Charge Controller Questions

GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
I have a few n00b questions about charge controllers.

I know they stop you from overfilling your batteries.
I hear they also stop you from discharging your batteries at night.

Q1: Where's the V cutoff? It's been overcast for the last WEEK in
otherwise sunny CA. I want to know am I getting a little power
or no power?

Q2: They have Amp Ratings on these charge controllers. I have a
120W @ 12V solar panel otherwise known as 10A. My Charge controller says
it's a 10A charge controller. However I know that my solar panel at peak is
producing WELL-over 12V and probably more than 120W. ALSO, what is that
rating referring to? Gauge of wire? why would it be limited to 10A and is there a
fuse in there that could be blown when the sun instantly doubles in intensity?
(hypothetical ;) ) Is it just a relay in there? The paper that comes with the
charge controller is very unsatisfying.

I have been using Sunforce.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,641 admin
    Re: Charge Controller Questions

    Gloves,

    I will take a shot at trying to answer your questions. First, here are a couple quick FAQ's for solar charge controllers:

    All About Charge Controllers
    Read this page about power tracking controllers
    Gloves wrote: »
    I have a few n00b questions about charge controllers.

    I know they stop you from overfilling your batteries.
    I hear they also stop you from discharging your batteries at night.
    More or less correct on the first point--Depending on what you are asking about--preventing solar panels from discharging battery bank vs an option on some controllers for the second (but not usually very useful).

    The solar charge controller typically is inserted between the solar panels and the battery bank. It is the job of the Charge Controller to allow maximum current flow (bulk charge) until the battery reaches the "absorb" voltage (~14.4 volts or so). Then it starts reducing current (typically by PWM--Pulse Width Modulation--Turning a transistor on and off hundreds of times a second. The transistor is on nearly 100% of the time for maximum current flow, and near 0% of the time for minimum current flow).

    The 14.4 volts is used to "quick charge" the battery bank and mix the electrolyte (some gassing). However, it is not good to keep charging a battery at 14.4 volts for days/weeks/months at a time. It will tend to overcharge the battery and cause it to fail sooner than if it is kept at ~13.6 volts for periods between use.

    Depending on the controller and the amount of "smarts" and hardware it has--It may time how long the battery remains at 14.4 volts (and/or monitor the amount of current flow). In several hours at 14.4 volts, the controller may decide to enter float mode (~13.6 volt) to maintain 100% state of charge and not "boil" the battery (overcharging).

    There are a couple of things that match your "prevent batteries from discharging at night).

    The first is that solar panels will allow a bit of reverse current flow from the battery bank if they are in darkness. The charge controller is usually designed to stop reverse current flow. But, it is not needed for 12 volt battery banks and definitely needed for 48+ volt battery banks. If no controller is used, a simple blocking diode may be used instead (i.e., small/cheap setups).

    The second is called LVD--Low Voltage Disconnect. This is a terminal on some charge controller where you attach your loads. Typically limited to 8-10 amps or so, the charge controller will turn off the external loads at 10.5 volts (some controllers are programmable to turn off at 11.5 volts too). It is not a great way to protect your battery bank from discharging below 20% state of charge (and run the risk of killing your battery bank right then and there with 0% or reverse charging 1 or more battery cells).

    The LVD output is usually too small of current rating to run AC inverters (may damage charge controller). And many devices have their own LVD shutdowns (like AC inverters).

    I have never understood if the LVD was intended to protect the battery bank (10.5 volts is a dead battery and probably damaged too). Or, if it is intended to protect the loads from running on too low of battery voltage.

    In either case--LVD is usually not very useful.
    Q1: Where's the V cutoff? It's been overcast for the last WEEK in
    otherwise sunny CA. I want to know am I getting a little power
    or no power?
    If you are asking about solar panels and battery charging... A typical "12 volt" solar PV panel is designed for Vmp~17.7 volts and Voc~22.1 (Vmax-power and Vopen-circuit voltages).

