New system, anyone feel like checking my math?

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unyalli
unyalli Solar Expert Posts: 121 ✭✭
in Solar Beginners Corner #1
Sunsaver MPPT 15L
2 group 24 marine flooded
2 solarland 90 watt in series
Samlex Pure Sine 300 watt
Old HP laptop

I ran the batteries down to 11.8 volts last night with the Inverter/Laptop load of 1.6 amps.

It's full cloud out there right now and in MSView I'm seeing 22 watts array power at 34 volts. 22 / 34 = 0.62 amps.
I'm showing battery voltage at 12.1 volts, charge current at 1.7 amps and load current at 1.6 amps.

With a battery voltage of 12.1 shouldn't my charge + load currents equal somewhere around 7 amps?

12.1 * 0,6 = 7.26 amps.

Thanks, Jeff
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: New system, anyone feel like checking my math?

    I don't know how you got separate charging and load currents. Much depends on how and where Voltage and current measurements are made.

    Power going in to controller: 34 Volts * 0.64 Amps = 22 Watts
    Power coming out of controller: 12.1 Volts * 1.7 Amps = 20.57 Watts

    Considering the varying accuracy of the method of measurement and losses in controller + wiring it looks about right.

    If the charge and load currents are separate then the total output from the controller would be the two added together: 3.3 Amps * 12.1 Volts = 39.9 Watts. Since it is impossible to get more power out of the controller than goes in, I'd say that's not the way the readings are being taken.
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  • unyalli
    unyalli Solar Expert Posts: 121 ✭✭
    #3
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    Re: New system, anyone feel like checking my math?

    I see the mistake, I have to remember the morningstar boast, watts in will equal watts out. 22 watts in at 34 volts is around .6 amps. 22 watts out at 12.1 volts is around 1.8 amps.

    I'm measuring inside the controller via msview and serial cable.

    Morningstar's help file states the charge current is the current fed to the battery bank and the load current is the current fed to loads. I'm guessing this is not true. The charge current is the total current at output battery voltage being produced by the MPPT circuit. The load current is subtracted from this to see whats going into the battery.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: New system, anyone feel like checking my math?

    They mean loads attached to the LOAD terminals. If the loads are attached to the battery terminals they won't read separately. As it is I'm not too sure how accurate the difference is on the MPPT 15. Probably not complex enough internally to discern between the two. Same software is used on the MPPT 45 and 60, which are far more elaborate controllers.
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  • unyalli
    unyalli Solar Expert Posts: 121 ✭✭
    #5
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    Re: New system, anyone feel like checking my math?
    Cariboocoot wrote: »
    They mean loads attached to the LOAD terminals. If the loads are attached to the battery terminals they won't read separately.
    I understand this. My inverter is connected to the load screws and the battery to the battery screws.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: New system, anyone feel like checking my math?
    unyalli wrote: »
    I understand this. My inverter is connected to the load screws and the battery to the battery screws.

    Unless your inverter is <150 Watts take it off the LOAD terminals. They can only handle the maximum current of the charge controller: 15 Amps. This is meant for light DC loads, not powering an inverter.
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