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How long will this batt last? (consumption)

  1. Aug 3, 2012 #1
    Hi, i 'm new here. (I 'm from Panama city, Panama).
    I know very, very little from electricity and those stuffes. I hope you can help me out.

    If I have a battery, say: 12v / 7ah

    How long will it last (in ideal scenario), if I power a device that is: (one at the time).

    1. 7 amp
    2. 3.5 amp
    3. 35 amp
    4. 70 amp

    Also, why do people say, that when you charge any rechargable battery, it should be from 10 to maximum 15% of its total ah?? However my cellphone (NOKIA N8) is: Standard battery, Li-Ion 1200 mAh (1.2ah) (BL-4D) and it's charged (from full discharge) in exactly 1 hour and 45 mins = 105 mins? that means that: 11.43 ma get in per minute, and 685.71 ma per hour (about 57.14% per hour). Why that exception???

    So, can I charge a car 50ah battery in 30 minutes by charging it with 100amp???
    50/100=0.5hours

    Thanks
     
  2. jcsd
  3. Aug 3, 2012 #2
    The first one sounds like a homework question (which we cannot answer directly), but the second doesn't -- are you asking for schoolwork, or not (if not, then I'll answer much more directly)?

    For the first question, determining battery life is done entirely through current. A 1 amp-hour battery can deliver one amp for one hour. It can also deliver half an amp for 2 hours or 2 amps for half an hour. In fact, unless you exceed the battery's current-sourcing ability, a 1 amp hour battery can source any amount of current for any amount of time as long as the two multiply to 1 amp hour.

    Does this help you enough?

    For the second question, the answer is that it depends on battery chemistry. I'll go ahead and introduce a notation that I've seen before that helps to answer this.

    1C is the current required to drain any battery in one hour. For your example Nokia battery, that would be 1200mA, because the battery's capacity is 1200mAh. A fraction of multiple of 1C, such as 0.1C or 2C, is simply a multiple of this current. In other words, for your phone, 0.1C is:

    120mA = 0.1 * 1200mA

    For NiMH and NiCd batteries (which used to be by far the most common type of rechargeable battery (car batteries excepted)), a charge rate of 0.1C (that 10% you mentioned) is about right. Unless your specific battery is designed to be charged faster, this is the fastest you should go or you risk damaging the cell.

    For Li-ion and LiPo batteries (such as the battery in your Nokia), a typical charge rate is 1C (or slightly lower). Further, most batteries (this applies to NiMH, NiCd, Li-ion, LiPo, and others) are about 2/3 efficient during charge. This inefficiency multiplies charge times by about 1.5-1.6.

    As your phone is a Li-Ion battery, it is probably being charged at 1C (or slightly under). That, plus that inefficiency, suggests that it should take 1.6 hours (or a little more) to fully charge. This is fairly close to the charge time you give.

    That car battery? It's Lead-Acid, and should not be charged at rates anywhere near 1C. You will damage that battery (at 100 amps, an explosion is a possibility). Plus, even if that battery can somehow take that charge rate, that 2/3 charging efficiency will make the charge time close to 45 minutes, not 30 minutes.

    Please respond if you are still unsure about anything.
     
  4. Aug 4, 2012 #3
    Thanks for answering Crosstalk, I think I should slap you in your face really hard, for suggesting I wanted you to answer my "homework". And yes, it's a homework, but for me. Since I want to get into solar panels stuff.

    In some other forum I asked some "experts", if by any combination or mix (paralell or series), could you manage to turn on and keep for 10 seconds a small a/c with "n" number of 1.5v batteries @ 1200 mah?
    They called me crazy, hehehe, but, in theory, is it possible?

    a/c = 700w @ 220v

    For 220v = 147 1.5v batteries.
    For 3.18amp (700w / 220v = 3.18 amp) = about 3 1.5v @ 1200mah batteries.

    Is it theorically possible with my combinations?
     
  5. Aug 4, 2012 #4
    Okay that makes sense -- and I asked because I'd rather not directly answer a homework question for anyone (plus it's against forum rules).

    You mention a 220v air conditioner. Most home appliances run on AC current (alternating current). Batteries produce DC current (which doesn't alternate). Unless this air conditioner is able to run on DC current, then you cannot simply power it off of AA batteries.

    Just for your information, the name for a device that converts DC into AC is an "inverter" -- so if you can find an inverter with the right specs then you could run the a/c through that inverter (as opposed to directly off of the batteries).

    If this air conditioner runs on DC, however, then it's theoretically possible. If those batteries were actually putting out 1.5 volts, then your calculation of 147 batteries is correct. However, battery voltages actually sag under load. This drop in voltage can be compensated for -- if you know about resistance, then look up "internal resistance".

    Your calculation for current however isn't quite accurate. That 1200mAh rating is capacity -- that calculation shows that you would need three strings of batteries, each putting out 220 volts, in parallel to get 1 hour of battery life. The number of these 220v strings you would need depends on the current-sourcing abilities of AA batteries, which I am less familiar with.

    Also, in practice, batteries in parallel have a tendency to try to reverse-charge each other. I don't have enough experience or knowledge to comment on whether or not this would be an issue, however.
     
  6. Aug 4, 2012 #5
    The behavior of battery chemistries varies thr charging method. You should also consider the application of the battery. This site may help:
    http://batteryuniversity.com/
     
  7. Feb 9, 2013 #6
    I have a load of 3kW.
    2 Batteries in series - each of 12Vand 42Ah
    Solar panel - 450W - 12V charging batteries with MPPT.

    Calculation to be done on the basis of charging i.e. taking care that battery is charging at the time of discharging.

    I need the calculation of the total time i can run the load which charging and discharging the battery.
    Plz explain how to calculate.
     
  8. Feb 9, 2013 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Yours is a common reaction but nothing like as common as the number of 'idle students' who write in and want us to do their assignments for them.
    I think the way to approach a forum like this is to write your actual problem down (some practical details, rather than stating it in the same form as a typical School question. (Which you did very well) That will establish your bona fide because no lazy student would ever bother to do that. :wink:
     
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