Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

jagecjagec Solar Expert Posts: 157 ✭✭
Like many owners of a grid-tied system, I am slightly bothered by the idea of someday finding myself in the middle of a lengthy blackout with kilowatts of panels on the roof that I can't use. Now, the power in the city doesn't go out often enough to justify spending thousands of dollars on a "what-if" (and giving up some efficiency in the process) just to keep a huge battery bank on float service 99% of the time. But after a minor basement flood, I decided that a couple of critical loads did warrant a small backup system, and I bought a single 100 Ah deep-cycle battery to power a backup sump pump and a couple of DC lights. The battery sits on an AC charger, and all is well.

I have been thinking about whether it would be possible to keep that battery charged with the panels in the (unlikely) event of a long-term power outage. I was considering simply reconnecting the array wiring in such a scenario so as to parallel two of the panels, leaving the others disconnected and dropping the "array" Voc to 37 or so, which is low enough to use a small PWM charge controller. Inefficient, and even 37 volts is too high for some controllers, but workable in a pinch.

However, recently I've been looking at the Vicor DC-DC converters that are being sold on ebay. I picked up a 24V 200W "batmod" unit, which takes an input voltage from 200-400VDC, and allows me to control the output voltage AND current. It's designed to be a basic float charger already, and can be used as the heart of an advanced multistage charger with the proper control circuitry. I was thinking of connecting it to the panels in parallel with the grid-tied inverter, setting the appropriate voltage and current limits, and connecting it upstream of a traditional PWM charger, which would then connect to the existing battery.

The nice thing about this emergency backup system is that I can ignore many of the rules of off-grid systems...since my array is so oversized compared to the battery, it would still provide a full charge even on cloudy days (so I wouldn't need multiple days of backup), and since the battery will be sitting on float the majority of the time, I don't have to be as careful about staying below 50% DOD if I do face a long-term outage.

Is there any reason that this wouldn't work or wouldn't be safe?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    The only possible problem I can see is the Vicor "load" interfering with the MPPT function of the GT inverter. This (if present) could be solved by judicious monitoring of the battery with "manual" charging as needed (either by shifting the panel connection or using a standard charger) and switching to "full automatic" only if the grid is down.
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    I like this idea. Low cost, no hassle way of tapping into the array to provide battery charging for a small off-grid inverter to power critical loads.
    Can wire into the H.V. DC right at the GT inverter (Maybe use a manual switch on the output of the DC-DC converter to engage it only during outages).
    Then use a low cost PWM charge controller to feed the battery. Could have a $1000 answer for the "you mean - I'm not going to have power during an outage from my $$$$ solar" objection. Would not even be that inefficient either.
    Exactly which model of Vicor are you looking at? I see that Digikey sells them as well for a reasonable ~$150.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    seems to me that it may work, but the fact that the batteries will need to be maintained even when there's no outages simply makes this a hybrid system and will therefor have a lower sell back of power to the utility and therefor a lower overall efficiency. now if you could maintain the batteries without the connection to the main hv pv system then it won't degrade the gt production and efficiency during normal operating times. a small pv setup with a pwm cc could keep them in float charge with a current output to the battery in the 1%-2% range of the battery capacity total in ah. now 200w is not high power and would deliver roughly about 8a. for a 100ah battery bank this is about an 8% charge rate and leaving the pv setup for float charging connected will add another 1a to 2a during an outage making it charge at 9%-10% total which is good. the big question would be is this enough for you to operate much during a longer term outage?

    do note that if you opt for 4 standard 6v 220a batteries in series for a higher battery bank capacity on this little backups that the charge % seen by the combined v converter and float charge pv system could be under the 5% we normally recommend for charging even with designing the float charge pv system with the higher float currents needed by the 220ah bank.
  • jagecjagec Solar Expert Posts: 157 ✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system
    niel wrote: »
    seems to me that it may work, but the fact that the batteries will need to be maintained even when there's no outages simply makes this a hybrid system and will therefor have a lower sell back of power to the utility and therefor a lower overall efficiency. now if you could maintain the batteries without the connection to the main hv pv system then it won't degrade the gt production and efficiency during normal operating times. a small pv setup with a pwm cc could keep them in float charge with a current output to the battery in the 1%-2% range of the battery capacity total in ah. now 200w is not high power and would deliver roughly about 8a. for a 100ah battery bank this is about an 8% charge rate and leaving the pv setup for float charging connected will add another 1a to 2a during an outage making it charge at 9%-10% total which is good. the big question would be is this enough for you to operate much during a longer term outage?

    do note that if you opt for 4 standard 6v 220a batteries in series for a higher battery bank capacity on this little backups that the charge % seen by the combined v converter and float charge pv system could be under the 5% we normally recommend for charging even with designing the float charge pv system with the higher float currents needed by the 220ah bank.

    I don't think that there is any way of getting around the slightly lower overall efficiency, since some power will be needed to float the bank no matter what. BUT since the bank is small and the GTI is more efficient than most of the grid-aware inverter/chargers, this system should still be more efficient, and much cheaper, than a true hybrid.

    Having a separate PV panel/array just to float the batteries seems like it wouldn't be worth it given that grid power is available for floating most of the time. If the Vicor interfered with the MPPT circuitry of the GTI (not sure if it would), an AC float charger would probably be the solution, leaving the Vicor charging circuit disconnected until an outage occurred.

