Inverter Problems

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Solarsurfer
Solarsurfer Registered Users Posts: 2 ✭
in Solar Beginners Corner #1
Hey Guys & Girls,
I'm pretty new to this solar game, apart from fitting out my VW campervan which works great, so can I ask for some advice.
I have launched into going off-grid in our house, and have installed 1600w of solar panels (so far), a 400ah battery bank (so far) along with two 40amp solar charge controllers.
I have a 12v to 240v inverter/charger (sorry I forgot to say I am UK based, but the theory is the same).
The inverter/charger can be connected to the incoming mains to run the charger when it detects the batteries are low, but as I said I am trying to go 'Off-grid' so it is only connected to the battery bank which is charged via the solar. The solar is working fine at charging the batteries, even though we are suffering from our early year grey skies at present.
If I change over to the inverter from the mains (Grid), the base current draw is between 2.5 & 3 amps, fridge/freezer, heating boiler (gas fired), LED lights (if they're on) and a couple of laptop chargers plus maybe the kitchen radio.
The inverter runs fine, but, if I try to use anything with a reasonable current draw, even if this is a resistive load of only 1200w, the inverter shuts down immediately, then starts up again. This continues until the additional load has been turned off, then the inverter returns to its happy state running the 2/3 amps.
The inverter is rated at 4000w, 9000w surge and is pure sine wave. It has overload protection, battery under voltage protection, battery supply over voltage protection, and more.
The 100ah batteries are LiFepo and connected in parallel, the terminal size is 8mm (5/16"), which roughly equates to a cross sectional area of 32 square mm.
The battery parallel cables are 32 square mm (16x2), and the inverter supply cables from the battery bus bar are also 32 square mm (16x2). Each 16mm cable supplying the inverter is protected by a 100a dc circuit breaker (2x100), each cable from the solar charge controllers to the battery bus bar is protected by a 40a dc circuit breaker.
When doing an over 2/3a test, as described above, none of the circuit breakers trip, and there is no dip in the battery bank voltage.
I am trying to return the inverter under warranty, as I think it is the inverter that is at fault, and in fact I video'd the inverter display while the wife turned on the load downstairs. As soon as she turned on the load, for a fraction of a second, and I mean a fraction of a second, the display showed the 'overload' warning and then the inverter turned off.
The manual says, if the light is flashing there is a fault, if the light is a constant on, there is a fault within the inverter.
My question to all you Solar Guru's is, as the light appears to be constant, albeit for a fraction of a second, would I be right in agreeing with the manual and that the problem is within the inverter?
Kindest thanks to anyone who manages to get to this point.
Regards
Solar Surfer
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,443 admin
    #2
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    Welcome to the forum Solarsurfer,

    Do you have a volt meter you can measure the voltage on the battery terminals and on the 12 VDC input terminals of the AC inverter?

    And how long are the 32mm^2 (roughly 2 AWG) cables from the battery bank to the DC input of the inverter?

    You have a 4x (?) of 100 AH Li Ion batteries? What is the output current rating (guessing around 100 Amps max per battery)?

    I do not know if your inverter is bad--But I suspect you are expecting more current than your battery bank + DC wiring can provide...

    I am in the US, so I will be using feet and AWG--But it should be pretty clear and easy to understand from your side of the pond too.

    When working with solar power--We have to be sure to understand that 3 amps @ 230 VAC (I assume this is your home "base load") converters to @ 12 volt battery bus current of:
    • 3 amps * 230 VAC = 690 Watts
    • 690 Watts * 1/0.85 AC inverter eff * 1/10.5 volts inverter battery cutoff voltage = 77 Amps @ 12 VDC bus (sort of "worse case" current)
    32mm^2 wiring converts to ~2 AWG cable in the US:
    https://www.multicable.com/resources/reference-data/cross-reference-awg-to-mm2/

    And using are National Electric Code standards (for house wiring), 2 AWG is rated (in conduit) for around:

    https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

    95 to 130 Amps (depending on insulation temperature ratings, ambient temperatures, etc.)....

    Assuming ~115 Amp rated wiring, the maximum suggested AC load for your setup would be:
    • 115 Amps * 0.85 AC inverter efficiency * 10.5 VDC battery cutoff voltage = 1,026 Watts
    Your 4x 100 AH Lithium batteries may be able to output 100 Amps maximum each, or 400 Amps total--But your DC wiring is not capable of "safely" carrying that current--Note, this is a "conservative" current estimate--Exposed high temperature cable (not in conduit) can carry more current--But you are getting into a potential hazard zone--You need the exact ratings for your cable insulation, and electric codes for your country--Insurance companies (in the US) may not cover a house fire if it started in the solar power system.

    There is also another possible issue... That is the voltage drop of your 12 VDC wiring from the battery bank to the DC input of your inverter... For a 12 volt system, suggest 0.5 volt maximum drop from battery to DC inverter input. That gives us 11.5 volts for battery under load, -0.5 volt drop for wiring (and fuses/breakers), and 11.0 volts to inverter input... And allows 2x current (surge) to 10.5 volts at inverter, or the (typical) 12 volt inverter battery cutoff voltage...

    Say you are trying to draw 115 Amp DC, and you have 2 AWG cable that is 4 feet (one way cable run for this calculator) from battery bank to inverter... Using a simple voltage drop calculator we get:
    https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=7&necconduit=pvc&necpf=0.99&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=12&phase=dc&noofconductor=1&distance=4&distanceunit=feet&amperes=115&x=69&y=18&ctype=nec

    Result

    Voltage drop: 0.17
    Voltage drop percentage: 1.45%
    Voltage at the end: 11.83

    Or 0.17 volt drop... Which is "OK" for 115 amps or ~1,000 Watts of invert load... But if you have longer cable runs, and/or try to pull more current, the voltage drop will increase (2x more current, 2x more voltage drop).

    This is an "issue" with 12 volt solar power systems... They are "difficult" to wire (i.e., lots of heavy, thick, stiff, expensive copper cable) and keep voltage drop low enough... Typically suggest that a 12 VDC system be limited to a maximum of 1,200 to 1,800 Watts... People do design larger systems--But it is better to go with higher DC battery bus voltage... 24 or 48 VDC is typical for larger systems. For a home system, suggest 48 VDC.

    But this can get into issues with your Li Ion batteries and their BMS (Battery Monitoring System) --- If there are any in your system. You have to make sure you can put 4 batteries in series, safely, to operate at 48 VDC (upwards of 60 VDC when charging)...

    Anyway... We are dissecting your power system from the middle of a "running" system. I highly suggest that we start from the beginning... What are your daily loads (max Watts, and Watt*Hours per day). That drives the battery bank sizing (voltage and Amp*Hours)... Then design the solar array to keep your battery bank charged and meet your daily load requirements... Then pick an AC inverter that works with your battery bank and your AC loads...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Solarsurfer
    Solarsurfer Registered Users Posts: 2 ✭
    #3
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    Thanks for the info Bill, I'll do some cable length measuring and some voltage readings at various places and get back to you. Bob
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