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Curious About Battery Charging

  1. Jul 1, 2015 #1
    So I learned that outlets actually provide a lot more voltage than small electronics like phones need but that transformers are used for voltage optimization.

    Is the sole reason that this is the case is so that the battery generates less heat and doesn't affect the battery as much? Or is there another reason?

    If there was a way to cool the battery, could batteries charge super fast with the voltage or is something else stopping it?
     
  2. jcsd
  3. Jul 1, 2015 #2
    Batteries need to be charged at a voltage that is slightly larger than the electro-chemical potential of the materials used in the cells. A cell usually has a potential ie voltage of about 1 to 2 volts. You can connect them in series to raise the voltage of the battery. Batteries up 12 V are commonly available. You can connect them in series to get higher voltages though.

    Most solid state devices run at voltages at less than 12V. Trying to charge a battery at a voltage that is roughly more than 10%- 15 % greater than the terminal voltage can damage the battery or shorten its life. This percentage is dependent on the type and style of battery. The maximum rate of charge is dependent of the type and construction of the battery and again excessive charge rate can damage the battery.

    Batteries come in a great variety of voltages, constructions, sizes, capacities, and sensitivities, and tolerances. Batteries of a given voltage are not necessarily suitable for all applications because of their characteristics.
     
  4. Jul 1, 2015 #3

    davenn

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    Gleem gave great battery charging info there :smile:

    but to take a step back and answer your earlier questions .....

    the mains wall outlet, depending on the country, supplies 240, 220, 120 Volts of AC ( note - AC = alternating current ). this is what is needed for your stove, bar heater, jug for boiling water etc

    TV's DVD, mobile phones, computers etc all require lower voltages and DC ( note - DC = direct current )

    So that plugpack for your phone, laptop and many other devices, not only transform the high mains voltage down to the lower voltage, but also have circuitry in them to convert from AC to DC ( a rectifier) and then usually other circuitry to regulate the voltage

    some devices like your TV, DVD player etc don't have an external plugpack, instead there is circuitry inside the unit that reduces the voltage and converts from AC to DC

    you may now be seeing that yes there is other reasons :smile:

    The big 2 are
    1) you cannot put AC into a device that is designed to run on DC, it will fail
    2) you cannot put excessive voltage from an AC or DC source into a device that requires a low voltage AC or DC source, again, it will fail

    eg, you cannot put 220VAC into a device that requires 12VAC

    just the same for DC or a combination of the two


    Dave
     
  5. Jul 2, 2015 #4
    I see, thanks gleem and davenn!

    I'm still curious though, is the reason you can't put 220VAC into 12VAC because the battery would overheat? And if it didn't overheat, would it charge 220/12 (18.33x) faster?
     
  6. Jul 2, 2015 #5

    davenn

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    it would explode, along with the device DONT TRY IT

    a device, regardless of if it has an internal battery or not. requires the stated voltage

    D
     
  7. Jul 2, 2015 #6

    CWatters

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    +1 to that.

    Inside a battery chemical reactions occur (during both charge and discharge phase). There is a limit to how fast these chemical reactions can occur safely. In some batteries the chemical reaction generates gases which are normally reabsorbed. If you were to charge a battery too fast it might generate these gasses faster than they can be reabsorbed resulting in the pressure in the battery increasing - possibly so much so that the battery explodes.

    Battery chargers are much more complicated than simple transformers. They are designed to suit the chemistry of the battery and ensure the chemical reactions present in the battery occur at the right and safe rate.
     
  8. Jul 2, 2015 #7
    So what determines this limit? For example, I read an article stating than iPad charger could charge a phone 4x faster since it offered a current of 2.1A compared to a USB charger of 0.5 A (without damaging the battery). What determines the limit at which point fast charging turns into exploding? I read something about a C-Limit, but that had to do with heat I think, which is why I asked about it above.
     
  9. Jul 2, 2015 #8
    The C rating relates to the discharge rate of the battery. For a given voltage, the more current you have flowing the higher the discharge rate and the higher C rating required. You may have two 5VDC 1000mAh batteries. The one with the higher C rating suits the higher power device.

    It has very little to do with the voltage the battery is designed to run at.
     
  10. Jul 2, 2015 #9

    Drakkith

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    Ultimately it's the cell chemistry. What the cells are made of and how they are constructed.
     
  11. Jul 2, 2015 #10
    It may be true that you can charge some cell phone batteries 4x faster but did the article make any mention of the effect that it might have one the batteries life? The recommended charging current is meant to safely maximize the life of the battery. And as stated before too high a charge rate can quickly destroy your battery or even cause an explosion. Higher charge rate chargers should have special safety feature that monitor charging conditions. To avoid possible problems use a charger manufactured by the same company or approved by the same company which built the battery. I would strongly advise not arbitrarily mixing batteries and chargers.
     
