safe discharge current of batteries confusion

Discussion in 'General Electronics Chat' started by count_volta, Jan 16, 2013.

  1. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    This problem has been bothering me for a while. I read many articles about it, and I'm still confused.

    According to my understanding (from reading the articles and experience), lets say we have a 1200mAh battery. This means the battery will deliver 1.2 amps for 1 hour and then become discharged.

    This implies that a current of 1.2amps will not harm the battery. So if the battery is 1.5V, it can deliver 1.5V*1.2A = 1.8Watts of power for an hour.

    Now we have something like this. http://www.newegg.com/Product/Product.aspx?Item=9SIA07Y0BH0758

    Capacity 220 mAh.

    Continuous Standard Current 0.2 mA.

    Continuous Max Current 0.4 mA.

    High Drain Pulse Current 12 mA.

    Do you see my confusion? Its rated for 220mAh, so from my previous example the battery should be able to deliver 220mA for 1 hour safely. And yet the product specs say that the max continuous current is 0.4mA.

    How does this make sense? Why do battery specifications have to be so confusing!!!
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Why do you randomly pick one hour as the appropriate discharge period? The battery you picked at 0.2mA max discharge rate is intended to be discharged over a period of 110 hours.
     
  3. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    Usually batteries are rated only in mAh and voltage. Its very hard to find ratings like "continuous max current". You go to the store and buy some duracells, the only information you will get is their voltage and their capacity (if youre lucky).

    How is a person supposed to know what the maximum current that the battery can safely handle is? It seems like an obvious rating.

    Think of any computer power supply. Look on the back of it. Output voltage = X Maximum Power = Y. And then you can figure out the current from those two pieces of data. Why don't they do this with batteries?

    Its almost as if though they they expect you to use batteries only in situations where you have an instruction manual that says something like "insert 6 AA batteries into this hole".

    What if you are a hobbyist or engineer that wants to use the battery as a power source and needs to know what the smallest load he can put on the source is. I don't get why this has to be so confusing.

    Where do I get this information? Why is it so hard to find it.

    So youre saying that even though this 3V lithium battery is rated at 220mAh, I can't connect it to a load that will draw 220mA and discharge the battery in an hour?

    To answer your question, I chose an "hour" arbitrarily. This way I can just read the rating of capacity directly without having to do any math.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Simple answer: check the specifications, or even better grab the data sheet!

    Here's an energizer CR2032
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    You are trying to assign a meaning to "ampere-hour" that is not implied. An ampere-hour is just a convenient and standard way of stating battery storage capacity and has no particular relation to what the maximum battery current can be. Even small button cell batteries, which can deliver no more than a few milliamps maximum, are characterized by their ampere-hour (or perhaps milliampere-hour) capacity. It could be also stated as amperes per 10 hours, etc. but that is not the convention. The convention is ampere-hour.

    Actually most batteries will not meet their stated storage capacity at a 1 hour discharge rate due to excessive internal impedance losses (being proportional to the square of the current). Most are rated at 10 hours or more.

    Battery capacity could also be stated by its energy storage as watt-seconds or Joules, (which actually may make more sense) but that is seldom done. And that would still not say anything about the maximum current capability.

    So the only way to reliably determine the maximum current the battery can supply is to look at the manufacturers data for a particular battery. Anything else is just guessing.
     
  6. count_volta

    Thread Starter Active Member

    Feb 4, 2009
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    Fair enough. But why don't they just write this information on battery packages in the store? Half the time they dont even write the capacity value.

    It feels like battery manufacturers are trying to conceal this information. Yes you can find it online (most of the time), but that is too much work in my opinion.

    Yes the average user doesn't care and will just buy a AA because the instruction manual for his camera said to use AA's. But not everyone is the average user. Why not just write this information? Is the extra ink really that expensive?

    Imagine if you were to go and buy a power supply for your computer at Walmart, and it did not have the power rating on the box. Just said output = 12V DC. Now you have to spend time looking for the datasheet online before you can decided which power supply to buy.

    Its just annoying is all I can say. Sometimes its really hard to find battery datasheets online.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    I think they don't supply the capacity info since they don't want buyers to be able to easily compare the relative capacity of batteries from different manufacturers. They want you to buy them based upon their ads which promote style (think Energizer Bunny) over substance.

    Interestingly, although none of the non-rechargeable cells contain this info, most of the rechargeable ones I've see, do. Don't know why the difference. :confused:
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    about 15 years ago, Duracell and Energizer batteries were very close to the same ampHour rating but one was better for fast discharge devices (camera flash packs) and one was better for long term storage and slow discharge rates (smoke detectors). Now, they are virtually identical in performance.

    I agree, they still do not make Datasheets readily available but they do exist.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    For many years I used Energizer AA alkaline cells. Then I needed some but only Duracells were on sale so I bought and used some. When they ran down THEY LEAKED all over the place so I will never use Duracells again.

    The cheap Chinese AA battery cells at The Dollar Store are leaking in their packages on the shelf before they are sold.

    It is EASY to find Energizer datasheets on their website. Click on Technical Info at the bottom of the first page.

    It is funny to see that the NEW, IMPROVED and MORE EXPENSIVE alkaline cells have exactly the same specs as the cheaper older cells.
    Some new alkaline cells had TITANIUM and were expensive. Hee, hee they weighed the same and performed exactly like the cheaper older cells.
     
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