Amps not adding up: 2 panels in parallel

eatmorespinacheatmorespinach Registered Users Posts: 2
edited July 2019 in Solar Beginners Corner #1

Happy Fourth of July!

I'm struggling to get my two 175w 12V Newpowa solar panels set up in parallel and read correctly by my 40A MPPT smart controller :'( .

tl:dr - How come my amps aren't around 18A with two 9.18A panels?

Is it the panels or cables? - I wired the two panels together in parallel. We tested them with a volt-meter. They produced the proper volts (around 18 volts), but amps were NOT added together as they should be with two panels. We only got around 8 amps total when testing the two panels in Parallel.

We then tested wiring them together in series, it produced 37 volts (normal), but for amps, it produced 18 amps, double the number of expected amps, which doesn't make sense as amps aren't multiplied in series wiring.

Both panels, when tested individually, produced the proper readings. When we wired to the smart controller, it only reads 8 amps too when two panels were wired in parallel under direct sun.

Maybe it's the Smart Controller? -  My smart controller says 24V for "Rate.Vol (image below)?" What does that mean? Shouldn't it be 12V since my panels are 12V?

Thank you! I will drive out to whomever can help with my converted van and shower you in cookies and beer!

Thanks, Drew


  • mcgivormcgivor Solar Expert Posts: 3,431 ✭✭✭✭✭
    When the panels are in series tha voltage doubles, current measured on the PV wiring side remains the same. The current on the output however will increase, this is what MPPT controllers are supposed to do, convert excess voltage to current. So if you are reading the current value on the controller display, that would be the battery charging current, not the PV output current.  
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • BB.BB. Super Moderators, Administrators Posts: 30,715 admin
    What is your battery bank voltage?

    Very roughly, on a cool/clear day, near noon (solar panels directly facing sun), and battery bank less than 80% State of Charge, the typical (more or less average best harvest):
    • 2 * 175 Watt panels * 0.77 panel+controller deratings (real life, not marketing specs) = ~270 Watt "average best harvest"
    • Power = Voltage * current
    • Assuming you have a 12 volt battery bank
    • Current = Power / Voltage = 270 Watts / 13.8 volts (charging a discharged battery bank) = 19.6 Amps into battery
    Notice that with MPPT charge controllers, the actual current into the battery bank (i.e., "dead battery" at 12 volts vs a battery approaching full at ~14.75 volts (absorb stage charging--steadily declining charging current).

    And, for an MPPT charge controller, on a 12 volt battery bank, you really want two "12 volt' (Vmp~18 volt) panels in series for Vmp-array ~ 35-40 volts.

    Placing the two panels in parallel (Vmp-array~18 volts) charging a 12 volt battery bank, MPPT controllers are not at their most efficient and you will probably get less than "optimal" charging current.

    Vmp-array falls as the array gets hot under full sun... And your Vmp-std of ~19 volts can fall to 80% of Vmp-std on hot/sunny days or ~15.2 volts Vmp-hot. You want to charge the battery bank at ~14.75 volts and need a few volts drop through wiring and MPPT charge controller... You need more "voltage headroom" for the MPPT charge controller to operate optimally.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • eatmorespinacheatmorespinach Registered Users Posts: 2
    Thanks @BB. and @mcgivor ,

    I mis-typed my description. - In Parallel wiring, the amps were NOT added together as they should be with two panels. We only got around 8 amps total when testing the two panels in Parallel (I've updated my post to reflect this) - Is this the problem?

    @Bill - I think my battery bank voltage 12v? Here are my 2 batteries I have

    Here's an image of what is displayed in pretty-much full sun with 2 175w panels wired in parallel. Does this seem accurate? Should 2 175w panels be able to power my 12v dometic fridge?

  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    With the battery at 14.7v it's likely in the latter (absorb) stage of charging, and will only take as much current as needed to hold that voltage.

    Small fridges can be remarkably high power consumers.  Whether 2 panels would do it depends on lots of things (ambient temp, door openings, available sun, etc), but it's probably marginal.
    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
  • PhotowhitPhotowhit Solar Expert Posts: 5,242 ✭✭✭✭✭
    You can test @Estragon 's theory, by adding loads until the voltage drops below 14.7, then you should see what total power is available.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do
  • PhotowhitPhotowhit Solar Expert Posts: 5,242 ✭✭✭✭✭
    Here's a basic description of how charging works;

    The voltage you are seeing is the system voltage and not the battery voltage. If you are connected to charging or a load it will effect the system voltage.

    During charging, there are basically 3 stages of charging, Bulk, Absorb, and Float.

    First thing when charging starts you will be in bulk, the voltage rises from what ever the system voltage was to a set point, around 14.5 volts. At that point the Charge controller stops the voltage from rising. Higher voltage can damage sealed batteries.

    Once the battery hits the preset point the charge controller keeps it at that point. Your batteries are roughly 80% full. Flooded batteries will start accepting less current at 80-85% full AGM/Sealed may go a little longer before accepting less current.
    On many controllers you can set this point, Some will have different presets for Flooded, and sealed batteries, or flooded, AGM, and sealed batteries.

    The charge controller has a couple ways to know when to switch to float, Most inexpensive Charge controller are just timed for 1.5-2 hours. Some will also see less current flowing through the charge controller and shut it down when minimal current is flowing through the controller. On more expensive charge controller. You can set battery capacity to give the Controller a better idea of when to stop. you can also set a longer Absorb time. Or set 'end amps' a amount of amps flowing through the charge controller to stop Absorb and switch to the final stage.

    Once the Controller has determined the battery is fully charged it reduces the voltage to a point where very little current is flowing to the battery. This will prevent the battery from over charging and heating up.

    While in 'Float' the charge controller watch for voltage drop, which would indicate a load. If the voltage begins to drop the charge controller will allow as much current to flow from the panels/array to compensate and maintain the voltage. If the voltage can be maintained, the load will in essence be running directly off the array/solar. If the voltage drops below the preset float voltage, the controller may start a whole new cycle if it stays there for a period of time.

    The system voltage drop you see at night when the sun goes down is the charge controller moving into a resting mode with no energy to contribute to the system.

    The morning voltage may reflect a load present that is effecting the voltage level. With sealed batteries, you would want to disconnect the battery from the system and allow it to 'rest' for a while to get an accurate idea of it's SOC (Sate Of Charge) from the voltage
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do
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