Warning on the wind & sun web store that I don't understand.

MPaulHolmes
MPaulHolmes Solar Expert Posts: 35 ✭✭
in Advanced Solar Electric Technical Forum #1
I was looking on the online store, and there was this warning:
"It is ALWAYS a bad idea to run an inverter from the LOAD output, as surges can blow the controller."

I'm not sure I understand what that means. Right now I have the inverter input connected to the 48v battery pack AND inverter input connected to the output of the MPPT charger. Is that the bad idea that they are talking about?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,601 admin
    #2
    Re: Warning on the wind & sun web store that I don't understand.

    Many (especially) smaller controllers have a Vpanel+Vbatt+Vload set of terminals.

    The Vpanel goes to the solar array, and the Vbatt go to the battery terminals. The Vload terminals are (usually) connected to small loads (typically 8-20 amps maximum).

    Some controllers monitor the current of the Vload terminals (as part of their logging). Some (most?) have a programmable battery voltage to allow the controller to "turn off" the Vload terminals (i.e., turn off at 11.5 volts, turn back on at 12.7 volts for example).

    The load terminals are somewhat good for turning on/off loads that may be voltage sensitive (appliances that may stall at low voltage and burn out a motor).

    But for AC inverters... Even a 300 watt inverter can take a lot of current at 12 VDC:

    300 watts * 1/0.85 inverter efficiency * 1/10.5 inverter cutoff = 33.6 amps at full load/minimum voltage

    And an AC inverter can draw ~2x the rated power for seconds to minutes at a time.... Way over the typical 8-20 amp rating of controllers with "Vload" outputs. The Vload terminals (in some controllers) is supposed to be protected against over current--But damage may still occur.

    Also--AC inverters usually have LVD (low voltage disconnect) any way (inverters will attempt to draw more current as the DC bus voltage falls and probably brown out the AC loads)... So most AC inverters these days have some sort of Low Voltage shutdown anyway--So connecting to the controller's Vload terminals is not really useful anyway.

    Here is a MorningStar PWM controller with Vload terminals.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • boB
    boB Solar Expert Posts: 1,032 ✭✭✭✭
    #3
    Re: Warning on the wind & sun web store that I don't understand.
    MPaulHolmes wrote: »
    Right now I have the inverter input connected to the 48v battery pack AND inverter input connected to the output of the MPPT charger. Is that the bad idea that they are talking about?

    I ~think~ so. I thought, (like BB), that it had to do with some "load" output but I think you've got the idea.

    So, instead of connecting the charge controller directly to the inverter battery terminals, connect the charge controller closer to, (if not right at), the battery terminals.

    When the inverter breaker is switched on or a really large surge load turns on, the inverter will try to get its current from the charge controller rather than the
    battery if CC is wired too close to the inverter terminals... This could over load some charge controllers to the point of breaking, especially if they
    are running with a high voltage input (like your MPPT CC). Wiring the CC closer to the battery terminals insures that the inverter and its input capacitors
    get its surge current from the battery. This is also better for the 120 Hz (or 100 Hz) ripple current that a lot of inverters may pull from their battery
    terminals.

    Of course, not all CC's will have a problem here but it's still a good practice to keep the battery the central tie point.

    The chargers will also be seeing more closely the battery voltage rather than the voltage farther out from the voltage drop of the battery wires.

    boB
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,601 admin
    #4
    Re: Warning on the wind & sun web store that I don't understand.

    As boB says--You should always connect the inverters (and home run all heavy cables/loads/charging sources) back to the main battery bus. The battery bank supplies/sinks all surge currents and keeps the battery bus at a stable XX volts.

    If you put a heavy load (or even large charge controller) on the Solar Charge Controller battery connection--The extra electrical noise of those loads/sources can confuse a charge controller and cause it to behave less than optimally (lower output current, less accurate charging voltages, etc.).

    You should not "daisy chain" heavy loads/charge controller cabling (battery to charge controller A to AC inverter, etc.).

    Each cable should be home run to battery bank (charge controller A to battery bank, AC inverter to battery bank, charge controller B to battery bank, etc.). And each cable leaving the battery bank should have a fuse/breaker rated to its capacity (i.e., 14 awg wire with 15 amp fuse/breaker).

    I don't think, in normal operation, daisy chained connections between charge controllers/AC inverter will hurt anything--Unless a fuse or breaker pops--For example, if the battery breaker opens, the charge controller and AC inverter have lots of power/capacitors/inductors/heavy current that could create a several hundred volt "kick back" and damage something.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Warning on the wind & sun web store that I don't understand.

    The warning is specifically about those charge controllers with LOAD terminals; they are not designed to handle the kind of current a typical inverter can draw.

    However, connecting an inverter directly to any charge controller's output terminals is little better. Usually the current from a controller to the batteries is much lower than that which an inverter would draw. As such the wiring would be smaller.

    Even if the wire is sized properly, by tapping the output at the controller you will get an inaccurate Voltage reading at that point due to inverter draw. This means the controller may not function accurately because it needs correct battery Voltage at that point to determine charge (and that is inaccurate enough already).