  12. Jul 3, 2015 #11
    I read it here http://www.forbes.com/sites/eliseac...apid-charging-damage-your-iphone-6-or-6-plus/ and it gives the reasoning for it not affecting battery life is that the extra heat generated isn't too high and the C-Rating. The information in this article I found elsewhere too.

    And about the cell chemistry, is there any place that I can find specifics on say, iPhone batteries, just to know the construction/limits and such?
     
  13. Jul 3, 2015 #12
    Lithium batteries are a special breed. They have multiple chemistry's and construction details including the built in electronic monitoring circuits. So you cannot expect any two brands of batteries to be equivalent. I expect Apple's batteries are proprietary and construction details are not available for scrutiny by unauthorized persons.
     
  14. Jul 3, 2015 #13

    CWatters

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    Typically it's the battery design. You have to refer to the makers data sheet. Batteries designed for fast charging typically have lower capacities than those designed for more normal charge rates. There is usually a trade off to be made.

    The charger circuit in the phone will charge the battery as fast as possible but limited by:

    a) The battery: Batteries have a maximum charge current (typically 1 - 2 Amps).
    b) The USB adaptor. The USB adaptor can deliver a maximum current (typically 0.5 - 2A).

    So if you switch from a cheap 0.5A mains adaptor to a 2A iPad charger it _might_ charge the battery faster. However it depends if the battery can actually accept a 2A charge current. If it can then it will charge 4x faster because before it was limited by the charger. Some batteries will only charge 2x faster because the max charge current is limited to 1A by the battery or charger circuit.

    Some of the very small USB sockets on phones are quite fragile and I don't recommend unplugging a 2A USB adaptor from the phone while it's on charge (eg while 2A is flowing through it). Best switch it off at the wall first.
     
  15. Jul 3, 2015 #14

    sophiecentaur

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    On the topic of exploding rechargeable batteries, there have been a lot of examples, in the news, of chargers for electronic cigarettes producing explosions and causing fires. I guess it must be to do with the fact that people require very rapid charging and are not prepared to wait a bit - as users of drills, computers and radios seem to be.
    Smoking can damage your eyebrows. lol
     
  16. Jul 3, 2015 #15
    The most volatile mass produced battery AFAIK is the lithium-polymer. They (possibly) could be designed to perform better than the lithium-iron batteries that are commonly used in everyday devices. Li-Fe batteries do not have the C-rating for some higher power devices for example RC craft. They use Li-Po's... If you want to learn about batteries talk to the RC guys.
     
  17. Jul 8, 2015 #16
    Another question-I was reading an article about how the batteries were getting larger in phones (like 1800 mAh to 2200 mAh) so the battery lives were getting longer. Is the relationship between mAh and battery lives shorter? For example if I tried to use a 300 mAh battery in a phone, would it have 1/6th of the battery life of a 1800 mAh phone, or does the phone require a certain size battery, are there other factors...?
     
  18. Jul 8, 2015 #17

    CWatters

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    In a phone the main difference would be the need to recharge it roughly 6 times more frequently.

    In some applications (eg power tools) the maximum recommended current you can draw is limited by the capacity. For example if C is the Amp Hour capacity then the max recommended current draw is typically limited to some multiple of C, perhaps 10C. So if..

    C = 300mAH then you might be limited to 3A.
    C = 2000mAH then you might be limited to 20A.

    There might also be similar restrictions on the charge current.

    You could draw more current but that might cause damage that reduces the capacity or reduces the battery life (number of cycles).
     
  19. Jul 8, 2015 #18

    Drakkith

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    All else being equal, a battery with 300 mAh of capacity would have 1/6th the life of a battery with 1800 mAh. In reality it would probably be a little less since you usually don't want to discharge a battery completely.
     
  20. Jul 11, 2015 #19
    Sounds good. Just wondering, what if the battery was 3 times the volts? Like 1->3V?
     
  21. Jul 11, 2015 #20

    Drakkith

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    There would be no change in the amp-hours. The difference is that a higher voltage battery provides more power per unit of charge. (Power in electrical circuits is P = VI) So if your 3 volt battery provides a current of 1 amp to a circuit, the power provided is 3 watts. In contrast, the power provided by your 1 volt battery at 1 amp is 1 watt.

    This means that a higher-voltage battery can potentially last longer than a lower voltage battery with the same amp-hour capacity because your electrical device wouldn't need to draw as much current in order to run at the same power. But the real world is complicated. You can't always change up your electrical devices to run at the new voltage, so you may need to convert the voltage into one that the device can use, which will introduce losses. The reverse is also true. The lower voltage battery may be better for the device and run longer because it matches the voltage than the device needs. That's one of the reasons we have many different types of batteries and voltages.
     
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