    I have 2 Vicor modules, which can be paralleled for 400w. I'm currently leaning away from buying a separate charge controller, since these modules don't really need much circuitry in order to function as proper 3-stage chargers. By simply setting a current (bulk) and voltage (absorb), and lowering the voltage setpoint to float as soon as current output dropped to 1-2% of battery capacity, they would be as good as any (non-temp compensated) PWM charge controller. Here is what they can do:
    Attachment not found.
    So, I should be able to get a charging current of 14.5 amps, which I might have to turn down slightly for the 105 Ah battery bank (isn't C/8 an upper limit for FLA?). Unless the weather is VERY bad indeed, the 5kW array should be able to push that current all day long without even breaking a sweat. That's a solid 2 kWh a day, almost no matter what! The battery bank capacity isn't much higher than that at ~2.5 kWh, but since these batteries are only getting used in emergencies I'm willing to flirt with >50% DOD.

    More than enough to run a few lights and a refrigerator, or even to start the blower for the gas furnace. Now I just need to figure out how big the inverter needs to be in order to start those inductive loads. Could I get away with a 600W (TSW) inverter if I added a start capacitor to the fridge?

    Here is my current plan for all of you to critique:

    Attachment not found.

    solarix, I bought them both at ebay for an astonishingly reasonable $26 each. But for any voltage other than 24V, digikey would probably be the place to go.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    all i can say is try it. at worst you'd need to buy at least 30w in pv at 24v (nominal) to maintain it, right?
  • jagecjagec Solar Expert Posts: 157 ✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    I spent some time designing a circuit to control my Batmod modules, and the parts finally came in on Friday. Over the next couple of weeks I hope to etch my circuit board, solder everything together, and start my tests.

    These Batmod units are very easy to design around. Just by using some resistors, a few trimpots, and a voltage comparator+MOSFET, I've got a charging circuit which will do a constant-current bulk, constant-voltage absorb, and then switch to a lower float voltage once charge current drops below a setpoint. All values can be adjusted by the trimpots, and I've got a small LCD display that I will use for setup+monitoring. Once the batteries get drained enough for the charge current to climb back above the switchover point, they'll get booted back into Bulk, then Absorb. Voltages are sloppily temperature compensated using an external "linearized" thermistor. The circuit certainly won't be as smart as the full-blown retail units, but it should be smart enough for my purposes.

    I'm tempted to toy with a grid-sourced DC source, since it would be very nice to use the same system to keep the batteries charged from the grid OR the panels. I might cannibalize a computer PSU as a high-voltage DC power source.

    My next hurdle is finding an inexpensive switch that can handle high voltage DC. I could just buy a DC disconnect intended for GTI use, but I wonder whether a cheap Chinese knife switch like this one would work:
    Attachment not found.
  • john pjohn p Solar Expert Posts: 814 ✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    DO NOT USE A KNIFE SWITCH FOR HIGH VOLTAGE AND OR HIGH CURRENT DC.
    VERY DANGEROUS.
    I f you are going to switch high voltage DC buy a real switch designed and rated for your needs. But be warned they are a bit EXPENSIVE.
  • BB.BB. Super Moderators, Administrators Posts: 28,186 admin
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    The knife switches in the upgraded GT Inverters were spring loaded... About 1/2-2/3 of the travel was loading a spring--Then the final point released the blades for a quick disengagement.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • john pjohn p Solar Expert Posts: 814 ✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    Then they have been designed for DC But cheap electrical store knife switches like the one shown are only designed for AC
  • jagecjagec Solar Expert Posts: 157 ✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    Update: I have finished etching/drilling/soldering the circuit board, and I tested the circuit (carefully!) using low-amperage 340VDC from an ATX power supply. That way I could kill the AC power, and the DC voltage would collapse. Everything seems to be working.

    I now need to work on a proper enclosure and mounting for the charger, and once the DC disconnect gets here I can wire everything up. I don't think that this charger will interfere with the MPPT circuitry of the Sunny Boy, but I will run tests with both connected to see how things look.

    If the charger does interfere with the GTI, then I'll need to run it from the grid most of the time, and only connect it to the panels when the power goes out. That complicates things, since a standard rectifier bridge/voltage doubler circuit produces +/- 170 VDC. For a grounded battery bank, I need 0/+340, so I'd have to use an isolation transformer to float the AC side. Then I have to make sure that I don't have any exposed live metal anywhere.

    Once I have everything buttoned up I can post some pictures and more technical details.
  • 65DegN65DegN Solar Expert Posts: 106 ✭✭✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system

    If interferrence is a problem you might try a band pass filter to ground once the frequency of the interferrence is determined.
    Or you might be able to swamp the noise with an old TV CRT yoke with the windings removed and some turns from the GTI input wrapped around the torroid.
    Hams use this trick to keep their high power amps out of the phone line.
  • mike95490mike95490 Solar Expert Posts: 8,033 ✭✭✭✭
    Re: Vicor Batmod as an inexpensive alternative to a full-blown hybrid system
    jagec wrote: »
    Update: I have finished etching/drilling/soldering the circuit board, and I tested the circuit (carefully!) using low-amperage 340VDC from an ATX power supply. That way I could kill the AC power, and the DC voltage would collapse. Everything seems to be working.........


    What's the outcome ?
    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 ,

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