Building a Solar Generator - What Am I Doing Wrong?

solartimesolartime Registered Users Posts: 4
I've learned a lot from the forums and have been reading here for about a year, so I figured it's time to participate. I am looking for some help and advice as I build out my first solar generator. I have my Amazon list in a link at the bottom. I have used the solar calculator quite a bit to understand more about what would be needed capacity wise : http://www.batterystuff.com/kb/tools...alculator.html

I plan to use a 12v fan for 7 hours at 1.1amp draw (high is 3amp): http://www.amazon.com/Fan-Tastic-011.../dp/B0000AY2Z6
I would also like to use a 7" lcd tv if possible for up to an hour (can't find the specs but I will just use the multimeter)

1) Is it better to have one 35 ah agm or two 18ah batteries? Space isn't a massive concern with the box I am planning

2) Is a 30w Instapark enough in central Texas to generate enough juice to refill a 35ah agm? It seems light based on my calculations, but other solar setups I have seen online aren't packing massive panels. I am looking for the right balance in portability and need.

3) I can't decide on a solar charger controller. I do want an lcd read out but I also don't want the top of the line. I would love to be able to monitor in real time as I am learning without having to carry the multimeter around all the time. Eventually I want to hook this up into the arduino system I have started as well for real time data that is logged. Are the ones in my wish list on Amazon too large? 30a seems ridiculous for my size setup.


Here is my Amazon list. I have 2 charge controllers in there, but do not know which one I want yet. http://amzn.com/w/2OYLB1QYWAUH4


Thanks a lot for any help!

Comments

  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Mornigstar makes some reliable low amp CCs. The typical really cheap imported CC is not to be trusted, and may not be UL listed for safety and legal use in the US in a home.
    SMA SB 3000, old BP panels.
  • BB.BB. Super Moderators, Administrators Posts: 29,720 admin
    Power usage is what you design you off grid power system around. I found one 7" LCD model that say 11 watts maximum (11 watts / 12 volts = ~1 amp).
    • 1 amp * 7 hours = 7 amp*hours
    • 1 amp * 1 hour = 1 amp*hour
    • 8 AH * 12 volts = 96 Watt*Hours per day
    Sizing the battery, roughly 1-3 days of storage (your choice), 50% maximum discharge. Call it two days:
    • 8 AH * 2 days * 1/0.50 maximum discharge = 32 AH @ 12 volt battery
    So, your suggested 35 AH battery is fine.

    Note about small Lead Acid Batteries (AGM and otherwise)--I would only plan on it lasting about 1-2 years of use. Small lead acid batteries just do not tend to last very long (at least for me).
    1) Is it better to have one 35 ah agm or two 18ah batteries? Space isn't a massive concern with the box I am planning

    In general, I would would suggest on large battery vs two smaller batteries in parallel. Lead Acid batteries in parallel can have issues sharing current (corroded/loose connection, bad cell, etc.). If you get a DC current clamp DMM (like this one from Sears--also handy for debugging/experimenting), and measure the current to each battery once in a while, you can make sure that all is going well.
    2) Is a 30w Instapark enough in central Texas to generate enough juice to refill a 35ah agm? It seems light based on my calculations, but other solar setups I have seen online aren't packing massive panels. I am looking for the right balance in portability and need.

    Two calculations for solar charging... One is based on what Lead Acid batteries need to charge, and second based on amount of sunlight you get.

    First--Recommend 5% to 13% rate of charge. 5% is OK for summer usage, weekend/occasional usage. If daily usage, I would suggest 10% or greater rate of charge:
    • 35 AH * 14.4 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 33 Watt minimum array
    • 35 AH * 14.4 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 65 Watt nominal array
    • 35 AH * 14.4 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 85 Watt "cost effective" maximum array
    And then based on hours of sun per day. Using PV Watts near Abilene Tx, fixed array tilted to 33 degrees from horizontal (of course, you can point at sun throughout the day and improve harvest):
    Month    Solar Radiation (kWh/m 2/day)
    1      4.80     
    2      5.44     
    3      6.06     
    4      6.43     
    5      6.12     
    6      6.17     
    7      6.29     
    8      6.22     
    9      5.72     
    10      6.10     
    11      5.24     
    12      4.57     
    Year      5.76
    

    So, if you need the system during bad weather/winter, on average you would get a minimum of 4.57 hours of sun per day (December):
    • 96 Watt*Hours of DC power per day * 1/0.61 DC off grid system eff * 1/4.57 hours of sun per day = 34 Watt array
    So, your 30 Watt array is probably going to be OK for your usage... And as you see, even a realitivly small DC load requires a fairly large solar panel + battery pack to power it.

