Small system design questions

dgc2018dgc2018 Posts: 3Registered Users

The attached diagram describes a portable back-up on a small cart, mainly used for 12v power in a prolonged grid down situation. We are all electric here and will be camping inside our passive solar home. I am not intending to use this system for any high wattage appliances. The inverter would be used to run a skill saw for really short periods of time if I needed it.

Data:

PV 1 and 2 are 100W Renogy panels. Data listed on the panels follows.

PMax = 100W

VOC = 22.5V

Optimum operating voltage VMP = 18.9V

Optimum operating current IMP = 5.29A

ISC = 5.75A

Max system voltage = 600VDC

Max series fuse rating = 15A

 

All 3 switches are SPST appropriately sized for amperage. S1 & 2 = 30A DC,  S3 = 20A DC

Information:

The 12V outlet will be used for LED lighting, and to run a 1 GPM pump, 2.5A max.

USB ports for charging cell phones if cell service is working.

The 7AH batteries are for LED lighting.

Questions:

1.      Having decided on an MPPT controller, could you suggest a brand and model that would work well, while keeping an eye on future expansion of the system.

2.      If I added another panel or two, keeping the total VOC below the maximum for the controller, would that require more batteries also?

3.      My guess was to get a 30A controller. Should it be larger?

4.      Is the 30A fuse for the panels correctly placed and the correct amperage rating?

5.      Could I attach my 18V drill motor battery to the MPPT battery ports to recharge it, after disconnecting the 12V system?

6.      Is the correct order of connections as follows?  For turn-on system:  Battery to MPPT first, then Panels to MPPT.

For turn-off system: Panels from MPPT first, then Battery from MPPT.

I appreciate your help and welcome your questions and advice.

Sincerely, David

 

Comments

  • EstragonEstragon Posts: 2,776Registered Users ✭✭✭✭
    1. Something like a midnight kid may work well.
    2. Depending on battery type etc., 110ah bank would take ~20a or so. Your 2 panels will likely do 10-12a or so, leaving some room for expansion of charging.
    3. 30a seems okay to me. You can easily add a second one in parallel if need be to charge a larger bank.
    4. Personally, I would replace both the pv switch and fuse with a 15a breaker. A proper DC rated breaker can serve both functions.
    5. No, don't try to charge a drill battery directly with DC unless you really know what you're doing. There are different charging algorithms for different types of drill batteries, and you really need to understand the these before attemping to charge directly.
    6. Yes.

    As with point 4, I would replace s2 and fuse with a dc rated breaker. I would replace 10ga going from controller to batteries with the biggest wire that physically fits on the controller, and size the breaker according to CC manufactures spec.

    14ga to the 12v outlet might be okay if the run is short and loads are really light. Personally, I'd use heavier wire.

    A 1500w inverter could pull 250+ [email protected] A skill-saw might draw [email protected] sustained, and considerably more on startup or when binding, so you'll want beefy (like 1gauge or heavier) wire from batteries to inverter, and good connectors (not clips). Even then, [email protected] battery may have a tough time.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • dgc2018dgc2018 Posts: 3Registered Users

    Dear Estragon. Thank you so much for your response. I was educated in electronics but this subject of PV panels, charge controllers, and DC values is foreign to me. I need your help.

    A.      I apologize for not stating the battery type. I was planning on AGM unless you suggest otherwise.

    B.      You said that the 2-battery bank at 110 AH would take approximately 20A or so. I need further explanation about what you mean. The ISC of a panel is 5.75 A so I concur with you about the fact that they would produce about 10 to 12 amps. I don’t know enough about batteries in general to understand what you meant about the batteries being able to take approx. 20 A or so.

    C.      I like your suggestion about using breakers instead of a switch and a fuse. That’s perfect.

    D.     I don’t understand the values though. How can a switch and 30 A fuse be replaced by a 15 A DC breaker? It seems like it should be a 30 A breaker.

    E.      Your suggestion about using the largest wire that will physically fit into the charge controller battery port is great and to use a breaker as spec’d by the CC Mfg’er. Thank you, I was hoping for this kind of help.

    F.       I will change the wire size to the 12 V outlet to 12 awg unless you think it should be 10. Thank you.

    G.     My skill saw amp rating on the tag says it’s 13 amps AC. I used my Kill-A-Watt meter on it and it read 17 amps AC at start up and free running with no cutting at 11 amps AC. How does that relate to your estimate of 80 amps DC?

    H.     You said the 1500-watt inverter idea might pull 250+ amps @ 12 VDC. I picked 1500 as the inverter size based on 120 VAC x 13 amps AC = 1560 watts. Could you please help me to understand how it might pull 250+ amps @ 12VDC?

    I.        I really don’t have very much intended usage for an inverter in my disaster scenario, primarily because I don’t have much of a battery bank and won’t want to be using high wattage items with this system.

    J.        Is there a basic consensus that relates battery bank size in AH and CC amp rating and inverter amps value? For my initial 110 amp hour bank value should it be more as per the diagram? Or are the total expected load amps the driving force that would determine the total amps of the CC? And then as a second question, the total load amps for the inverter would need a certain size of battery bank? I’m just trying to relate all three of these factors.

    I am really enjoying learning more in this “electric” field of study, that I didn’t learn in school. Thank you so much for your help. I really appreciate this valuable resource for those who have questions.

    Sincerely, David Culver

  • EstragonEstragon Posts: 2,776Registered Users ✭✭✭✭
    dgc2018 said:

    Dear Estragon. Thank you so much for your response. I was educated in electronics but this subject of PV panels, charge controllers, and DC values is foreign to me. I need your help.

    A.      I apologize for not stating the battery type. I was planning on AGM unless you suggest otherwise.

    AGMs do have advantages.  They tend to be more expensive and not last as long as flooded, but do some have features which may or may not be worthwhile in your application.  Some have quite low self-discharge rates, which is useful if they might sit for long periods with no charging source.  Some also can handle sustained higher charge/discharge rates.  You may want to look for a true deep cycle version though, not a starting or marine type, if you expect to need to repeatedly deep cycle them.  If you expect to only need them for occasional emergency use though, a starting or marine type may handle the saw load better (deep cycling will not be good for a starting or marine type, but if only rarely used, they'll likely die of old age first anyway). 

    B.      You said that the 2-battery bank at 110 AH would take approximately 20A or so. I need further explanation about what you mean. The ISC of a panel is 5.75 A so I concur with you about the fact that they would produce about 10 to 12 amps. I don’t know enough about batteries in general to understand what you meant about the batteries being able to take approx. 20 A or so.

    A battery will only handle charge at a certain rate.  True deep cycle batteries have thick plates, and it takes time for them to absorb more than a surface charge.  Starting batteries, and many AGMs can handle higher rates of charge.


    C.      I like your suggestion about using breakers instead of a switch and a fuse. That’s perfect.

    D.     I don’t understand the values though. How can a switch and 30 A fuse be replaced by a 15 A DC breaker? It seems like it should be a 30 A breaker.

    The breaker is sized to protect the wire.  Although the wire from PV to CC is sized to minimize voltage drop, so has lots of ampacity (and could otherwise be protected by a bigger breaker), this is likely not true for the panels.  You should have a spec for max breaker size on the spec label on the panels, which is likely 15a. 

    E.      Your suggestion about using the largest wire that will physically fit into the charge controller battery port is great and to use a breaker as spec’d by the CC Mfg’er. Thank you, I was hoping for this kind of help.

    F.       I will change the wire size to the 12 V outlet to 12 awg unless you think it should be 10. Thank you.

    It really depends on what sort of loads you plan to run.  Presumably the run will be short to the plug, and you wouldn't want to run any sort of high amp draw loads off a 12v socket anyway, so 12ga may be okay.

    G.     My skill saw amp rating on the tag says it’s 13 amps AC. I used my Kill-A-Watt meter on it and it read 17 amps AC at start up and free running with no cutting at 11 amps AC. How does that relate to your estimate of 80 amps DC?

    At 12vdc nominal, current will be 10x (or possibly more) the current at 120vac for a given load.  13aac at 120v becomes 130a at 12v.  It can be worse for a couple of reasons.  First, there are losses in the inverter.  A psw inverter should run the saw motor more efficiently that a msw one, but there will still be losses.  Second, battery voltage will sag with increased load, and wire losses will increase.  As voltage sags, more current is needed to run the load.

    H.     You said the 1500-watt inverter idea might pull 250+ amps @ 12 VDC. I picked 1500 as the inverter size based on 120 VAC x 13 amps AC = 1560 watts. Could you please help me to understand how it might pull 250+ amps @ 12VDC?

    That would be a surge current that will happen to spin up the saw, and if the blade binds.  A good PSW inverter should be able to handle a surge of about 2x rated for a short (few seconds) time.  You have to be sure the wire and connections can also handle that kind of current. 

    I.        I really don’t have very much intended usage for an inverter in my disaster scenario, primarily because I don’t have much of a battery bank and won’t want to be using high wattage items with this system.

    I'd consider the saw a fairly high wattage load.

    J.        Is there a basic consensus that relates battery bank size in AH and CC amp rating and inverter amps value? For my initial 110 amp hour bank value should it be more as per the diagram? Or are the total expected load amps the driving force that would determine the total amps of the CC? And then as a second question, the total load amps for the inverter would need a certain size of battery bank? I’m just trying to relate all three of these factors.

    In a solar system, we normally shoot for something like 2 days autonomy (for cloudy weather), and a maximum drawdown of 50% of battery capacity.  With 110ah, that would be about 25ah/day.  Note that just having a 1500w inverter on idle for 24hrs could use that or more, so you'll want to keep it off when not running the saw.  If using the saw for more than a couple of quick cuts, more battery would certainly be a good idea.  As noted above, the voltage will sag under load and will take time to recover.  A bigger bank would likely be needed to do a series of longer cuts.


    I am really enjoying learning more in this “electric” field of study, that I didn’t learn in school. Thank you so much for your help. I really appreciate this valuable resource for those who have questions.

    Sincerely, David Culver

    You're welcome.  There is a lot of real-world experience on this forum.  I'm still learning lots!


    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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