    For the most part, solar panels will reach >Vmp when they are exposed to week sunlight or under heavy clouds (middle of day). However, they will not have very much (if any) current output... As the sun light becomes "stronger"--they will begin to output more current up to their rated current (Imp at Vmp) on a cool day near noontime with the panel facing the sun (within ~10 degrees).

    Something to understand is that solar panels are "current sources" as opposed to voltage sources (i.e., the storage battery we all know and love). For the most part, the output current from a solar panel (at Vmp and below) is proportional to the amount of sunlight striking the face of the panel.

    If a panel is rated at 7.6 amps at 17.7 volts and full sun (1,000 watts/sqmeter), it will output around 8.4 amps at 0.0 volts (into a dead short).

    Also note that Vmp and Voc is proportional to the temperature of the solar panels--Hot panels depress Vmp/Voc. And panels in full sun and very hot days may run 80% of Vmp (17.7 volts * 0.8 ~ 14.16 volts). So--we do want good air circulation and low voltage drop wiring+controllers to charge the battery bank (and why we have 17.7 volt panels instead of 14.4 volt panels).

    So--long way around, what you are looking for is Vmp > Vbatt-charging+voltage-drops (wiring and controller).
    Q2: They have Amp Ratings on these charge controllers. I have a
    120W @ 12V solar panel otherwise known as 10A. My Charge controller says
    it's a 10A charge controller. However I know that my solar panel at peak is
    producing WELL-over 12V and probably more than 120W. ALSO, what is that
    rating referring to? Gauge of wire? why would it be limited to 10A and is there a
    fuse in there that could be blown when the sun instantly doubles in intensity?
    (hypothetical ;) ) Is it just a relay in there? The paper that comes with the
    charge controller is very unsatisfying.
    For a simply PWM controller--it does not care what the Vmp of the array is... Just that it is over 14.4 volts + 1 volt controller drop. What the controller passes to charge the battery bank is the current from the panel (simply an on/off switch). Remember that solar panels are current mode devices and can connect to a dead short without any issue. (You would not want to connect a 12 volt battery directly to a 6 volt battery very long--the 12 volt battery will output a whole bunch of current to the 6 volt battery--and eventually overheat).

    Of course there are controller limits too... You could connect a "24 volt" panel (~35 volt Vmp) to a PWM controller and it will work OK (only at 50% efficiency of a 17.7 volt Vmp panel) as the controller is only passing current. However, transistors and switches have maximum working voltage ratings--so there is an upper limit on Voc of the solar panel.

    Now, the 10 amp rating of the controller. If it was designed to follow NEC type codes... the controller should have a 1.25 safety margin:
    • 10 amp * 1.25 safety factor = 12.5 amp actual never exceed rating
    And, the NEC actually requires another 1.25 safety factor for charge controllers (account for extra strong sunlight?). So, from an NEC point of view (as I recall--I am not a code expert here), they would want a maximum of:
    • 10 amps / 1.25 safety factor = 8 amps of solar panels
    Did your vendor take this into account? I don't know.

    One reason they are so careful about the amount maximum current in NEC/safety standards is to remember that
    • Power=Current^2 / R
    So, if you double the current, the heating effect (too much current in the wiring or too much current in the charge controller), the amount of power released is 4x... So code is very careful to keep current under control (by specifications, fuses, breakers, design, etc.).

    Some more complication--A PWM controller really does not have any control over the amount of current flow through it... The Transistor can be thought of just like a simple light switch or relay (older controllers probably had a relay). The switch turns on--and current flows (based on solar panels and battery/loads). There is probably no fuse inside the controller. If you have too much solar panel current (Imp/Isc) for the charge controller--it may be damaged.

