Discussion in 'General Electronics Chat' started by Mathematics!, Jan 10, 2010.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    OK , I found that the capacity ratings for AAA , AA , C , D , and 9-V NiMH are 1000mAh , 2300mAh , 5000mAh , 8500mAh , 250mAh

    I have read about C ratings and read that most portable batteries are 1C meaning they draw a current equal to the rated capacity that takes one hour.
    For example
    1000mAh AAA battery can deliver 1000mA for 1h
    if it had a 0.5C rating then it could deliver 2000mA for 30 minutes

    My question is
    Is this C-rating an approximate rating for the max amount of steady current that a battery can deliver per hours?

    Or for example in the case of AAA above could the battery supply 2000mA for 30 minutes even if the C-rating was 1C and not 0.5C because if this was the case then what would be the point of having this rating???

    If it is away of approximating the max steady current for an hour then for the above batteries assuming 1C

    They would beable to deliver a max current of approximately 1000mA , 2300mA , 5000mA , 8500mA , 250mA for 1hour

    So all of these batteries even the 9-v NiMH can supply a device that draws 100mA for at least 2hours or more....

    But if we had a device/load that drew 300mA then based on 1C rating the max the 9volt could supply steady current for is 250 for 1hour so this battery would not work for this load....

    I also have looked up the internal resistance of the batteries and they are
    approx below at 100% fully charged or new values
    9V zinc carbon 35ohm
    9V lithium 16-18ohm
    9V alkaline 1 to 2 ohms
    AA alkaline 0.15ohm
    AA NiMH 0.02ohm
    D alkaline 0.1ohm
    D-NiCad 0.009ohm
    D SLA 0.006ohm
    AC13 zinc-air 5ohm
    76 silver 10ohm
    657mecury 10ohm

    So for a AA alkaline which was 2300mAh with 1C => it can deliver 2300mA for 1hour.

    Going by the internal reistance and the nominal voltage rating we have by ohms law => 1.5volt/0.15ohm = 10amps => 10,000mA

    So why the difference 10000mA to 2300mA that is nearly 5 times less then the theoritical ohms calculation ....

    So I guess I am wondering if a AA with the above ratings can only supply steady max current of 2300mA for 1hour but with if it wasn't supply steady current it could supply more then 2300mA and a max of 10,000mA if shorted.... (minus wire resistance)

    Just trying to piece this stuff together I know a more exact model is Peurkert's equation but it is hard to work with this all the time so maybe internal resistance , mAhours , and C-rating is a better thing to use for approximation???

    Thanks for clearing an or this up.
  2. Paulo540


    Nov 23, 2009
    Well, you're partially right.

    C rating is purely a charge/discharge rate rating. Basically, a 1c 1000 mAh battery can discharge 1A continuously without melting. Same with charging, it can be charged at 1A while staying within safety limits.

    The Ah ratings you found for the respective batteries are not set in stone by any means. There are plenty of cells with much higher and lower ratings. There are some batteries rated at 20-30....+ C as well.

    So, C isn't really based upon the time a battery can 'put out', its merely a maximum steady operating current.

    The other thing to know is that a battery's Ah rating is not at all linear. Its based upon a certain amount of current discharge over time and thus if you ask a 2Ah battery to supply 2 amps, it won't last an hour. If you ask it to supply 100mA over the course of 20 hours, then it should do that.
  3. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    well I read that most portable batteries have a c-rating = 1C
    which would imply if you have the mAh rating for the battery it would be the same mA rateing that the battery can steadly supply for 1hour.

    As for c-ratings is this the max steady current a battery can supply.
    Assuming it is what would happen if a device drew more then the steady c-rating current.... would it still be steady but for a shorter time or would it not be steady or damage the battery...etc?

    I am just wondering because knowing the max rating of steady current for one hour doesn't not imply you can figure out the steady max current for a 1/2hour or 10 minutes ...etc it is not linear I believe...

