Why ? please explain.

Discussion in 'Tech/Mech General' started by haganap, Nov 25, 2008.

  1. haganap

    haganap Funster Life Member

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    If you have a leisure battery, of say 110 amp. and you wsh to fit another, Why do people state you have to have two new ones? why can you just not join the two together?
    If you did what is the Dr pepper? (worse that could happen).
    after all the way i see it is that if one battery is fullly loaded after its charge as is the other, power is drawn from one then the other? or am I wrong?
    Please enlighten me.
     
  2. olley

    olley Funster

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    Hi Haganap, I have asked this question myself and the "brains" have said the two batteries will have different internal resistances, meaning that one will charge/discharge different to the other. One particular poster when pushed for more detail said that he didn't have the time or inclination to teach me electrics, and I should just take his word for it.

    Personally I think they overstate the case, because if they are closely connected their voltages must remain the same, meaning to my mind that the different internal resistance is not particularly important.

    I know that many do use different age/size batteries and appear to have few problems.

    Olley
     
  3. Jim

    Jim Ringleader

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    For 4 years my RV was happy to have a bank of 4 batteries consisting of 2 x 110 and 2 x 85:Smile:
     
  4. stagman

    stagman Deleted User

    [Your Batteries will charge up to your weakest battery .If they are both close to the same potential no problem.But if one is weak other strong not much point.Have them tested first .
     
  5. scotjimland

    scotjimland Funster Life Member

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    Sounds like a GT quip ....:Rofl1::Rofl1:


    There are perfectly sound electrical reasons why it's BEST to have batteries of the same type, size, age and brand, Mixing and matching different sizes and ages will affect the performance.

    Wrong, the batteries are connected in parallel so current is drawn from both. Lets say you have an 85ah in parallel with a 110ah , call them battery A and battery B
    In use, battery A will reach 50% of it's capacity before battery B , continued use until battery B is at 50 % will have seriously discharged battery A

    Conversely when charging, battery A will reach full charge before battery B , the charger will stop charging so battery B will never reach full capacity..

    I agree with the previous posters that many do mix and match without problem but they are not getting the best performance from their batteries.
    BUT

    Never mix TYPES ie Gel, AGM or FLAs .. (flooded lead acid)

    I hope that made some sense, I'm not great at explaining things..

    Try here for a more detailed read:
    Batteries -- and Other Electric Stuff by phred
     
  6. Pat4Neil

    Pat4Neil Read Only Funster

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    Hi Haganap

    We have just gone down that route, we had two 105amp batteries, and felt that one was letting us down, more so when we had no EHU and the heating was on pulling on the batteries. Because also we are about to go away for 6 weeks and not always on EHU we decided to change both batteries.

    It is such a shame because one of the batteries is still great, but after research and advice from techies I was happier to change both. I choose to also upgrade to 125amp batteries at the same time, which gives us 40amp more. I went with Elecsol batteries but cant comment on them at the moment only just fitted (but expensive). I think it came £270, but I wanted to have peace of mind. We also have a solar panel which helps in topping up the batteries. I think also when its a new van to you, you would not know how they run, until you use it and winter is a good test for batteries.

    Have you decided on your new motorhome then?

    Kind regards

    Pat
     
    Last edited: Nov 25, 2008
  7. olley

    olley Funster

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    Hi Jim, no me and GT used to get on quite well, and if pushed he would explain things, sometimes in to much detail. :BigGrin: This guy used to put his qualifications on his signature, I think he used to teach, boy with an attitude like that I bet his students loved him. :Rofl1:

    The thing with batteries is that if you use the same type then their voltages give a good indication of their state of charge irrespective of their size or age, if they are closely connected then they will both reach 50% discharge at the sametime, I can't see how it can be otherwise.

