool the controllers into thinking the battery is charged higher than it really is.

    Thanks again

    Kevin
    Calgary, Alberta

  10. #10

    Default Re: Best Vmp for use with SolarBoost 2000E

    Here is a youtube link to my Boler trailer project.

    http://ca.youtube.com/watch?v=NH9HITYmtSY


    Note that some of the battery wiring has since been upsized as I switched from a 10 amp AC charger to a 20 amp charger.

    Thanks again for the help.

    Kevin
    Calgary, Alberta

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