battery issue

Discussion in 'General Electronics Chat' started by shahnawaz, Jul 1, 2011.

  1. shahnawaz

    Thread Starter Active Member

    Dec 30, 2008
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    Dear all,
    I have one issue which needs your help. Our contractor in one of the site they have put two strings of batteries. One is 105AH and the second one is 150AH.What is the effect of put two difference capacity of batteries? remmember each string is 48vdc.

    thanks .
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    It depends on where each string is connected. If they are on different circuits that may be ok.
    If they are in series I would not do it. You will have different charge and discharge rates and will destroy the batteries.
     
  3. shahnawaz

    Thread Starter Active Member

    Dec 30, 2008
    37
    0
    its connected in parallel with same charger.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Ideally, you want all the batteries to be the same. As the batteries age, when one battery needs replacing all the batteries should be replaced at the same time.
    I do not have a definitive answer with two strings in parallel. My guess is that they will be ok while under load.
    But while charging, the lower capacity string will reach terminal voltage first and prevent the higher capacity string from being fully charged. Just my guess.
    Mind my asking what is your load and charger? Is this a PV solar panel system?
    Can you ask your contractor to install same batteries?
     
  5. shahnawaz

    Thread Starter Active Member

    Dec 30, 2008
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    load is 96 amps and the charger is 120amps output. its basically an hybrid site where batteries are connected with both charger and the solar supply. a genset is also available for the charger during night.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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  7. SgtWookie

    Expert

    Jul 17, 2007
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    Well, if all of the batteries in a string were the same AH capacity, then it would be OK; eg: one string were all 105AH, the other were all 150AH. However, you should not mix different AH capacities in a single string; as the lower AH capacity batteries would wind up being discharged too deeply, and badly overcharged.
     
  8. steveb

    Senior Member

    Jul 3, 2008
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    In parallel it may be OK. The real question is whether the person who installed the system knows what he is doing and did an engineering analysis to make sure the design is suitable for the application, or if he followed the advice given by a qualified person or given by a qualified source of information (such as the battery manufacturer). If he just slapped whatever he had together and isn't qualified to do an analysis, then this may not be the best option.

    I agree with the above advice to use the same exact battery when combining. This is because you don't have to do as much thinking or analysis. Actually, combining different capacities in series would be a very bad idea. However, when combining in parallel this is just a good rule of thumb, not a strict rule.

    In this case, parallel batteries would require the same battery type, such as all Li-ion, or all NiMH etc. This ensures that the batteries will not be damaged by the voltage profile of charging and discharging. If the batteries are of the same type, then the different capacities and different internal restistances can work together to provide automatic feedback that adjusts charging and discharging current. Note that I just said "can work together", not that they "will work together". An analysis of the design within the application is needed to say that they "will work together".

    Anyway, I think it is most likely OK, but you are wise to ask the question and investigate further.
     
  9. chimera

    Member

    Oct 21, 2010
    122
    2
    Do not mix different capacity batteries together, in any configuration. Ive learned this the hard way and with lithium battery. They WILL EXPLODE on you. Ended up using a fire extinguisher..first time in my life..was fun. But seriously, don't do it. Try getting the same capacity batteries and then configure them together, either is series and parallel. I checked out your profile and it states that your working for a telecom company. Im guessing your going to beworking with big size batteries. The charge algorithm changes when dealing with higher rated batteries. View the data sheet provided for the batteries (assuming you arent working with car batteries :p joke). The data sheets need to be thoroughly read and understood. The current rating on the said batteries is enormous . So safety is an utmost priority. Post a picture of the schematic that your working with. It'll be easier to understand where you are coming from. Best of luck.
     
  10. steveb

    Senior Member

    Jul 3, 2008
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    It just occurred to me that it's possible the person who installed the system is dyslexic and mistakenly used the 105 instead of a 150. I'm not trying to make a bad joke here. This could happen very easily. Actually, I've made similar types of mistakes when grabbing components, and I don't even have dyslexia.
     
  11. steveb

    Senior Member

    Jul 3, 2008
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    This is definitely a good rule of thumb and good advice. It takes a qualified battery systems engineer to design a multicell system and to design the monitoring, charging, balancing and safety circuits needed for a given application. Only such a person should consider making any rare exceptions to the rule. Being an electrical engineer, designer or technician is not enough in itself. Expertise in this specific area is critical. So the real question is what is the qualification and what due diligence was involved in this installation? Also, irrespective of that question, was a simple mistake made in installing the wrong component?

    Actually, we all need to consider the "good" advice itself ... "use only the same battery type with equal capacity". Such a rule is impossible to apply in the real world because of the normal manufacturing tolerances and aging difference with all battery types. No two cells are created "equal" and "no two cells age equally". Hence, caution and design analysis are needed generally. This is not something a novice should play around with.
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Good point. Perhaps there is simple explanation. Where did OP get the information about 105AH? Did someone actually verify the batteries installed?
     
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