    However, there are the MPPT type solar charge controllers. These are more expensive and typically used for higher power rated systems. One of the "neat things" about them is they are usually "buck mode" switching power supplies. Besides the ability to do MPPT (Maximum Power Point Tracking), the controller can also limit its maximum output current to its design limits... For MPPT controllers you can connect way more solar panel energy than a similar PWM controller and not have any problems as the MPPT controller will simply output no more than XX amps (maximum current rating).

    There are some more details (fuses/breakers for panels/battery connections) than the simple explanations/requirements I gave above--but the function is as I describe.

    Please feel free to ask more questions--I am trying to not be too confusing.:roll:;)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: Charge Controller Questions
    Gloves wrote: »
    I have a few n00b questions about charge controllers.

    I know they stop you from overfilling your batteries.
    I hear they also stop you from discharging your batteries at night.

    Q1: Where's the V cutoff? It's been overcast for the last WEEK in
    otherwise sunny CA. I want to know am I getting a little power
    or no power?

    Not totally sure what you mean exactly, but given the context of the question I'm wondering if you mean at what point the controller "wakes up" and starts charging?
    If so, it does not take a lot of light to produce sufficient voltage and current to start charging, but the amount of power that will be produced will be low... as will your total energy harvest. The best way to get a better handle on all of this stuff is probably to invest in a good battery monitor like the trimetric or some other unit that will show you the charging and discharging amps, and voltage. If nothing else the unit will probably more than pay for itself in having a better handle on how much total energy you are generating and using; and learning how the system as a whole behaves. A very cheap source of "education" in my book.

    Your charge controller sounds like a pretty simple unit, meaning it will simply supply a 14V(looks like this is it's charging voltage?) charge at whatever current is coming from your panel. So if the VMP your "12V" nominal panel was in the 17.5V range, and the IMP was 10amps, under ideal conditions your charge controller could put 10amps @14V (or 140watts, instead of 175watts).

    As for the second question I think Bill made that abundantly clear.

    If you have more numbers then you could certainly get more specific accurate answers to your questions. I couldn't see much info at all on your Charger controller at amazon, what kind of solar panel do you have?
  • BB.BB. Super Moderators, Administrators Posts: 32,641 admin
    Re: Charge Controller Questions
    hillbilly wrote: »
    As for the second question I think Bill made that abundantly clear.

    Was that a good thing or not? :confused::roll:;)

    -Bill :D
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,582 ✭✭✭✭✭
    Re: Charge Controller Questions
    Gloves wrote: »
    Q1: Where's the V cutoff? It's been overcast for the last WEEK in
    otherwise sunny CA. I want to know am I getting a little power
    or no power?


    Overcast woes. My 3,000w array, on a cloudy day, produces about 300W Time to wire the genset up for backup.
    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 ,

  • GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
    Re: Charge Controller Questions

    Ok, I want to restate what you both had just said
    In a way that is simple enough for me to understand.

    - In overcast situations, my solar panel and charge controller
    Will continue to charge my batteries, however at a significantly
    Lower rate.
    (I am interested in your recommendations on a device that
    Would monitor; power collected, power stored, power used)

    - I have a 130W 12V PV and a 10A rated charge controller. I'm
    Tempted to just set it up. However I don't want my house burnt
    down.

    (I am interested in your recommendations on a charge controller
    For a 6-PV @ 120W each setup. Although I might just go grid-tied
    With that many PVs.)

    My wind/Solar setup is linked in my signature. Thanks for the help.

    :typed on BlackBerry
  • GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
    Re: Charge Controller Questions
    mike90045 wrote: »
    Overcast woes. My 3,000w array, on a cloudy day, produces about 300W Time to wire the genset up for backup.

    That's an excellent testimonial. You have found it to collect ~10% of full sun.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Charge Controller Questions
    Gloves wrote: »
    That's an excellent testimonial. You have found it to collect ~10% of full sun.

    not so as he said overcast and that is not full sun.
  • BB.BB. Super Moderators, Administrators Posts: 32,641 admin
    Re: Charge Controller Questions
    Gloves wrote: »
    (I am interested in your recommendations on a device that Would monitor; power collected, power stored, power used)

    For a full-up monitoring system--A Battery Monitor (the Trimetric as a low cost unit has good reviews here). Also, Victron Energy has a couple Battery Monitors that have a couple positive comments here too.

