Is Charge Controller Necessary?



  • thesolencothesolenco Registered Users Posts: 6
    Re: Is Charge Controller Necessary?

    please Suggest me a good PWM charge controller for my system as i stated below all the specifications.
  • BB.BB. Super Moderators, Administrators Posts: 30,737 admin
    Re: Is Charge Controller Necessary?

    "thesolenco" sent me a PM and asked how to do a 1,000 watt solar panel system... I will attempt to answer it here.

    First, a little analogy--See if this helps.

    Have a small ship next to the dock with a ladder from the doc to the ship. 1' (0.33 meter steps) and at high tide, the top of the ladder just matches the dock. Everyone is happy.

    Now the tide goes out--down 1 foot, there is one "final" step up--But things sort of work OK but may be a bit harder to use ladder/move cargo between ship and shore.

    Now down 2 feet, can take large step at top--very hard to use ladder.

    Now down 5 feet, cannot use ladder to access dock from ship.

    That is the Vmp=30 volts (cold panels) with 29 volt battery charging vs Vmp=24 volts and 29 volt charging with no current flow. (most panels operate "hot" unless in sub freezing weather.

    Take two ladders and make it twice as long--Now will always be able to access doc--But cargo traffic wise--Two ladders and one stream of traffic--So less than optimum solution.

    MPPT controller... Buy "magic ladder" that is always the correct length for the job. MPPT controller is what does the conversion (MPPT controllers are really the DC Equivalent of a variable AC transformer or arc welder controller--Match 230 VAC 60Hz to the 20-90 volts and variable current for arc welding--another analogy).

    Don't remember battery bank capacity--But say you want a 24 volt battery bank with 1,000 watt solar array... Nominally, 5% to 13% rate of charge (i.e., 100 AH battery bank would use 5-13 amp rate of charge):
    • 1,000 watts * 1/29 volts * 0.77 panel+controller losses * 1/0.05 rate of charge = 531 AH @ 24 volt max battery bank
    • 1,000 watts * 1/29 volts * 0.77 panel+controller losses * 1/0.10 rate of charge = 266 AH @ 24 volt nominal battery bank
    • 1,000 watts * 1/29 volts * 0.77 panel+controller losses * 1/0.13 rate of charge = 204 AH @ 24 volt minimum cost effective battery bank + panel combination

    The above is based on an MPPT charge controller or with Vmp~35-39 volt arrays (i.e., 2x ~17.5 volt Vmp panels in series).

    A 1,000 watt system operating in sunny season of Pakistan (high/dry desert climate?) would be around 5 hours of sun per day (again proper MPPT or PWM with correct Vmp array):
    • 1,000 watt * 0.52 system efficiency * 5 hours of noon-time equivalent sun per day = 2,600 Watt*Hours per sunny day

    Now, lets look at a system that uses 2x 250 watt 30.1 volt Vmp panels in series with PWM type charge controller. Effectively, a 1,000 watt array (2 panels in series, 2 strings in parallel for Vmp-array=60.1 volts and Imp=16.66 volts standard test conditions)...
    • 16.66 Amp charging * 1/0.05 rate of charge = 333.2 AH @ 24 volt battery bank maximum battery bank
    • 16.66 Amp charging * 1/0.05 rate of charge = 166.6 AH @ 24 volt battery bank nominal ttery bank
    • 16.66 Amp charging * 1/0.05 rate of charge = 128 AH @ 24 volt battery bank minimum battery bank

    And the useful amount of energy from the 1,000 watt + PWM array:
    • 16.66 Amps * 29 volts charging * 0.85 inverter eff * 5 hours per day = 2,053 WH per sunny day

    Now, the actual design... You have several options (assuming no MPPT charge controller). First is to find a 48 volt rated PWM controller that will charge a 24 volt battery bank. Here is one == A Xantrex C40 controller (40 amp rated)

    Schneider Electric C40 40 Amp Solar Charge Controller

    Connect Solar array (2 series x 2 parallel strings) -> charge controller -> Battery bank -> AC inverter -> AC loads

    There is one other option for you--If these type of controllers are easier for you to find. Wind turbines use "Shunt/Diversion/Dump" controllers. You connect the unregulated charging source to the battery bank (wind turbine, or in your case your 2x2 250 watt solar panels). And then get a Shunt Controller---Basically a PWM controller that turns "on" when the battery voltage > 29 volts and turns "off" when the battery voltage falls below 28 volts (for example). And connected to the "output" of the shunt controller is a resistor/heater bank (dumping waste heat to air or water, etc.).

    So, in this case, the connection is:

    Solar array -> Battery bank -> Shunt Controller -> Dump load

    And the AC inverter is also connected to the battery bank.

    In this case, you only need a 24 volt shunt controller. Some PWM Solar Charge Controllers can be re-configured to operate as a Shunt Controller. The C40 is one of those that can be a PWM series or set up as a "diversion" charge controller.

    Shunt controller systems are "less desirable" and the battery charging is "less ideal".

    All three of the above systems will work, with the second two PWM systems "wasting" more of the solar array's output power because of the "poor match" between Vmp-array and Vbatt-charging.

    Does this help/make sense?


    As Marc said--It is possible to design/build a "boost" charge controller that can increase battery charging voltage higher than the solar array input voltage--But there is only one commercial solar charge controller that I am aware of and it is a smaller unit (not sure it does 24 volts either).

    In the end "buck" (down converting) charge controller are cheaper and more efficient. A "boost" (up converting) controller with the same capabilities would usually cost more.

    So--it is easier to put two panels in series (can save you money as you need less copper wire from the array to the battery charger--higher voltage, less current, less copper wire diameter to carry less current).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Is Charge Controller Necessary?

    Or in short form; a PWM type controller isn't going to work here.
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