    After you use the AGM battery for a while and get tired of the weight, you might want to look at LiFePO4 (lithium iron rechargeable) battery. It will be much lighter than AGM, more expensive, and may give you better life/service.
    3) I can't decide on a solar charger controller. I do want an lcd read out but I also don't want the top of the line. I would love to be able to monitor in real time as I am learning without having to carry the multimeter around all the time. Eventually I want to hook this up into the arduino system I have started as well for real time data that is logged. Are the ones in my wish list on Amazon too large? 30a seems ridiculous for my size setup.

    You only need a ~2 amp rated charge controller.... 30a is ridiculously large--Unless you want the display or some sort of logging function.

    The only small device that is not too expensive is the Doc Watson type units (there are a lot of knockoffs out there too). The one big issue with these type is that they only read AH/WH in one direction (i. e., they count up, not down--So if connected to a battery that is charging/supplying loads, they will not keep track of the "net" AH into/out of the battery). But otherwise, these types of units will save you bringing a multi meter and will give you AH or WH usage--Something that a multi meter does not.

    You have a 400 Watt AC inverter on your list too... Is that something you are thinking of connecting to your portable power system? A 400 Watt AC inverter will draw something like:
    • 400 watts * 1/0.85 AC inverter eff * 1/10.5 volts cutoff = 45 Amps
    Or it will kill (aka "murder") your battery in less than 45 minutes at full power. If you want to supply AC power--You really need to look at your AC power needs and design a system around that.

    In general, stay way from a large(ish) AC inverter and your present system design.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solartimesolartime Registered Users Posts: 4
    You have no idea how helpful this is. Thanks a lot Bill. Your responses are always so thorough and I know you took a lot of time to generate this one and I really appreciate it. I have literally printed out your response and generated an excel sheet for these algorithms. Hopefully this will help many others down the road as well. The praise aside, I have a few follow up questions:


    I would absolutely love to go with a LiFePO4 battery like Battery Tender or WPS but I figured they are so expensive for what I am getting. I would love the added space but I am really learning right now. Even if I only get a year out of a cheapo battery, it is money well spent. For under $100, can you think of any better battery? The LiFePO4 ones would only be 7ah for my budget.

    I really have no explicit need for an AC plug, but wanted one incase I needed it for small electronics. 400w is overkill I know - I should have looked more like 100w or less. I plan to have rocker switches on the inverter anyways so its not just using electricity.

    I am also looking for idea on containers that are less ghetto than a plano box, but not as expensive as a pelican case. Any ideas for a 35ah battery with twice as much footprint to spare?
  • solartimesolartime Registered Users Posts: 4
    Ok, so I have actually decided for this, I will stay away from AC conversion and stick to DC sockets. With AGM batteries, is there a need to store them with ventilation or only have them vented as they are being used? I can install some plastic downward vents on the side, but I am not sure of the size to make them. Also, does anyone have an suggestions for 35ah LiFePO4 batteries? Ideally ones that balance cells internally like the WPS?
  • BB.BB. Super Moderators, Administrators Posts: 29,720 admin
    In general, agm batteries will only vent if over charging and/or the batteries are failing (when charging).

    For 12 volt plugs, look at Anderson power pole connectors. Try to stay away from cigarette lighter type outlets.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solartimesolartime Registered Users Posts: 4
    Thanks Bill. I actually have some Anderson connectors I can use. I might just put in a small vent - eventually I will move some a sturdier case like an old Pelican I have but for now I'm going cheap ol Plano with some foam.
Sign In or Register to comment.