    Olley
     
  8. scotjimland

    scotjimland Funster Life Member

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    Hi Ian

    I dunno mate.. I'm not qualified in the 'black art' of batteries, in fact I doubt if anyone REALLY understands their inner workings.. (except the guy you refer to), like you I got on well with GT, I enjoyed his posts, brusque at times but always informative.. never got banned on RVF:Wink: but that's going off topic :Laughing:

    Back to batteries, lets take an extreme example.. Bat A = 10ah Bat B = 200ah

    what will happen when charging, would Bat A not get cooked ?
    and discharging, will Bat A not be totally flat long before Bat B ?

    Maybe I'll ask GT .. :Laughing::Laughing:

    Jim
     
  9. ruffingitsmoothly

    ruffingitsmoothly Read Only Funster

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    Sorry but I have to ask who is GT?

    And where is he now?

    Regards Pat
     
  10. Jaws

    Jaws Funster Life Member

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    A point that is regularly missed is that the chances of two brand new batteries being identical is actually fairly remote !

    Whilst technically I have to come out on the side of those that say it is better to replace both at once, in real terms I have no issues with either fitting one new one or indeed, fitting odd ones as I have done in my own vehicles over the years
     
  11. scotjimland

    scotjimland Funster Life Member

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    Hi Pat

    sorry for using initials..

    GT or George Telford , infamous for being banned on sites, you may find some of his posts on here, MHfacts, RVF, SBMCC, etc

    Last I heard he had given up motorhoming .. he has a site of his own.
     
  12. scotjimland

    scotjimland Funster Life Member

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    Indeed, I recall when GT was sourcing batteries for his self build bus he actually visited the battery manufacturer to purchase, just to ensure they were made at the same time and had consecutive serial numbers .. he was that kind of person.. a stickler for detail.
     
  13. olley

    olley Funster

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    Hi Jim, battery A could only be cooked if its voltage rose above battery B, but as they are closely connected that can't happen. The way I see it is if I charge them at 13.8v then they will both finish up in a similar state of charge, likewise if I discharge them to 12v they will be in a similar state of discharge.

    Don't we get warned about when connecting batteries to make sure they have similar charges, connect two together which don't, and you get a flash, and then the higher charge battery, charges the other one until the voltages balance out.

    I think its a bit like reverse polarity, yes it exists but is it really a problem? :Doh:

    Olley
     
  14. haganap

    haganap Funster Life Member

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    Well thanks for all your help guys. My head is still spinning. But I thank you all for your contributions.

    In esscence, this is what I will do. When I get my van next week it has 1 battery of 125amp, I will then lay another next to it of 125amp connecting positive to positive of course.

    I will then see how it goes.

    Surely at the worse case scenario I will have no less power than if I had just the one battery.

    so I will go with that and see what happens.

    I am sure as scotjim has suggested and me GT of course, this is probably not the ideal, but then driving at 75 on a motorway is not neither. but many do.
     
  15. Tony Lee

    Tony Lee Read Only Funster

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    Lot of misconceptions around.

    A battery can be thought of as a tank of water - so a 100Ah 12V battery could be represented by a large diameter tank say 8' high holding say 1000gallons. Similarly a 10Ah 12V battery can be regarded as a small diameter tank 8'high holding 100gallons. If this was the case and they were both 8'high and both standing on the same bit of level ground and connected together at the bottom then they would both fill with water at the same time so your total capacity would be 1100gallons. Makes absolutely no difference what capacity each tank is (this is analogous to Ah of a battery), as long as all tanks are exactly the same height (analogous to battery voltage) they will all reach their full capacity together and they will all be empty at the same time and you just add all the capacities together to get the total available capacity.

    SO - back to batteries - if they are from the same manufacturer and have the same plate composition and same construction then you can parallel different capacities (Ah) together with no problems whatsoever. The total will be the sum of the individual ones.


