Fast vs slow charging li-ion batteries (0.8C vs 0.4C)

Discussion in 'General Electronics Chat' started by Grayham, Jun 27, 2010.

  1. Grayham

    Thread Starter Member

    May 18, 2010
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    I am charging a mobile phone 1150mAh battery and I have the option to charge it via USB (500mA which would be 0.43C) otherwise using its AC power adapter (which charges at 900mA which is 0.78C).

    For those who don't know, a 1C charge would charge a battery in 1 hour (at 1150mA constant current), where a 0.5C charge would do in 2 hours (1150mA/2 constant current)

    Can someone tell me which one of these charging methods will give me the longest "runtime" after the charge if I were to drain the battery down to nothing?

    I am aware higher 'C' rates decrease lifespan of battery, but I'm only concerned about runtime for now.
     
  2. sceadwian

    New Member

    Jun 1, 2009
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    Lithium chargers don't work like that, they charge with a constant current until the peak cell voltage is reached and then they're charged at a constant voltage till the current drops to between 2 and 10% of C.

    If you constant current charge only up to the cells maximum voltage that cell is only approximately 80% charged, it will take nearly as much time as the first stage took to charge the cell up the remaining 20% so even at 1C you're looking at an hour and a half or two minimum to fully charge the pack. If you want maximum capacity out of the cell you can't rush the charging process, the final charging stage is the most important for maximum capacity.

    You can tweak a little more capacity out of a cell by increasing the maximum cell voltage the charger allows but you'll lose capacity really fast.

    Keep in mind you can't be only concerned with runtime, runtime and lifespan are linked, aggressive charging will decrease lifespan which means decreasing maximum capacity after a number of charges. If you want just one really good charge out of a battery that's one thing but you'll never get repeatably higher capacities from a cell by pushing it's limits. If you REALLY want more capacity buy bigger cells, the more gently you charge and discharge the battery the longer it will last and the higher the overall capacity will be.
     
    Last edited: Jun 27, 2010
  3. Grayham

    Thread Starter Member

    May 18, 2010
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    2
    I'm only asking because I'm going on a 8 hour flight and want my phone to last me through it (reading PDF's etc). It's a one off thing for this phone to need to do. Just survive the trip :)

    It's a Nexus One and charges 900mA when plugged into AC, and 500mA when plugged into USB. Wondering which is best charge method to do before my trip.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why don't you buy a spare battery, charge it at the 500mA rate overnight, and take it with you?

    As you've probably been charging your existing battery using the 900mA charge cycle, its' capacity will already be reduced from when new.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Lithium batteries have a limited number of charge-discharge cycles, maybe only 300.
    After only 100 cycles you will notice that the run time is shorter than a new battery.
     
  6. Norfindel

    Active Member

    Mar 6, 2008
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    Isn't cellphone use forbidden during flight anyways?
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Some cellphones (iphone for example) can switch off the RF part to enable you using it on an airplane or in hospital.
     
  8. sceadwian

    New Member

    Jun 1, 2009
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    No, not during flight, it's typically only disallowed during takeoff and landing, though that's not necessarily the case everywhere. Every cell phone I know of has an airplane mode or private mode. I think it's required by law to be able to turn off the Rf section, at least in the US. There is no law in hospitals that I'm aware of they're just not recommended.
     
  9. sceadwian

    New Member

    Jun 1, 2009
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    audioguru I'm sorry but your limit of 100 charge/discharge cycles on a lithium battery is false, there is no such limitation on any battery technology I know of. The number of charge and discharge cycles you can expect from a battery is dependent on the charge scheme and the maximum discharge current, and can't be generally applied to chemistry as a simple number.

    I had a cell phone with a 3 year old battery that had 75% original capacity, at least 300 charge/discharge cycles.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    Full discharge/recharge cycles are different from "I have 2 bars left and Im plugging the charger in"
    Batteries DO have enough chemical in them for a particular number of charge/discharge cycles.

    This is determined by the manufacture of the battery.

    If you do not use ALL of what is available of the battery before you re-charge, then it isn't a complete charge-discharge cycle.
     
  11. sceadwian

    New Member

    Jun 1, 2009
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    retched, what you're missing is the law of diminishing returns =) You can stretch them out for YEARS, using only 80% of their available capacity, or use the entire cell up inside 1 year using 110% of it's available capacity. If you use it gently, the total energy you get in and out of the cell is 2-5 times higher if the cell is abused that lasts only 1 year.
    The charge and discharge cycle rates as well as the ultimate voltage high and low that the cell is charged/discharged to determines it's ultimate lifespan and capacity not number of charge discharge cycles.

