Newb Questions MPPT'S

ZakarumeZakarume Solar Expert Posts: 143 ✭✭
Been looking at the MPPT Charge Controllers

1. http://www.solar-electric.com/inverters-controllers-accessories/chco/misoclchco/misoclli/misoclli150m.html

2. http://www.solar-electric.com/inverters-controllers-accessories/chco/misoclchco/midnite-solar-kid-mppt-charge-controller/midnite-solar-kid-mppt-solar-charge-controller.html


I like them both, but drawback on kid is only go to 400 watts maybe 500 Watts. Now the other i can upgrade to 800 watts. Still in the beginning stages of solar. If you were to buy 1. Which would you get?
1460 Watts Solar @24v. 675 AH Battery Bank using 12 6v Trojan T-105. 1 Midnite Classic 150. 1500 Watt 24v Samlex Pure Sine Inverter

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newb Questions MPPT'S

    What you need to decide is how big the system ultimately will be.

    30 Amps (the Kid) on 12 Volts is actually about 468 Watts of PV. But off-grid systems aren't sized by PV but rather by the Watt hours that the battery bank can supply (load needs). The size of that determines how much PV and what controller you need to recharge the batteries.

    You can in fact parallel two Kid controllers if you want to expand that way.

    You can also go up in system Voltage to expand: the controllers will handle their rated output on 12, 24, or 48 Volts.

    If your system size is fixed at around 200 Amp hours & 12 Volts you don't need more than a 30 Amp controller. As it is your 400 Watts of PV & an MPPT controller would produce about 26 Amps of peak current, which is fine for 200-ish Amp hours of battery.

    The step up to the Classic is a pretty big one, as it can handle 90-ish Amps on a 12 Volt system. That's 3X the Kid's capacity.
  • nyarelathodepnyarelathodep Solar Expert Posts: 99 ✭✭
    Re: Newb Questions MPPT'S

    Hey , coot... so the kid can handle a pair of 235w panels at its maximum load into a 215ah 12v bank? And if one were to later expand their system into a 24v 215ah bank and add another pair of 235w panels and another kid, they would have a good, workable system in both cases (assuming of course the loads were appropriate)? And what might be additionally required in that upgrade when doubling the array and bank, on the panel to controller part of the system?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newb Questions MPPT'S

    Two 235 Watt panels at 77% typical efficiency / 12 Volts nominal = 30 Amps peak current. The Kid is good for 20% over (before you're really wasting money on PV) and will limit current to that peak. If the system doesn't demand it, the current won't be at peak of course.

    Switch up to 24 Volts and you'll need to add the other two panels. This is where it gets interesting: the panels have a Vmp around 30 which is plenty for 12 Volts but not enough for 24. So on the 12 Volt system you'd wire the panels in parallel (lower Vmp to battery Voltage ratio = higher controller efficiency). Change to 24 Volt system, just add the two new panels in series with the old ones: two parallel strings of two in series. No messy, complicated wiring changes. Just plan for it in advance and make sure the new panels have identical Imp.

    You would not need a second Kid for four 235 Watts on 24 Volts: 940 Watt array @ 77% / 24 = 30 Amps.

    You only need a second Kid if you need to go to 60 Amps (i.e. the 940 Watt array on a 12 Volt system with larger battery bank).

    Of course if you go up to 24 Volts you'll need to change the inverter. In either scenario you'd be adding batteries, so make sure the old ones aren't too old or used/abused.
  • nyarelathodepnyarelathodep Solar Expert Posts: 99 ✭✭
    Re: Newb Questions MPPT'S

    Ah, good to know about the kid handling 4 panels. So, for the first stage of this system when the bank is at 12v, do the panels need a combiner box? Have neglected learning anything about the panels side of things up till this point. Basically, i would have to wait til round February to get a larger, complete system but the lady of the house desires a more immediate upgrade to our current one (don't ask... its pitiful, heh), and i know such a thing can be tricky and typically cost you more in the end. So I figured with the kid being modular (and the whizbang being cheap enough), i could get to where i wanted to be in two neat and clean phases, so to speak. The inverter won't really be a loss changing up (just figuring a suresine, which can always find a use somewhere when i get the ob3524). Curious though about what a pair of kids could do on a 24v system, ie how many watts of panels could you get out of two of em. In rethinking all of this, this two phase route could actually cost me a substantial bit less and be more appropriate than the bigger kit i have been looking at, while still having a little more room for expansion.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newb Questions MPPT'S

    Two panels or two strings of panels in parallel would not need a combiner box.
    You need the combiner & fuses/breakers when you have three or more parallel connections and need to protect against one of the strings shorting.

    Two kids on a 24 Volt system would handle 60 Amps, or about 1880 Watts (as two 940 Watt arrays).

    This is right at the tipping point to using a different controller:
    Kid at $285 x2 = $570 for 60 Amps
    Outback FM60 $499 for 60 Amps (no shunt monitor control like the MidNite)
    MidNite Classic Lite $500 for ~80 Amps (no built-in display like the FM60; use computer)
    MidNite Classic 150 $610 for ~80 Amps (built-in display and shunt control)

    So if you know in advance you're going up to 60 Amps or more and can swing it you may as well by the single, larger controller to start with. The exception to this would be if you can see into the future and know you won't be able to get matching panels: the two Kids could handle two mismatched arrays.
  • nyarelathodepnyarelathodep Solar Expert Posts: 99 ✭✭
    Re: Newb Questions MPPT'S

    Two remaining questions come to mind, given everything you've said coot. Are two kids compatible with any of the OB figured EZ panels by midnite? And if the maximum array that a pair of kids can handle ends up being all that i would need, is there any substantial benefit the classic 150 would have over them other than further panel expansion?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Newb Questions MPPT'S

    That depends on how you mean "compatible": the MidNite panels will accommodate just about anything with a bit of work, but I'm not sure they're specifically designed to mount one or two Kids. You can always ask them at MidNite because they're very good about answering such questions. :D

    If your only need is the charge controller then two Kids are a bargain compared to a Classic. But the Classic has additional abilities such as the AUX controls. Otherwise it would be a matter of current handling. No sense buying 80 Amps of charge controller for a system that isn't going to go beyond 60 Amps. But do think about the future carefully.

    Curiously I bought the MX60 way back when figuring I could go up substantially in battery power from my initial 200-ish Amp hours. That was 2008, and I haven't needed to expand yet. I have added panels to catch light and start charging earlier in the day. It's amazing how much you can do with a little battery an a lot of thinking. :D
  • nyarelathodepnyarelathodep Solar Expert Posts: 99 ✭✭
    Re: Newb Questions MPPT'S

    That's why i find myself valuing your input as much as I do sir. You seem to do a lot with a system size very similar to what I have been aiming for. Can't thank you (and the rest of the NAWS forum regulars, of course) enough for answering the same questions day in and day out for us neophytes. Without your consultations, I'm sure a great many of us would be making very costly errors, so Cheers!
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