    Besides these cautions there is also the issue of having the right fusing in the right place. On the whole, inverters should always be connected to their own wiring directly to the battery bank or bus bars/common point connections.
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  • Slappy
    Slappy Solar Expert Posts: 251 ✭✭✭✭✭✭
    #6
    Re: Warning on the wind & sun web store that I don't understand.

    Off topic. But if this is the same MPaulHolmes congratulations on the DIY Ev controller. If not I apologize for mentioning this.
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  • MPaulHolmes
    MPaulHolmes Solar Expert Posts: 35 ✭✭
    #7
    Re: Warning on the wind & sun web store that I don't understand.

    Thanks for all the helpful clarifications on that! Hi Slappy! Yep that's me! There was so much in common with EV controllers and MPPT controllers and inverters that I just couldn't help myself once we moved to the desert. haha. But I'm so used to really high power, that everything sort of turned out to be in the 100kW range LOL, even though I only need like 5kW. Those 600v 600amp IGBT half bridges are just so easy to work with!
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #8
    Re: Warning on the wind & sun web store that I don't understand.

    DIY EV controller? As in charge EV battery directly with solar? Details please...
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  • MPaulHolmes
    MPaulHolmes Solar Expert Posts: 35 ✭✭
    #9
    Re: Warning on the wind & sun web store that I don't understand.

    Washington?? What part? I just moved down to AZ a few months ago from Olympia. I did an open source DIY motor controller for electric cars. It takes any voltage from 0 to about 150v DC, and has a maximum motor current of about 500amp. So, people can drive 80MPH instead of that weenie 25MPH. haha. I also did a 3 phase inverter, and need to just finally finish the Field oriented control software, but life keeps getting in the way.

    I just finished a MPPT charge controller that can take any voltage in that you like up to around 350-400v, and can charge any battery bank on the output, as long as it's less than the input voltage. It wouldn't have any trouble charging a car's battery pack. You'd just have to tell it the voltage to stop at. The weak link is the inductor, which is rated for 200 amps continuous. Still, if you have a 300v battery bank, that's like 60kW. But I'd probably have to mill some channels to water cool the baseplate, or get a much more serious fan. My dang shop is around 120degF right now, so that's unhelpful too. Now I have a swamp cooler blowing on it and the inverter. It has been like 8% humidity around here, and now as soon as I got it, it's like 40%. So, it's like a glorified wet fan.
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #10
    Re: Warning on the wind & sun web store that I don't understand.

    I'm just outside of Port Angeles.

    I don't have an EV yet. I'm contemplating getting a Volt and would love to be able to directly charge with PV, avoiding the efficiency losses inherent with going through my current system to house battery bank, inverter, etc. I think Doug Coulter and some others have experimented with DIY controllers to do this. I haven't heard of anything commercially available yet. I'd think there would be a market for such a thing. IIRC the Volt's battery pack is around 400 V. Perhaps the Xantrex 600V Mppt controller could be used?? Maybe yours?
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  • MPaulHolmes
    MPaulHolmes Solar Expert Posts: 35 ✭✭
    #11
    Re: Warning on the wind & sun web store that I don't understand.

    The nice thing about DIY is, if you need a higher voltage, then get higher voltage components! haha. The driver board for the IGBTs can easily drive 1200v ones as well. They tend to produce a little more heat though, but that's OK.

    Lithium is really touchy. They'll last forever, as long as you keep them below like 4v and above 2.5v or so. But when they are finishing their charge, the voltage shoots up really really fast. So, when there are a lot of them in series, you need a way to know the highest voltage in the string. The volt probably monitors each battery, but I'm not sure. I bet communicating with the battery management system is almost impossible. I could be wrong, but car companies are notorious for making things hopelessly complicated.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #12
    Re: Warning on the wind & sun web store that I don't understand.
    MPaulHolmes wrote: »
    The volt probably monitors each battery, but I'm not sure. I bet communicating with the battery management system is almost impossible. I could be wrong, but car companies are notorious for making things hopelessly complicated.

    Getting output information over a standard diagnostic connector should be pretty straightforward. There are kits to do that for the Prius Hybrids, and Prius hackers are a dedicated bunch. But sending commands to the BMS will probably not be allowed!

    (Side note: The BMS in the Boeing 787 measured the voltage of each cell, but it did not pass that information along to the charger or any recording device. Just a bunch of go/no-go indications instead.)
    SMA SB 3000, old BP panels.
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #13
    Re: Warning on the wind & sun web store that I don't understand.
    inetdog wrote: »
    Getting output information over a standard diagnostic connector should be pretty straightforward. There are kits to do that for the Prius Hybrids, and Prius hackers are a dedicated bunch. But sending commands to the BMS will probably not be allowed!

    Right, but the standard Voltec chargers used on the Chevy Volts are pretty simple devices. Have you seen the pics of what's in them? Pretty basic stuff. I doubt they are doing any sophisticated communication with the BMS - if any at all - though I don't really know.

    I think there's lots of potential for a charge controller designed specifically to charge EV batteries directly from PV. I suppose there must be some that exist - though I haven't heard of any commercially available.
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