    If the charge/discharge cycle is gentle it will sustain a DRAMATICALLY higher rate of charge/discharge cycles compared to a cell that was used hard. The overall more useful amount of extractable energy will go to the gently used cell.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    I said, "Lithium batteries have a limited number of charge-discharge cycles, maybe only 300.
    After only 100 cycles you will notice that the run time is shorter than a new battery.


    Battery manufacturers say a Lithium rechargeable battery lasts for at least 300 charge-discharge cycles but don't say the resulting reduction in the run time (half?).
    Products designed for Lithium rechargeable batteries have a low voltage cutoff to protect the battery reasonably, but if the cutoff voltage is higher then the run time is shorter and the battery will last for more charge-discharge cycles. The total run time might be the same for both amounts of discharge times.
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    [ed]
    I was posting before I saw Audiogurus post
    [/ed]
    @sceadwian
    You are not talking about a complete discharge/recharge cycle.

    You are talking about a partial discharge/recharge cycle.

    As I said, battery manufacture is what it is dependent on. If you buy a 100Ah battery and only use 20 Ah, you are being smart, but using the batteries rated use, the discharge/recharge cycles are set.

    Batteries are manufactured to provide a particular amount of current. This is why lead acid batteries have thicker plates.

    It depends on manufacture.

    Just like your car is rated for MPG, you can increase this by driving slower.

    It is rated for highway MPG at 55mph. At 75mph, your MPG decreases. At 25mph, it increases.

    The law requires a manufacturer to rate a battery, then guarantee it can last X discharge/recharge cycles at a particular rate.

    So, YES if you undermine that rate, you change the life.
     
  14. Markd77

    Senior Member

    Sep 7, 2009
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    Unfortunately the petrol car is a bad analogy. At low speeds MPG decreases as well. There is a sweet spot somewhere around 40-60 mph where MPG is highest.
    http://www.metrompg.com/posts/speed-vs-mpg.htm
    (scroll down a bit for a petrol graph).
    An electric car would be a better example because it doesn't waste so much power just running, but that would confuse things.
     
  15. Norfindel

    Active Member

    Mar 6, 2008
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    IIRC, li-ion batteries last longer if you have them charged to 40% capacity, and keep them cool. 100% charge for extended periods wear the battery faster. That's why notebook batteries last only a year, or so.
     
  16. Grayham

    Thread Starter Member

    May 18, 2010
    79
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    From what I understand. Heat or full discharges do shorten the total lifespan of a lithium battery. Best to keep topping it up with charge before it gets lower than 60% or so.
     
  17. Markd77

    Senior Member

    Sep 7, 2009
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    Also check if you can turn down the brightness on the phone. It makes a noticable difference to battery life.
     
    Last edited: Jun 29, 2010
  18. sceadwian

    New Member

    Jun 1, 2009
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    retched, with lithium cells you have to talk about partial discharge/charge cycles. If you fully discharge a lithium cell once, just ONCE that's it the battery is toast and can't be used again. Chemistry strain also occurs the closer a pack gets to it's maximum theoretical capacity. So the charge/discharge scheme for a lithium pack will determine it's number of charge cycles and actual usable capacity. These ranges vary with device maker, battery maker and even manufacturing variances in the components.
     
  19. retched

    AAC Fanatic!

    Dec 5, 2009
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    Ok, friend, maybe you dont understand what Im saying.

    The manufacturer defines the discharge level. The MAX DISCHARGE LEVEL. And A MAX CHARGE LEVEL.

    So, 1 COMPLETE charge/discharge cycle is discharging the battery to the MANUFACTURERS DISCHARGE LEVEL and then recharging the battery to the full charge level.

    The manufacturer uses these numbers to give us the so-called number of cycles lifespan.

    I am completely aware of how to abuse and baby a battery. We are talking about a number given by the manufacturers. Not by you.
     
  20. sceadwian

    New Member

    Jun 1, 2009
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    What you didn't get retched is that the makers of even the same exact physical format and chemistry of batteries will give you different numbers. I've seen maximum cell voltages ranging from 4.1 to 4.3 volts per cell and minimum discharge limits from 2.9 to 3.1 volts, and the constant voltage secondary charging mode a lithium anywhere from 2 to 10% of the cells maH capacity, on identical chemistry, and it's difficult to attribute the decrease in capacity as a cause of temperature from steep discharges. This seems trivial, but you'll get 100 charges one way and 300 the other to the same 80% of original capacity of the battery. If you have a cell out of variance by so much as a single percent you have the difference between 6 months and 5 years of usability. Lithium's are not like many other types of batteries.
     
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