    The problems come when the tanks are not the same height (=very slight differences in voltage or different type or one is getting old) or one tank has a very small pipe going to it (=high internal resistance) - or one has a rust hole part way up (dud battery). IN this case you can't fill all the tanks fuller than the height of the lowest tank or the height of the hole - and in the case of a tank with a very small pipe going to it, this will always lag behind the others.
    In batteries of different constructions, or different compositions,different ages or different manufacturers the cell voltages at various stages of the charging cycle will be different and this can lead to all the batteries being charged up only to the point dictated by the lowest. In the case where one battery has a shorted cell (hole in the tank), all the batteries will never reach anywhere near full charge. [if one tank has the tap turned off (=one cell open circuit) it will never fill up and so will not contribute to the total capacity

    Of course this analogy is not perfect and the charging cycle is more complex than merely filling a tank but basically there is no harm in adding an identical battery to one that is still performing well. It is not hard to do a basic load test on the current battery to check the performance. It may not be a perfect solution, but the experts are not the ones forking out double the money for two batteries.
    -----------------------------------------------------
    If I was wanting to double the capacity of my battery bank and needed to buy two batteries, I would get two 6V batteries, each of double the Ah of the original one and put them in series. This configuration, while having slight disadvantages in certain situations, is considered a better system than two 12V batteries in parallel.
     
    Last edited: Nov 25, 2008
  16. stagman

    stagman Deleted User

    In simple terms as I said earlier your batteries will charge up to the potential of your weakest.To keep it simple it is down to VA volt x amps.If ypu have a 85a battery with a voltage of 13.5 volts your VA will be 1147.5 VA .If you have a bat of 110a and 13.5v you will have a VA of 1485 VA .If you bank these 2 bats together the charger will only charge up to the potential of the 1st bat which will be like having two 85a bat so in effect the 110 amp is a waste of money space and weight.Another example is if you have two 85a bats one has a voltage of 13.5 volts which is good the other is down to 12v getting bad the charger will charge both up to potential of the weakest which is 12v x 85a = 1020VA a loss of 127Va on just one of the bats:: .Did try and keep it simple but got out of hand:thumb:
     
  17. Beachbum

    Beachbum Read Only Funster

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    I have been reading all your technical jargon on batteries. So this afternoon, I went out to my MH diconnected all three 110AH batteries which are in a row. I then connected 1st Batt: it read 13.4. Disconnected it and connected the 2nd Batt: that too read 13.4. And so did the 3rd. Point I am trying to make, all the batteries were charged simentaneously. I take it when in use all will run down accordingly. I haven't the fainest idea about Batts: or electrics, just thought I would see what the charge would be one at a time. Perhaps you guys may have something to remark on regards my stupidty.:Rofl1:
     
    Last edited: Nov 25, 2008
  18. itinerant

    itinerant Deleted User

    I don't wishto be confrontational but I feel that the non-techincal response you have made requires some explanation. It is my opinion that:
    1/ The battery with the lowest internal resistance will will reach the fully charged state first as its charging current (I = V/R) will be higher.
    2/ The (duff?) battery will acheive this state slower but will eventually reach fully charge (within it's capability).
    3/ On discharge the more capable battery (lower intenal resistance) will provide a larger percentage ofthe current and as this reduces the charge level the battery with the higher internal resistance will chip in so to speak within it's more limited capability.
    4/ This situation will only change if for instance one battery developes a short in one or more cells. This is a rare failure mode.
    Your argument would however stand in a case where the batteries were connected in series which is clearly not the case here.
    I would welcome constructive discussion on this issue.:Wink:
    Regards
    Itinerant
     
  19. stagman

    stagman Deleted User

    If they are all reading 13.4 v then that is great and should run down equally together .It is worth doing the test you carried out once a year to keep a check if any of the batteries you have are deteriorating.
     
  20. scotjimland

    scotjimland Funster Life Member

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    If they are reading 13.4v that's the 'surface charge' after being charged, you must leave them disconnected for at least four hours or overnight before measuring to get an accurate reading. Even duff batteries will read high when just taken off charge.
     
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