    Because if it where linear then a battery that is rated at 8000mAh with 1C
    implies it can supply 8000mA for 1hour but not 4000mA for 2hours...
    in theory if it was linear the above should hold....

    So my many question's are given a time how can you figure out the max steady current for that particular time limit based on the amH or C-ratings,...etc

    Obviously if you are given the 1C then you know the correct amount for 1hour or if you where given 0.5h you would know the correct amount of max steady current for 2hours and so on but not at any other reference time....Like if you where given 8000mAh and a 1C but wanted to know the max steady current the battery could supply for a 5 hour period?

    The other question is given the internal resistance of a battery and it's nominal voltage by ohms law you could caculate the max current if no external resistance was imposed on the battery other then the external resistance of the wire... but this current is way higher then the max steady current curious if you are not worried about steady current could you reach approximately this value V/Rinternal = max current with no load in theory? But I am assuming this calculation is skewed based on the fact that over time the internal resistance of the battery changes as well....
    Like AA alkaline 0.15ohm they said in my chart that normally AA have a internal resistance of 0.30ohms when 50% used but for exact resistance charge % look at the spec's

    But based on this and looking at the spec's a battery charger would be nothing more then testing for a particulary internal resistance and turning on a led based on it when charged (other then a few special batteries that must be gradually charged or pulse charged or something ...)

    Curious to know if the first question about the max steady current for a given time could be answer with Peurkert's equation ? Or if I can use the Peurkert's equation to find out how charged a specific battery is based on it's internal resistance measurements at a given time?

    Thanks for any help these battery questions are trick because you have some much variation in parameters...

    Also if you have a battery that has ratings 2000mAH with 1C and it is 1.5volts nominal
    Does this mean the total energy the battery has would be E = P * t
    E = 1.5* 2000mA *1H = 3Wh approximately could be more based on nonlinearity of the battery but it should never be less then this since 1C, 2000mAH => steady 2000mA current for 1hour.
    Note this is just an approximation the true value for Energy would be the integral of V(t)*I(t) dt from 0 to infinity

    Tell me if I am missing anything
    Last edited: Jan 11, 2010
  4. maxpower097

    Well-Known Member

    Feb 20, 2009
    Here's an example with lipo's.

    C Rating
    LiPo cells are also commonly given a C or current rating. This is the maximum average recommended discharge current for the cell. For example, the Thunder Power 1900mAh packs have a 6C rating. To determine the maximum recommended discharge rate multiply the capacity times the C rating. 1900mAh x 6C = 11,400. So the maximum recommended discharge rate would be 11,400mA or 11.4 amps.

    How long you are going to be able to hold that discharge rate will depend on the capacity of the battery.
  5. jpanhalt


    Jan 18, 2008
    No, you have it backwards. A 1000 mAH battery with a 0.5C rating can deliver at the rate of 500 mA without damage.


    Math error. See above.

    That is quite true. A NiCd can deliver much more current than its C rating would imply, as can most batteries (LiPo's included). The problem is that when you exceed the C rating, you risk damage to the battery. With NiCd's and LiPo's that damage can be spectacular. They can catch fire or explode. I believe that risk is somewhat less with NiMH batteries, but the C rating still should not be exceeded.

    See comments inserted above.

    Also, the comments above relate only to discharge ratings. Charging rates are also described for these batteries using C ratings.

  6. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok , but 1C , 1000mA means that the battery can supply max steady current of 1000mA for 1hour...

    But what happens if you want to find the max amount of steady current that this battery can supply for another given time.... like 2hours or 1/2hour , or 0.4hour ...etc if you only know the mAh and the fact that it is 1C how can you determine it for another time ? ( I am assuming it is not linear meaning you can just take 1000mA and divide it by 2 to get 500mA for 1/2hour.... ??? <--- Maybe this is where Peurkert's equation comes in?

