Need Help On Solar Panel/LiFePO4 Solution For Battery Powered Scrolling Billboard

mohknight
mohknight Registered Users Posts: 2
Hello,

I'm pretty new at this stuff (solar power, batteries, etc.) so I hope y'all will bear with me.

I and a friend designed a backlit (LEDs) scrolling billboard and had it built as a part of a hopefully successful business project. The power situation here in West Africa is not that good so we had to design the billboard to work without mains power for up to 3 days.

The batteries we are using are 12V-30Ah NiMH battery packs of 30 D-Cell NiMH batteries connected in series and parallel - we are using these NiMH battery packs due to the need to keep the battery weight as low as possible. At full charge they run the billboard for 3 days before needing to be recharged.

The current draw of the billboard is;
0.14A - motors stopped, lights off
0.56A~0.78A - motors rolling, lights off
0.18A - motors stopped, lights on
0.69A~0.91A - motors rolling, lights on
NOTE: The battery is depleted by about 6Ah every 24 hours.

So here's the situation; we've decided to incorporate a solar element into our design so we can go completely off-grid (the region gets at least 6-8 hours of sunshine daily in the dry season and 4-6 hours in the wet season), save ourselves some money in the long run and do some good for the environment.

We have been advised to invest in a solar panel system using 12V-30Ah LiFePO4 batteries due to their weight, longer lifespan, discharge efficiency and that they're more suited to solar applications than NiMH.

What we're trying to find out is what exactly will suit our needs? What size of solar panel (65W? 85W?) would serve to keep 12V-30Ah LiFePO4 battery fully charged? Do we need a BMS for the battery? What about the Charge Controller - what rating (Amp?) is required? What are the best brands?

Hope I'm not asking too much ...

Thanks!

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Need Help On Solar Panel/LiFePO4 Solution For Battery Powered Scrolling Billboard

    LiFePO4 needs battery management.
    NiMh will need it's own special charger.
    Sealed AGM lead acid battery would work with any major brand charge controller.

    What is security for the system (panels & battery & electronics) like, would someone be watching it, or are you likely to find an empty pole the next morning after the install ?

    LED display. Motors. Hmm. All LED displays I've ever seen, scroll electronically, no motor needed.
    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 ,

  • mohknight
    mohknight Registered Users Posts: 2
    Re: Need Help On Solar Panel/LiFePO4 Solution For Battery Powered Scrolling Billboard

    To clarify, the type of scrolling billboard I'm talking about is the type that displays a series of posters one at a time i.e. you see them mostly aot airports. I'm using an LED light panel to backlight it.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Need Help On Solar Panel/LiFePO4 Solution For Battery Powered Scrolling Billboard

    Sorry for the late reply--Computer ate my earlier post. :cry:
    mohknight wrote: »
    ...design the billboard to work without mains power for up to 3 days.

    The batteries we are using are 12V-30Ah NiMH battery packs of 30 D-Cell NiMH batteries connected in series and parallel - we are using these NiMH battery packs due to the need to keep the battery weight as low as possible. At full charge they run the billboard for 3 days before needing to be recharged.

    Are you planning on designing your own battery charging circuits or buying your own?

    When designing a solar power system, you are, frequently, limited by the power needs of the load/charging circuits... DC input? AC charger?

    And slow chargers (C/10--I.e., takes around 10 hours to recharge) for NiMH can be pretty simple (just current limit and timer/voltage cutoff, or even let the charging current continue with voltage/current limiting. NiCad may be a better/more rugged chemistry for simple charger circuits, but the Cadmium is very toxic if you lose control of the battery pack).

    Hi charge rates (C/2 or even C*4) take specialized chargers/electronics and is probably something you are not going to make in a weekend.

    Lastly, with many cells in series, it is very possible for a load to draw a bank "dead"... And what usually happens, is there is one "weak cell" that goes dead first and actually reverses polarity and begins to "reverse charge"--An instant death sentence for the cell. So, either you need to monitor the battery bank (i.e., turn of load at 20% state of charge before a cell can go dead), or what Radio Control guys do is "match" cells so that they all have about the same AH rating. That way all cells go dead at about the same time.
    The current draw of the billboard is;
    0.14A - motors stopped, lights off
    0.56A~0.78A - motors rolling, lights off
    0.18A - motors stopped, lights on
    0.69A~0.91A - motors rolling, lights on
    NOTE: The battery is depleted by about 6Ah every 24 hours.

    So here's the situation; we've decided to incorporate a solar element into our design so we can go completely off-grid (the region gets at least 6-8 hours of sunshine daily in the dry season and 4-6 hours in the wet season), save ourselves some money in the long run and do some good for the environment.

    Here is an equation with my guesses--You can change the numbers to better match your needs:
    • 6 AH * 16 volts recharging (10 cell strings?) * 1/0.66 batt eff * 1/0.77 panel+charger derating * 1/5 hours min sun = 38 watts minimum
    Note that "12 volt solar panels" have Vmp~17.5 volts typical. However, that is at 25C--A solar panel in full sun in Africa is going to be upwards of 30C above ambient air temperatures which suppresses the Vmp voltage of the panel. You will (most likely) need higher voltage solar panels--if you look around you may find ~20-24 volt Vmp panels, but not usually in that "small size" unless you order your own.

    Anyway, a ~40 watt panel minimum (with efficient matching of solar panel Vmp/Imp to efficient charging circuit). It could easily go to 80 watts if you end up with more losses (less efficient panel matching, less efficient charging system, etc.).
    We have been advised to invest in a solar panel system using 12V-30Ah LiFePO4 batteries due to their weight, longer lifespan, discharge efficiency and that they're more suited to solar applications than NiMH.

    Can make the battery sub-system much more expensive (battery plus more electronics to manage the battery bank on a cell by cell basis). But, if done right, could be a really nice choice.
    What we're trying to find out is what exactly will suit our needs? What size of solar panel (65W? 85W?) would serve to keep 12V-30Ah LiFePO4 battery fully charged? Do we need a BMS for the battery? What about the Charge Controller - what rating (Amp?) is required? What are the best brands?

    Yes, you would need/want a BMS for the battery bank. Lithium chemistry is very sensitive to minimum/maximum voltages (and states of charge). And can be easily damaged if it gets out of the proper range.

    An 85 watt panel is where I would start from if you can... At least for prototyping. Will give you lots of extra room to play with and experiment until you nail down the numbers. But--it really depends a lot of on the actual charging circuitry and what it will require (which could force non-standard Vmp voltage for the array to match your needs in a hot climate).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset