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Off grid system

Discussion in 'Electrics UK' started by Suited72, 5 May 2015.

  1. Suited72

    Suited72

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    I need someone with battery or solar off grid experience to fault find on an off grid system in Leicestershire area. Any recommendations of people or help appreciated. Trouble is I cannot find a trade who are experienced in this. the system was fitted by a solar electrician but he did make it clear to me that he wasn't familiar with off grid and would only be able to wire it up.

    4000w off grid smart inverter (8000w peak)
    4 x 105ah marine lead acid batteries in series 48v 420ah
    2 x 250w sharp solar panels used as chargers.

    It is all brand new.

    Thought it was working. Inverter showing everything is fine. Batteries get to fully charged state. Everything works but there seems to be a fault.

    At a fully charged state if I switch on the oven (draw is 55amp 2.4kw) the batteries last 30 seconds! The voltage on the inverter shows 51.9v and drops off very quickly. At 46v the inverter battery protection kicks in and shuts the system off.

    The batteries then show as low, but mysteriously end up back to fully charged again within 20 minutes?
     
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  3. Jackrae

    Jackrae

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    your 48v battery system has only 105AH capacity (you add volts , not AH for batteries in series)

    Are the batteries "cranking" or "deep discharge" type

    You do not specify the rated period of the 105AH, and it may be C20 (based on 5A discharge for 20 hours)

    So, if you are drawing 55A which is C2 (capacity discharge in 2 hours) their actual capacity will be no more than about 10 to 20% of the C20 figure

    Your system is recognising what is happening - your discharge is way in excess of the battery capability and the damage you have done has increased their resistance so lowering capacity even further.

    You could do some simple load tests to evaluate what is happening to them.

    Get hold of some 50W halogen lamps and connect one across each fully charged 12 volt battery. This will load the battery with somewhere around 4 amps. Now monitor individual battery voltage until their voltage falls to no lower than 11 volts and write down discharge period and volts at say 1/2hour intervals. Your batteries should last in excess of 24 hours but I suspect, if they are goosed, you'll reach 11 volts on at least one of them long before then.
     
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  4. Suited72

    Suited72

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    Hi. Thank you for that. I bought 4 of the following

    https://www.tayna.co.uk/Leisure-Battery-Powerline-110-CaravanLeisureMarine-Battery-P7714.html

    Doesn't give a c rating, and as they are the cheapest i assume you are correct. I think I'm starting to realise why caravans have gas ovens. The system was working very well with low power items until I put a Hoover or an oven through them.
     
  5. Jackrae

    Jackrae

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    You said your were 105AH but the ones you linked to are shown as 110AH. Irrespective, if they are leisure type then almost certainly they are C20 and not suitable for cooking the turkey. OK for lights etc but not heavy loads.
     
  6. leeco

    leeco

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    I have setup an off grid system on my boat and a couple of others.

    These are the best type battery's to use they are deep cycle ones.

    http://www.trojanbattery.com/reliantagm/

    It sounds like you are drawing too much power out of the bank and the inverter switches off.

    My inverter does it when my batteries are full and it's not sunny when I use my washer.
     
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  7. ericmark

    ericmark

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    I would tend to agree however a motor mover draws between 20 and 30 amp and battery required is 105 to 110 AH so there seems to be some other factor as well. I Seem to remember normal cut off voltage is around the 11.5 volt mark for 12 volt battery and so for the 4 should be around 46 volt.

    Clearly you need to double up on battery size but I wonder if they are in fact fully charged.

    For a lead acid battery we have a range of charging voltages. 13.2 volt is considered float but where the float charger is switched on and off then this is raised so with a car 13.8 volt more common. With a stage charger often it will go to 14.8 volt until the current drops to around 4 amp.

    But with solar panels the pulse charge is the more common method. These measure the battery volts between pulses to work out how large a pulse is required. The problem with stage chargers is drawing power while on charge can make it over charge battery.

    The solar panel charger is rather a clever unit it allows the panel voltage to vary to get maximum watts. It uses this to charge a capacitor then turns it into high frequency AC and transforms it to the voltage required for the battery at start if gives maximum output until the voltage between pulses reaches a set level when it starts to reduce the charge.

    As with any complex system it can go wrong and the first thing is to return to basics forget the voltmeter use a hydrometer they rarely lie. You are looking for three things.

    Around 1.07 is a flat battery (10%) and 1.27 a fully charged I say about as the acid used in hot countries vary from cold counties so fully charged could be 1.25 or 1.32 but it is still a better gauge to using a volt meter for a 48 volt battery 46 volt is 10% and 50.96 volt 100% open circuit voltage but needs an hour disconnected before these are true measurements. Where the hydrometer can be used straight away (with temperature correction).

    But as well as the single reading you can compare the cells. So a battery with uneven cells is highlighted.

    The last is colour damaged cells often have brown electrolyte.

    Although the battery is too small I would guess there is also either a faulty cell or they are not fully charged.
     
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  9. leeco

    leeco

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    It's not the motor that kills it it's the heating element but when it's sunny my inverter can handle it.

    On all the set ups it have done I have always fitted this solar controller.

    http://www.bimblesolar.com/offgrid/mppt/outbackfm80

    They cost a lot but they are one of the best types you can get and can equalise the batteries too which keep them in good shape.
     
  10. ban-all-sheds

    ban-all-sheds

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    So WT* did you use him?
     
  11. Suited72

    Suited72

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    Thanks, Anywhere particular to buy these?
     
  12. Suited72

    Suited72

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    Couldn't find any electricians that had installed off grid.
     
  13. leeco

    leeco

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    You can buy them on ebay mate search Trojan battery.

    Mine are all 6v but I'm running them at 12v.
     
  14. Jackrae

    Jackrae

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    55a for say a mean time of 1 hour =55AH ( 2 hours at 50:50 on/off heating) Pulling capacity to no lower than 50% means you might seem to get away with 110AH batteries, but that's at C2 rating. You really need to look at no lower than C5 rating so add at least 20% to capacity and you are now looking at around 130AH batteries (all based on no more than 1 hour of steady load) If you want to cook for longer then you need to increase AH rating accordingly

    Whatever you go for make sure they are "deep discharge" specified
     
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