    So in theory if you shorted the battery with a tiny copper wire minus the resistance of the wire the current should increase to
    Vnominal /Rinternal = Imax - internal resistance of wire. "In theory"
    Which exceeds the C-rating by far. Is their an equation for the battery to determine when it is fully charged based on it's internal resistance??????
    Because I know the internal resistance chart I listed was based on the batteries being fully charged but as the batteries become more used the
    internal resistance increases... for example the book said AA alkaline 0.15ohm <-when full but 0.30ohm approx <-- when 50% charged.

    And other then the fact that some special batteries need to be pulse charged or slow charged.... this should be a way of determining how depleted a battery is and being able to make a recharger based on a target internal resistance to be meet....

    Just wondering if their is some kind of equation to calculate the internal resistance of a battery based on it's percentage ????

    Ya I was typing fast and did some basic math calculations backwards sorry about that good call.

    could you give me an example of figuring out charge ratings by C-ratings?
    for example if I have 8000mAh and 1C then the max discharge rate would be 8000mA for 1hour but if I was charging this battery wouldn't the max charge rate by the same as the discharge rate ? What would make these different....
  7. jpanhalt


    Jan 18, 2008
    No there isn't. Internal resistance changes with time, cycles, and temperature. Probably some other variables too.

    Discharge and charge rates are not at all the same. A typical high-discharge LiPo may have a discharge rating of 25C or more. Its charge rating may only be 0.5C or less. The particular brand that I use for 11.1V and 2200 mAH is rated at 25C discharge. Charging at a max. of 1A is recommended, but even then, they get warm. So, I charge at 0.8 A (0.4C) and have had very good life from them.

    NiCd's have similar high discharge and limited charge ratings -- even the fast charge types do. Just check what the manufacturer recommends and use that for your absolute maximums. It doesn't hurt to be on the conservative side, though.

  8. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok , so how would you base % charged the battery is on (if not the internal resistance changes )

    They have simple battery checkers that say bad , good , great 100%
    They must base it on something I was thinking it was some internal resistance measurements. Because As the battery loses charge the resistance slowly increases over time.

    So in the battery spec's they should give you to C-ratings one for the charge rating and one for the discharge rating. Yes/No?
    And normally would the charge rating be the rate at which you charge the battery to 100% or do you decrease the charge rating as you get closer and closer to 100% ? (other then the fact about special batteries needing special charge ways )

    Thanks for the help

    would Peurkert's equation be the way to find the max discharge/charge rates for a given time other then hour when you have 1C...
    Would this be the way to do it?
  9. maxpower097

    Well-Known Member

    Feb 20, 2009
    I just want to add while you can over discharge a lipo, never do it. They poof up then explode in a ball of flames. Same goes for charging with to many amps. Like jpan said most lipo's want to be charged at 1A or under. I personally used to charge my 22.2.v 6000mah packs at .5A This tend to give you much more cycles. Also as was said Temp. is a big factor with batterys. If you have a cold pack you usually can only pull 1/2 power till it heats up.
  10. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok , yes I get all that and I usually would charge at least 20% under what they recommend in the spec's.

    But I am unsure what varible they use to determine how to charge a rechargeable battery is... The book I am reading say's it is by the increase/decrease in internal resistance from the chart I gave.

    They said always check the spec's to be exact, but given the 100% charged internal resistance and the fully depleted resistance then you would beable to calculate the percentage of charged based on this?

    I am wondering if this is how battery chargers or checkers know how charged the battery is..?

    Or if their is any other way of baseing this on in the spec's their is got to be away?
    I know they make IC for battery recharging but these IC must be based on something to determine the batteries state...
    I am thinking the only thing they could use is checking for some internal resistance values or something?

    Thanks for any help
  11. maxpower097

    Well-Known Member

    Feb 20, 2009
    In a lipo charger you just check the voltage. For instance a 11.1v pack at full charge it reads 12.6v , dead empty it reads 7.5v. So if it reads 10v your about half full. This is usually about as low as you want to take a lipo. Other type packs don't suffer this problem. You could take the pack down to 7.5v but its gonna die after 30-100 cycles. If you only take it down to 10v you will get 100-300 cycles. If you guys want to see the future in batteries check out A123 racing cells. They are 80% as strong as a lipo, don't explode or burn, and can get over 1000 cycles. I know a lot about high discharge packs from my RC heli days. These are really the only two types of packs with the power to fly a heli.

