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  • Offgrid caravan battery query

    Hi All

    I have just finished my modest installation on my caravan in Orihuela Spain.

    The setup is as follows:

    3 x 140w poly PV panels wired in series giving approx 62v (OC) and 8amps
    Tracer-4210RN 40 amp MPPT controller with remote display
    1500w (3000w peak) pure sine wave inverter 12v
    2 x 250amp/h c100 lead acid batteries wired together in parallel hopefully giving 500amp/h and 12v

    I've used earth spikes, a DC isolator for the PV, fuse for PV to controller, fuse for batteries to inverter. I also have a standard electricity meter to measure the kw/h consumption.

    Here in southern Spain and with a great orientation I am getting loads of productive sun. In the books it says 5 hours for this time of year but I'm getting way more than that, almost 7!

    All of the kit is brand new. Initially the batteries were charged quickly as I had only LEDS and an occasional 12v internal pump for the kitchen and bathroom taps plus 2 or 3 minutes of a 400 watt microwave once every 2 or 3 days.

    I connected a small medium sized (for US readers a micro fridge ) fridge with small freezer box. The fridge runs at about 150w at full power and 80-100 at the normal cold setting. Obviously its hot here so during the day the motor is kicking in fairly regularly but at night it is quite dormant on a low setting. The first 24 hours including starting the fridge from warm our total consumption was 1.8kw for the caravan.

    If I monitor the visual for the battery charge it seems to contradict the battery voltage. For example, last night at 9 oclock there was almost no charging input and I put a voltmeter across the 2 batteries and got 12.5v. This morning at 8 am the actual battery voltage was 12.4/5. To me that suggests I perhaps don't have a problem???? Trouble is the graphic on the Tracer remote display is all over the shop sometimes 40% sometimes 60% but never the 80% + that 12.5v should be?

    Not everyone buys the remote display, shall I ignore it and keep physically measuring the voltage on the batteries when not being charged nor under load?

    The inverter has low voltage protection so theroretically............I shouldn't be able to cycle my batteries more than 80%.........

    Thanks in advance

  • #2
    Battery voltages are pretty much useless to determine State of Charge Voltages (SOC) as they only apply to a fully rested open circuit voltage. When under load or charge the voltage swill be much higher on charge up to 14.4 volts, and slightly lower on discharge depending on the amount of discharge current. You do have some significant mismatches in your system.

    A 12 volt 1500 watt invereter requires a 12 volt 1000 AH battery.
    A 12 volt 500 AH battery requires a minimum 550 watts of solar panels.

    As long as you never load up the Inverter to full power you can get away with it, but your system sounds like it was just thrown together without any thought given to matching components. Working backwards from the the Inverter of 1500 watts would be a 24 volt 500 AH Battery, 60 amp MPPT Controller, 1500 Panel watts.

    Why did you opt for 12 volts? A 12 volt 1500 Watt Inverter requires a 150 amp circuit which is real dangerous.

    MSEE, PE

    Comment


    • #3
      Originally posted by Sunking View Post
      Battery voltages are pretty much useless to determine State of Charge Voltages (SOC) as they only apply to a fully rested open circuit voltage. When under load or charge the voltage swill be much higher on charge up to 14.4 volts, and slightly lower on discharge depending on the amount of discharge current. You do have some significant mismatches in your system.

      A 12 volt 1500 watt invereter requires a 12 volt 1000 AH battery.
      A 12 volt 500 AH battery requires a minimum 550 watts of solar panels.

      As long as you never load up the Inverter to full power you can get away with it, but your system sounds like it was just thrown together without any thought given to matching components. Working backwards from the the Inverter of 1500 watts would be a 24 volt 500 AH Battery, 60 amp MPPT Controller, 1500 Panel watts.

      Why did you opt for 12 volts? A 12 volt 1500 Watt Inverter requires a 150 amp circuit which is real dangerous.

      Thanks for the prompt response.
      I have an isolator on the PV and a 125amp fuse that I can open to give me a more accurate reading of the actual battery voltage?
      The inverter size was selected to provide some headroom, I could have done with a 1000 or 1200 but thought going slightly oversize would be better than a bit tight?
      The 550w minimum panel size surely depends on your geographical location and uninterrupted access to E S W arc? I'm from the North of England and would be struggling to get 3 good productive hours of solar insolation. I live in southern Spain and as of today I get 7+ with 320 days of cloud free skies. (Southern California) I used 5 hours x 420w when I was planning it but I knew I would be getting more sun than that.
      I got a good price for the inverter but going to 24v and 500amp/h is an option. Why a 60 amp controller? I can see your planning logic working backwards but don't you think for a small caravan with LED lights, a small fridge, occasional (a few seconds a day) 12v water pump and 2 or 3 minutes every couple of days on a 400w microwave 1500 panel watts (5 x 300w panels) is a bit unrealistic and enormously expensive?
      Given my time again I would go to 24v to reduce the cable sizes, they were very hard to work with.

      Hope it makes sense, I'm just a humble newby. Thanks

      Comment


      • #4
        Minimal Panel wattage is determined by th ebattery minimum charge current requirements to keep it from Stratifying. For FLA batteries that is typically C/12. For a 500 AH battery that is 500 AH / 12 H = 41.2 amps. Maximum is C/8 typically and for 500 AH / 8 H = 62.5 amps. So for a 12 volt battery using MPPT controller is 494 watts to 750 watts. Ideal is C/10 or 50 amps @ 600 watts.
        MSEE, PE

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