Building a small 6 V system - Ready to add a new panel

Hi,

I'm new at this so please forgive simple questions.

Currently have a 6v 4watt panel, 6V Charge Controller, 2 6V 4.5 Amp Hour batteries, and little motor for load. The charge controler only allows load to activate when sun goes down. This is just a small demo unit to help me understand PV Systems. 50 bucks so far.

I understand hooking the batteries in parallel to keep 6V and increase the amps of the system and why the batteries need to be the same.

Question:

I have a small solar powered water fountain system (6V 1.5watt Panel, 6v motor). Can I hook the two PV panels together in parallel (a 6v 4w and a 6v 1.5 w)?? If so then is this a good way to go? If not, why not. What can I get away with when combining PV Panels?

With Regards,
Scott

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Building a small 6 V system - Ready to add a new panel

    Yes, you can.

    Batteries are a "pain" to connect in parallel. The batteries need to be "matched' and the wiring between batteries too... Or one battery will carry most of the current.

    With solar panels they are actually "current mode" sources... More or less, they output a fixed amount of current based on available sunlight on the panel.

    You have to watch out for "non-standard" panels ratings... There is a Voc and Vmp voltage rating (voltage open circuit and voltage maximum power).

    For example, a 12 volt battery bank charges at ~14.5 volts (77F) and the "appropriate" rated solar panel would be Vmp~17.5-18.5 volts. The panel will have a Voc of around 21 volts.

    The reason for this is:
    • Battery charging voltage changes with temperature (cold batteries, higher voltage)
    • wiring drop (you only want ~0.05 volt drop between controller and batteries in your 6 volt system)
    • controller drop (frequently 1-2 volts)
    • Vmp of panel falls with temperature (hot panels will drop a volt or two).
    So--yes, you have the basics correct. And need to research the actual Vmp of your "6 volt panels" (probably need Vmp>9.5 volts to work "correctly").

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