    Senior Member

    May 15, 2009
    For what it is worth, I have been involved (design, repair etc) of battery chargers for a long time. All the decent design chargers (not el cheapo car battery chargers) that I have seen and repaired, work on the fully charged voltage system. That means that for a particular battery, the fully charged voltage will be "x" and on any decent charger that is adjustable.
    Proper chargers will also change the charging current as the votage builds up. (Ie less amps as you approach the fully charge voltage)
    Charging current is normally recomended at 8 to 10 percent of the capacity of the battery.
    Hope this helps
  13. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Actually , this does help alot.
    So are you saying that in the spec's of all rechargeable batteries they list the voltage when the battery is 100% approximatly and they give you the voltage the battery will be at approximatly when it is fully empty?

    If this is the case then I am all set.

    Also if this is the case one could measure the max current at the 100% and 0% levels and based on them and the voltages for the 100% and 0% level find the internal resistance at those levels....

    Then that gets back to the book I have and the talk about internal resistance and baseing it all on this would be the same as baseing it all on the voltages ...etc
    But either way doesn't really matter as long as I have a way of determining the % a battery is at with a circuit I create....

    Also I am wondering if the spec's for nonrechargeable batteries will give these voltages 100% or 0% so even if I cann't recharge them at least I know how used they are???

    I will be honest I haven't really check any spec's yet I will see if I can find these voltage levels for 100% and 0% that you are talking about....

    When recharging you mentioned you don't charge at a fix current so using a steady 1/2 amp or 80% of 1/2amp to charge the batteries would not be as good as varying the charge in someway...

    What is the ideal way to vary it is it linear y =mx+b or some nonlinear function to do it ?
    Because if it is linear that would be much easier to implement in a circuit.

  14. kubeek


    Sep 20, 2005
    You still miss the point that the 0%-100% of "full life" depends on how you use the batterries. If your circuit need 0.01mA current, then the "totally flat" side will be way deeper discharged than when you need 100mA.

    That means, to get the 0-100% rating you need to actually measure the curve of voltage through time while supplying current the actual load. When the voltage or current falls under your threshold, that is the 0% point. From the graph you can find the points where your 25, 50, 75% are, but you have to decide if you are talking about percentage of "capacity" or about percentage of battery life under the standard load.

    And please read this, preferably the whole page, word by word:
    Last edited: Jan 12, 2010
  15. retched

    AAC Fanatic!

    Dec 5, 2009
    Also those life meters on batteries, a lot of the time, are timers of sorts. And after a few cycles, are just plain wrong. There is usually a way to re-calibrate the life meter, but that is a crap-shoot. The microprocessor-based life meters sometimes use built in charts against cell temp after a charge cycle. That sets the drain time for the meter and it updates itself to is own aging by temp/voltage chartings at charge completion.
  16. Audioguru


    Dec 20, 2007
    The spec's for lead-acid batteries give the fully charged voltage and the minimum voltage.
    Ni-Cad, Ni-MH and Lithium batteries are different.
    Ni-Cad and Ni-MH battery cells are said to be 1.2V but they are 1.4V to 1.5V when still charging and are fully charged. Their voltage gets lower and lower as they are discharged but the datasheet just shows the curve and you must pick the minimum voltage yourself.

    Lithium cells have a max allowed charging voltage of 4.2V but a cell is only 70% fully charged when its voltage reaches 4.2V. Then it must continue charging for at least another hour until its charging current drops to be very low.

    It is difficult and wasteful to measure the internal resistance of a powerful battery.

    You must determine how much voltage drop your circuit can cope with.
    The datasheets for alkaline battery cells show how much the voltage drops at various currents at various durations.