Help choose charge controller

whackamole
whackamole Registered Users Posts: 18 ✭
in General Solar Power Topics #1
Hi

I have 5 20 watt panels I will use to charge a tent trailer's battery Eventually I may expand to 200 watts total to support two 100ah batteries. The panels are 16.8v 1.19amps each. (but the open circuit voltage is 21v and 1.29amps I assume that I need a controller that can handle 1.29 * 5 amps?

I am considering a morningstar sunsaver SS-20L 20 amp controller to allow for growth. But I see a xantrex C35 or C12 is not much more money (~$90 vs ~$110)

So, is "3-stage charging" of the xantrex better than the "PWM charging" of the sunsaver? Any other controllers I should consider? I assume I need the Low voltage disconnect feature to keep the battery from getting.

Any advice welcomed!
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Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: Help choose charge controller

    both are pwm, but the 3 stage would be better for your batteries like the c35 has. the c35 also has the battery temperature sensor ability and the sunsaver has it built into the controller. the battery temp is what needs to be watched and not the controller's temp. the sunsavers you could view as 2 stage as they allow for bulk and absorb. the 3rd stage of the c series allows for the voltage to cut back to the float charge.
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  • SolarJohn
    SolarJohn Solar Expert Posts: 202 ✭✭
    #3
    Re: Help choose charge controller

    I recently upgraded from a Sun Saver to a TriStar with a built-in meter. In the past I've used a DVM to look at panel voltage and battery voltage, but now I'm able to monitor many more parameters. That is really helpful, and worth the extra cost IMHO. And, unlike the Sun Saver, the TriStar is a 4-stage charger. It's self-consumption is low compared to other charge controllers, a big plus when you're talking about a small system.

    BTW: A remote temperature sensor is available for the TriStar.

    I've never tried the C35, so I can't comment on it.

    John
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  • System2
    System2 Posts: 6,290 admin
    #4
    Re: Help choose charge controller

    Better check that 5 amps....I was reading that there are times that there are surges so you have to overkilll the controller a bit.

    Something about the cloud Edge effect......The costs of the slightly bigger controller isn't hardly anything...so i would go to the larger side to be safe. Also i think it would run cooler.

    I was reading about mppt on here and they were saying that doesn't really work like it is supposed to. PWM was supposedly better....I just got a PWM Xantrax and I am hooking it up today 030108 on my RV.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: Help choose charge controller

    whoa there roger,
    most mppt controllers work just fine and it seems you are judging them all by the bad rap that the bz controllers are giving the industry. do note that all controllers are limited in their output current somehow, but your xantrex will work ok for you.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Help choose charge controller

    MPPT controllers, in general, do work just fine... However, they do have limitations...

    On the large controllers (like the Outback MX60 and such), they usually don't make sense on systems with less than about 400 watts of solar panels as they MPPT circuits consume more power for internal operations than a simple PWM does--so any extra energy they gather is "wasted" on its own operation.

    Also, if the solar panel and battery voltage is properly matched (i.e., 12 volt panels for 12 volt batteries) and the system is in a temperate area, then you don't gain much extra power over a simple PWM system (especially for smaller installations).

    MPPT controllers have, typically cost more than PWM controllers. Again, for smaller systems, you could buy more panels vs pay more for the controller and get roughly equal results (although, the price difference in recent years has narrowed).

    On the other hand, MPPT controllers are great for larger systems and those exposed to a wide temperature range. They allow you to design a system that will work in the hottest weather (where solar panel voltages sag and not properly charge the batteries) and in cold weather (where solar panel voltages rise well over battery voltage and that extra voltage can be changed into "extra power" for battery charging--which would be wasted with a simple PWM controller).

    MPPT controllers also allow you to run the solar panels at a higher voltage than the battery bus (without loss in power output) and use smaller (cheaper) wire--great for installations where the panels need to be a fair distance from the batteries.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • whackamole
    whackamole Registered Users Posts: 18 ✭
    #7
    Re: Help choose charge controller

    Thanks for the responses. I do have a follow-up question.

    I notice that a controller like the xantrex C35/40/60 and the Morningstar Tristar all function as a load, charge, OR diversion controller, (key word: OR) but that means I can't charge my battery and run a load at the same time using the controller. (I got this info from the xantrex manual and the morningstar website.

    Doesn't that kind of defeat the purpose of this device? If I don't have load control function (because I'm in charge control mode), don't I run the risk of running my battery too low since there would be no low voltage disconnect function? Do I need to then have a separate low voltage disconnect device between my load and battery?

    Other chargers like the sunsaver and xantrex C12 seem to be able to support both at the same time. What gives?
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  • boB
    boB Solar Expert Posts: 1,032 ✭✭✭✭
    #8
    Re: Help choose charge controller

    When used as load control, the controller (C40 etc) will "load" the battery down if its voltage gets above a preset threshold. The energy is dissipated in an external load resistor. The controller should still control using a decent charging algorithm, like 3 stage, but would only come into play if the battery loads that YOU supply don't load the battery down enough to keep that battery voltage down to the proper levels. When your loads are high, the C40 or whatever is basically out of the circuit because your loads ARE the diversion load at that point and the C40 sees that because the voltage is down below its threshold.

    does that make sense ?
    boB
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  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    #9
    Re: Help choose charge controller

    A LOAD control is basically just feeding from the battery to an outside load. It cuts the battery off at low voltage to save the battery from going totally dead, usually around 10.5 to 11 volts. In fact, you don't even need solar to use one, since it works from the battery only. We sell a few of the solar controllers to people with small UPS systems to use just as a load controller. The LOAD and charge control functions are totally independent of each other.

    A DIVERSION controller (a system which I hate, BTW) actually acts like a shunt controller. It looks at the battery voltage, and when ever it gets above a certain point, say 12.9 volts, it DIVERTS the solar panel power coming in to an outside load, such as a hot water heater element.

    IMO, diversion control in a system is a recipe for disaster (and we have seen some). The problem is that a diversion control is "fail unsafe" - if the diversion controller fails, your batteries are toast. You would think you could just use two controlers- one from the panels, to fail-safe, but then you have to set your diversion load voltage too low, so the batteries are perpetually not fully charged.
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  • boB
    boB Solar Expert Posts: 1,032 ✭✭✭✭
    #10
    Re: Help choose charge controller
    Windsun wrote: »
    A LOAD control is basically just feeding from the battery to an outside load. I

    Ooops on my post. Senior moment going too fast...
    Here's looking at you, kid.

    boB
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  • whackamole
    whackamole Registered Users Posts: 18 ✭
    #11
    Re: Help choose charge controller

    OK, so here is what I conclude, tell me if I'm wrong.

    If I buy a 3 stage PWM controller which operates in a charge controller mode (i.e. xantrex C35), then I also need to purchase another device to handle low voltage disconnect to avoid damaging the battery.

    I realize my dc->ac inverter has a 10.5v low voltage disconnect built-in (at least my cheapo modified sine wave inverter does). But if I am running 12v loads rather than AC loads (lights) off the battery at night, would I need to purchase a 2nd device on the 12v load to prevent over-discharging?

    Is this how RV'ers do it?

    Thanks in advance for helping out a newbie.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #12
    Re: Help choose charge controller

    You can use the LVD from the controller as a signal to a relay (or a buzzer/red light) to signal when to shut down your loads--but the real problem is that you generally don't want to discharge beyond 50% of capacity for a wet cell (or more than 80% discharge for a good AGM battery).

    And measuring voltage under load is really not a good way to do this. To find out a battery's state of charge, you need to let it sit for several hours (no loads or charging) and with an accurate voltmeter (and temperature gauge) need to read the resting voltage and look it up against a chart.

    The standard LVD is set for so low of voltage, the battery is basically dead at that point. And, if you set it higher, you run the risk of false triggering when starting heavy loads (fridge, fan, etc.).