    For a small system that you want to measure Amp*Hours / Watt*Hours--these units appear to be nice at $60 each. These only count one direction (they do not add/subtract amp*hours based on charging/discharging the battery bank). But they appear to be very nice when setup to monitor/experiment/qualify equipment.
    - I have a 130W 12V PV and a 10A rated charge controller. I'm
    Tempted to just set it up. However I don't want my house burnt
    down.
    A 130 watt panel may have Vmp=17.7 volts or so. Vmp:
    • Imp=Pmp/Vmp=130 watts / 17.7 volts = 7.3 amps
    So Imp and Isc are well under the Imax of the controller (it should be "safe" to run the controller at 10 amps--and its 1.25 safety factor should allow 12.5 amps--may pop an internal fuse/breaker--if it has one--is operated for hours at that current level). But, of course, we do not know the details of rating/testing performed by that company.

    So, unless you are doing something I missed (like 130 watt "6 volt" panel charging a "6 volt battrey"--Unlikely)--Your setup, with proper fuses on the battery bank should be relatively safe (assuming the vendor did their part too).
    (I am interested in your recommendations on a charge controller
    For a 6-PV @ 120W each setup. Although I might just go grid-tied
    With that many PVs.)
    The answer for Off-Grid vs Grid-Tied is just too different--Each deserves its own set of discussion and answers...

    For me, the question is what are you aiming at?
    • Portable or fixed system?
    • Emergency Power / Saving Money / "going green"
    • How much power / loads do you expect to operate (by season)
    • If Grid Tied--Does your utility allow grid tied systems?
    • If Off-Grid (and even On-Grid to a large degree)--Have you done every bit of conservation / power reduction you can possibly do before planning out your solar system?
    • With all solar systems--Location matters. No shading at least from 9am-3pm. Lots of sunny weather (and my system, I have had several days of 1% of rated output during dark winter storms). If you do not have a good location for Solar Array (in forest, neighbor's home, landscaping, etc. blocking array), solar PV will never be an answer.
    Based on the above answers, then we can address what are options given:
    • Off Grid Power--Most expensive, most maintenance ($1-$2+ per kWH)
    • Hybrid (GT+OG)--Advantages of Grid Tied (using utility as your "battery"). Efficient, perhaps a bit less maintenance costs vs off-grid. Battery+OG inverter for emergency power (long term off grid very possible+generator support). Call it $0.45-$0.75 per kWH
    • Grid Tied--Cheapest, most reliable, least maintenance. Power Costs can approach (or even beat) utility power costs--Roughly $0.30 to $0.15 or less per kWH.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: Charge Controller Questions
    Gloves wrote: »
    Ok, I want to restate what you both had just said
    In a way that is simple enough for me to understand.

    - In overcast situations, my solar panel and charge controller
    Will continue to charge my batteries, however at a significantly
    Lower rate.
    (I am interested in your recommendations on a device that
    Would monitor; power collected, power stored, power used)

    - I have a 130W 12V PV and a 10A rated charge controller. I'm
    Tempted to just set it up. However I don't want my house burnt
    down.

    (I am interested in your recommendations on a charge controller
    For a 6-PV @ 120W each setup. Although I might just go grid-tied
    With that many PVs.)

    My wind/Solar setup is linked in my signature. Thanks for the help.

    :typed on BlackBerry


    Ok, in order here:

    Yes your charge controller may still "charge" your batteries some on in certain overcast or partly overcast conditions, but probably not very much. That's the simple answer, the full answer is a tad more complex (hence the recommendation for the battery monitor so you can learn to better understand all this yourself): depending on just how much power your PV is generating, how big your battery bank is (2 T-105's I think?) and how full the batteries are; you may or may not receive any real charging (so little as to be essentially irrelevant to your batteries). The best way to know this is to measure the AH in and AH out.

