Help with 12 volt requiements

Discussion in 'The Beginner' started by spiritsfree, Mar 15, 2010.

  1. spiritsfree

    spiritsfree Read Only Funster

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    I`m not totally thick, but where electricity is concerned i have to admit, although i am trying, i`m not grasping it.

    My question, bearing in mind I wild camp, is about what i need to power 2 x laptop, a TV...and the occasional battery chargers......and how i can understand what power they use...and for how long ?

    I am in the process of upgrading from a Pilote Galaxy 75......to an A Class Dethleffs.....and want to get it right.....as i have a solar panel on the Pilote, and we generally head to Scotland early Spring and Late Autumn (for photography)....and most days find the inverter is "screaming at us" after say 4 hours of TV ( not 12 volt ). In trying to get it right this time, I`d like to add what is necessary to the new motorhome in order that we have peace of mind...and relaxation.

    I`m going to get a couple of "ciggy" connection adapters for the Pc`s...and maybe a 12 volt TV, as well as a second leisure battery.

    Now the man at the shop said that if the leisure batteries were running low, I could switch on the engine for say an hour and recharge the batteries......also that as we move around Scotland it will charge as we are travelling, (generally we`ve only stayed in one place for 3 days ).

    Obviously weather wise, Scotland is not the place and at that time of year, where solar panels are effective. This time I`d like any advice, in order to get it right, as to what I need to put our minds at rest. Our normal pattern is to wild camp for 6 days, and then stay over on a camp site, if that helps.

    I hope I`ve explained my concerns in a simple way, for what to me is unknown territory, and do`nt come across as a complete dork !! :Doh:

    Any advice , no matter how big or small, will be very much appreciated :BigGrin:
     
  2. pappajohn

    pappajohn Funster Life Member

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    if you look on your appliances there should be a data label with the wattage on it.

    add them all together then divide the watts by 12 will give you the amps used per hour.
    IE: 120watts divided by 12v = 10amps
    even though you use an invertor its still pulling 12v from the battery.

    an invertor is only around 85 to 90% efficient so if the stuff adds up to 10amps then the invertor will take around 11amps from the battery.

    a second battery will double your running time but will also need twice as long to recharge.

    running the engine will recharge the battery but it will need a lot longer than an hour.

    if you do proper wildcamping, as opposed to layby's and carparks, then a genny would be my first choice.

    more or larger solar panels will help but to replace the 10amps used you will need at least a couple of 120watt panels on a sunny day.

    unless your lappy's are 12v (mines 15v..many are 19v) you will need special power supplies (DC to DC convertors) to run off a 12v socket
     
  3. pappajohn

    pappajohn Funster Life Member

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    just a thought on your tv.....is it purely 230v or does it have a 230v to 12v power transformer (black box) in the mains lead ?
     
  4. JeanLuc

    JeanLuc Funster

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    To answer your questions, you need to calculate the daily usage of 12V electricity. Attached is a spreadsheet (it's in a zip file as xls is not an allowed format for attachments). First unzip the file and open with Excel (or similar). You will see a list of possible items on the left. You can edit these to suit your own needs. Then in the first green column B, input the wattage for each item. In green columns D and E input your estimated daily usage - there is a summer and winter option. Don't touch any of the red cells as these contain formulae.
    You can change the blue cells if you wish. The Truma current use is taken from the manual and represents the maximum usage in winter. In all probability, it will be a bit less. You then need to change the green Battery Capacity cell under the heading "Max" to match the Ah capacity of your battery(ies). Also, you may change the 80% "Usable" figure in the blue cell if you wish. Some people do not like to exhaust their batteries below 50% - I use 80% as Elecsols are supposed to be capable of taking deep discharge rates.

    You will then see how many Ah you will draw in a day, and how many days you can exist without any charging.

    Regarding the use of an inverter, leave out the 230V items that will be powered by it from the list in the spreadsheet. Then for the inverter-driven items, take the total Watts / 12V / 0.8 to estimate the current drawn by the inverter. The 0.8 is a factor to estimate inverter inefficiency - 80%. It can be better - say 85% or perhaps even 90%, but I would play safe and stick with the 0.8 factor.

    So, a 40W mains-powered TV would draw 40 / 12 / 0.8 = 4.17 amps, which, if used for 4 hours would take 16.7 Ah out of the battery.

    Once you have worked out your total inverter usage, you can write it in the spreadsheet below the last item in my column of appliances. Just put the total current draw in column C as per the entry for the Truma above. It will then be included in the total calculations.

    Finally, having worked all that out, you will see how much extra charge you need to get back into your batteries. And by the way, it is not generally regarded as good practice to sit with the engine running at idle to re-charge. Apart from the diesel consumption and noise, it can lead to cylinder bore glazing that shortens the engine's life.

    Hope that helps.

    Philip
     

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    Last edited: Mar 15, 2010
  5. pappajohn

    pappajohn Funster Life Member

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    line 28, column C needs changing to 50%.

    if you run the battery down to 80% used you will kill the battery.

    a 110ah battery will give you around 55amps usable power before cell damage starts.


    for some reason a can only open the document as 'read only' :Sad:
     
  6. JeanLuc

    JeanLuc Funster

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    I don't necessarily agree with your first point PPJ - it depends on your point of view. If you re-read my post, you will see that I have referred to that issue and suggested that the user might want to change it. I Use Elecsol batteries and they claim to be capable of taking deep discharge without damage.

    Don't understand the Read-only issue - works fine for me from the download. Maybe to do with the fact that I use a Mac. The file was created using Office 2008, but saved as an Excel 97-2004 version. It might work if you save it as a new file name.

    Philip
     
  7. pappajohn

    pappajohn Funster Life Member

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    agreed philip,
    but my point was a regular lead acid flooded battery will be damaged.

    as for the file....i noticed it was a mac format.
    not too bothered that i cant open it because regardless of the figures i'd still use stuff as usual and charge via solar or/and genny off hookup.

    but to stave off boredom i'll have a play and see if i can open it as it should be opened. :thumb:
     
  8. spiritsfree

    spiritsfree Read Only Funster

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    It downloaded fine for me on Vista :BigGrin:
     
  9. PeterandLinda

    PeterandLinda Read Only Funster

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    Hi

    We do a lot of wild camping and VanBitz supplied and fitted a Sterling battery charger which transfers power from the vehicle battery to the leisure batteries at a faster rate than the Electroblock fitted to our Hymer. Im sure that VanBitz can give a more technical explanation, but it works for us!

    P&L
     
  10. ArenqueRojo

    ArenqueRojo Read Only Funster

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    Are you using LEDs for lighting. If not, they will extend your off-hook abilities quite substantially unless you go to bed when it gets dark - make for an early night in Scotland, though:Laughing:
    Patrick
     
  11. spiritsfree

    spiritsfree Read Only Funster

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    We tend to use candles for the little amount of light we use when it`s dark.....but thanks for the tip. I`ve never stopped and thought of the lights like that before, assuming wrongly that they use very little of the power that`s there. :Doh: Yes we do tend to move our body clock a tad :BigGrin:
     
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