    If you have the funds (and plan on doing a lot of "battery" camping), then you should look at a real Battery Monitor.

    These guys are really like a fuel gauge--they are reasonably accurate under an normal conditions and let you plan your loads/charging accordingly.

    Others here who have used these can give you a good brand/model recommendation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    #13
    Re: Help choose charge controller
    whackamole wrote: »
    OK, so here is what I conclude, tell me if I'm wrong.

    If I buy a 3 stage PWM controller which operates in a charge controller mode (i.e. xantrex C35), then I also need to purchase another device to handle low voltage disconnect to avoid damaging the battery.

    Like I said in my last message:

    The LOAD and charge control functions are totally independent of each other.
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  • SolarJohn
    SolarJohn Solar Expert Posts: 202 ✭✭
    #14
    Re: Help choose charge controller

    Whackamole:

    You can use a voltage-controlled-switch to control a relay, which can connect and disconnect loads depending upon battery voltage. I use a Morningstar Relay Driver (which is actually 4 voltage controlled switches) to do the job. I like the fact that I can set High Voltage and Low Voltage thresholds. For example: I programmed it to turn my inverter on at 13.75 volts, and turn it off at 12.00 volts. Another nice feature of the morningstar relay driver is that you can set L to H and H to L threshold delays. Using that feature, you can prevent a high motor starting current (which can cause a brief voltage sag) from tripping the relay prematurely.

    You can find the voltage controlled switch, and the morningstar relay driver, in the Wind-Sun store.

    John
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  • System2
    System2 Posts: 6,290 admin
    #15
    Re: Help choose charge controller

    Hey SolarJohn,

    Ive been looking at the morningstar relay driver for awhile now.

    Does it have to be connected to a morningstar charge controller to work?? I just need the relay driver. Ive got a very small system and i like the idea of being able to shed loads depending on the battery voltage. Thank You.
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  • SolarJohn
    SolarJohn Solar Expert Posts: 202 ✭✭
    #16
    Re: Help choose charge controller
    Fred-H wrote: »
    Hey SolarJohn,

    Ive been looking at the morningstar relay driver for awhile now.

    Does it have to be connected to a morningstar charge controller to work?? I just need the relay driver. Ive got a very small system and i like the idea of being able to shed loads depending on the battery voltage. Thank You.

    Morningstar literature says you can connect it to a source other than a morningstar charge controller, but I don't know exactly how you would do that. It communicates with other devices via the "Modbus Protocol" and uses an RJ-11 connector. It seems as if you would have to be a programmer to get this to work for you. I hope someone else here can provide additional information, cause I would like to know how to do it myself.

    BTW, you can download the MSView software from the morningstar site and try it, even if you haven't purchased the relay driver. That might help a little.

    John
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #17
    Re: Help choose charge controller

    The MRD can be used without the TriStar controller, although with reduced functionality. You can download the free MSview software from the Morningstar website. Once an operational configuration is programmed (no special skills required), you just upload it to the MRD from your computer via RS-232.

    See: http://www.morningstarcorp.com/products/software/index.shtml
    and: http://www.morningstarcorp.com/products/RelayDriver/info/RD_Manual.pdf
    and: http://www.morningstarcorp.com/products/RelayDriver/info/RD_Applications.pdf

    HTH,
    Jim / crewzer
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  • SolarJohn
    SolarJohn Solar Expert Posts: 202 ✭✭
    #18
    Re: Help choose charge controller

    How to use the Morningstar Relay Driver when you don't have a Morningstar Charge Controller:

    Using MSView software, program one of the channels as "disabled". You can then use that channel as a voltage input. Apply battery voltage to the appropriate terminal block terminal for that channel.

    Program another one of the relay driver's channels for the "Threshold" function, and select the "disabled" channel as the input. You can then set the parameters, such as High Threshold, Low Threshold, H to L and L to H delays, and several others.

    As Jim said, no programming skills are required. This is easy to set up using the MSView software. I am very pleased with my decision to use this product. The manuals are very short on details though.

    John
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