    Your panel specs list a VMP of 17.3V and a IMP of 7A, so this shouldn't be a problem with your charge controller. The reality is though that I don't know that you'll be getting much charging out of that for your battery bank (a little on good sunny days, but not a lot overall). Safe is clearly a bit of a relative term, and I'm not going to be the one to say "GO" if you're not sure about it yourself. In terms of the charge controller and panel, I'm not as concerned; in general I'd worry a LOT more about the batteries side of things. From your pics it looks like the proverbial "bird's nest" in the battery box; not trying to be critical, and my own foray's into our off grid PV system were far from perfect, but I'd look to tidy that up a bit personally.

    As pointed out, off grid vs grid tied are very different issues, but I have to say that if you already have grid power onsite it seems hard to justify doing anymore than the smallest "off grid" system. This comes from someone who really LOVES being independent from the grid; I've found great satisfaction in designing our own personal "energy department", and being impervious to all the power outages we have up in the mountains. I still wouldn't personally try to go off grid in a grid supplied urban setting, except perhaps on a small scale experiment for fun/education (just my two pennies).

    IF you really do decide to keep with the off grid ideal, then I would say don't spend another penny on any more stuff just yet (ok, except for the battery monitor which would probably be a good buy asap). Do a lot more research and figure out exactly what sorts of loads you'll need to power for how long, tally up how much solar energy is available at your site, how you'll mount the panels, house the batteries and B.O.S. (CC, inverter, monitors, etc), calculate wire routing and conductor sizing etc. Every last detail can really hurt you (hopefully only financially) if you realize later on that you need to change something. Batteries are best purchased new and put into service all at the same time, so you may well end up buying more/bigger batteries upfront to allow for some growth in the future (generally our wants and needs tend to go up not down). Having a large battery bank means a lot of money invested, and kind of a "big hungry mouth to feed" in order to not damage or shorten it's lifespan. Etc, etc, etc. In the end the "sticker shock" may be discouraging, but if you do want to go for it at least you'll have a better idea just how deep that hole can get upfront.

    Good luck.
  • GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
    Re: Charge Controller Questions

    Been lots of help guys! I hope to have this panel installed this weekend. (weather permitting)
  • GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
    Re: Charge Controller Questions

    1 is the loneliest number, so I got the solar panel a friend.

    It's installed! I spent part of the day Sunday setting up panel #2.

    Here is some messy Henry i slathered all over the bottom side of the 2x4
    before drilling into my once, leak-less roof.
    2nd-panel-install1.jpg

    Wow, that's awesome... I'm already dreaming up ways to maximize add more power. I must hold back... this is putting a strain on my wallet!
    2nd-panel-install2.jpg

    Now, let me check the forecast...
    you-goota-be-kidding.jpg
    :cry:
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Charge Controller Questions

    i'm not so sure using 2x4s was a good idea. seeing as how you did go with them i would suggest waterproofing them. just coating them with roof coating could crack and leak it inside to the 2x4s so use something like thompsons water seal and reapply it every year or 2.
  • BB.BB. Super Moderators, Administrators Posts: 32,641 admin
    Re: Charge Controller Questions

    You need paint or stain/sealer--Thompson's Water Seal does not have any ability to prevent UV damage (as far as I know). Possibly a mold killer too (Copper Green or whatever is legal these days)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GlovesGloves Solar Expert Posts: 65 ✭✭✭✭
    Re: Charge Controller Questions

    Neal where were your ideas when I was planning this?

    So anything that protects deck wood will protect up there for a few years right?
    I might just go back around those 2x4s with some of the silicon, i did buy some.
    Some guy said that this Henry stuff seals everything.

    Next up, how do you guys handle the wiring? What are the best practices?
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