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I am new to solar panel system set ups, and I'm trying to make an off-grid DC-DC solar panel system where in I will be running a bunch of 12v loads which are mainly DC Pumps cause its gonna be something like a waste water management and recycling system.

Then, I saw this guy online selling his solar panel set up for a cheap price and he is near to my area, and so I decided to grab it.

Here are the specs of the solar panel system that I just bought:

  • 310W Solar Panel

  • 40A MPPT (ML2440) SRNE Solar Charge Controller

  • 150AH Sealed Lead Acid Battery

  • 1000W Pure Sine Wave Toroidal Inverter

And just before I bought it, I thought that the "40A" in the 40A MPPT SRNE Solar Charge Controller stands for the output the SCC(solar charge controller) can give from its load terminals. And now that I have it and have read the manual, I realized that its Max Discharging current is 20A only.. (or maybe I'm wrong.. please correct me, I'm just googling everything and watching YouTube videos)

Going straight to my problem number 1: I have calculated my estimated load, and it is around 23Amps.. I am thinking, is it safe to connect a buck converter to the battery bank directly in order to get more amps for my project?? because the SCC can only give 20A, and I don't wanna compromise the performance of my loads.. cause I have 5 loads to power, and I thought, if 4 of my loads sum up to 19A, I'll power those 19Amps with the power from the load terminals of the SCC, and then for the 5th load, I will just connect it directly to the battery bank and just use a XL4015 buck converter to regulate the voltage if its too high.. Is it safe to do so?? If not, how can I increase the 20A to 23Amps with the safest way possible.

my question number 2: If a load's specification let's say needs 6A, does it really use up 6A all the time? I know that you can run a 6A load with a 10A power supply but not the other way around, but I am genuinely confused, here's the story why.

When I met up with the guy to whom I bought my solar panel set up, I talked to him (I just assumed that he's an electronics and electrical stuff expert) about running some 12V DC motors and Arduino on a sealed lead acid battery that goes to 14.4V to a little less than 15V when fully charged.

I asked him if it is safe for devices to run them on voltages a little bit higher than its requirement of the load

Example: Running a 12V Pump with a battery voltage of 14V (lets just assume the sun is like really really bright for most of the day time)

and how do I increase the Load Current Capacity just incase I need to add more load to my project..

The guy then weirdly took the 12V DC Pump I was holding that time and then tested it directly to the load terminals of the Solar Charge Controller, please note that the battery voltage at that time was 14.4 volts, and he said,

"Just because it says 6Amps on the item's specification doesn't mean it takes 6A constantly"

I've read about overvoltage and that how dangerous it is, but I am now weirdly confused AF because of that guy's answer to my question to him. I've been googling all day and watching YouTube videos about my question but I can't seem to find a good answer to it.

Somebody please enlighten me and correct me. (T__T)

I might be wrong, I'm really sorry, I'm just trying to build this project idea on my own and I thought I could get some answers to my questions from the experts here. Any form of comment or suggestion and corrections to me will be highly appreciated.

Thanks! :)

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  • \$\begingroup\$ You CAN connect the load directly to the battery. And you pretty much MUST if it's a 1000W inverter. Then you can't switch it via the controller though, so an isolator switch and a fuse or breaker are your responsibility. \$\endgroup\$
    – user16324
    Commented Oct 29, 2020 at 12:27
  • \$\begingroup\$ thank you @BrianDrummond , I won't be using an inverter cause all my loads are DC. \$\endgroup\$
    – princessbubbles15
    Commented Oct 29, 2020 at 12:55
  • \$\begingroup\$ Separate buck converter should be OK. || Pumps will probably draw far more than rated current at startup. || 6A rated motor will PROBABLY draw ABOUT 6A at rated full load and less under lighter loads || 14.4V on a 12V motor for short periods is PROBABLY safe and in some cases long term is OK. Varies with brand, design, load etc. || What 'the guy' did is probably OK enough. \$\endgroup\$
    – Russell McMahon ♦
    Commented Nov 9, 2020 at 20:32
  • \$\begingroup\$ @RussellMcMahon, thanks for your explanation and confirmation, i now understand it much better :) \$\endgroup\$
    – princessbubbles15
    Commented Nov 9, 2020 at 23:00

1 Answer 1

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enter image description here

DC motors have a start current 8~12x the rated current, so a SMPS is a bad idea. If you went direct off the battery, that will support the power you need for some Ah period but you cannot then determine the cutoff current threshold to the battery as it will be also going to the load instead and you may end up drying out the electrolyte with Vch driving the load so your load current, start-stop surges, duration and charge accumulation is no longer balanced with external loads.

The SRNE2440 supports 12V/24V batteries with 40A charge and 20A loads.

You must define your pump motor DCR, Battery specs, regional Solarity power cycles and then a better idea of a direct solution is possible perhaps with soft start ICL's added to motors. Then include all this with a block diagram and power balance list on how you can manage to regulate preventing extended CV durations with external loads and under-voltage protection without the sun.

Otherwise you are limit to 20A loads.

Other REF (TL;DR)

AU EN translated https://www.rvpartsexpress.com.au/mpattachments/file/viewonline/id/64/product_id/1850/

https://lithium-car-battery.3dcartstores.com/assets/images/ML4860%20User%20Manual.pdf

http://www.srnesolar.com/Private/Files/637033792260708743146914845.pdf slow

https://azimuthsolar.ca/wp-content/uploads/2019/05/MC-SERIES-SRNE-MPPT-MANUAL-V1.01.pdf

http://www.srnesolar.com/Private/Files/6370337829631198461661594389.pdf

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  • \$\begingroup\$ thank you so much, i'll just find another way around my project and remove some of my load \$\endgroup\$
    – princessbubbles15
    Commented Oct 29, 2020 at 15:41
  • \$\begingroup\$ What way was that? \$\endgroup\$
    – D.A.S.
    Commented Oct 29, 2020 at 18:06
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 Can you provide a link to the source of the image? SE converts external links to imgur links and thus destroys citation paths \$\endgroup\$
    – Russell McMahon ♦
    Commented Nov 10, 2020 at 5:22
  • \$\begingroup\$ It was harder to find this time, I must have used Google instead of Duck-Duck-Go this time. You can look if you try. \$\endgroup\$
    – D.A.S.
    Commented Nov 10, 2020 at 6:29

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