Testing the mah rating

Discussion in 'Battery Issues' started by Renzuli, Jan 13, 2012.

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  1. Renzuli

    Renzuli Ultra Member Verified Member ECF Veteran

    Does anyone know of a way to test the mah rating of a battery ??? Say an EGO battery , say some one tells you it's 1100 mah , and in reality , it's only a 650 mah . Is there a way to test them , to find out ????
  2. Eddie.Willers

    Eddie.Willers ECF Wiki SysOp Verified Member ECF Veteran

    Supporting member
    Well, there's no easy way - that's for sure.

    The rating is an expression of the amount of time it takes to drain a battery by using a known load. It should be do-able but finicky.

    Do a thread search - I seem to recall some time back there was a suggestion that one could get a rough idea by firing an atty of known resistance in 5 second bursts and counting these 'pulses' until the battery worked no more.
  3. Duekster

    Duekster Senior Member ECF Veteran

    That is pretty much it, operate it under load over a period of time to see if the math works.
  4. stephpd

    stephpd Super Member ECF Veteran

  5. zoiDman

    zoiDman My -0^10 = Nothing at All ECF Veteran

    Supporting member
    But I thought that was some of the Mystery to mAh.

    What load is it being tested under and does Everyone use the Same Load when they come up with their mAh Rating?
  6. DarkAynjil

    DarkAynjil Senior Member ECF Veteran

    Actually, to determine the amp-hour (or in this case, milliamp-hour) rating of a battery all you need to know is how much current the battery supplied (in amps) over what time (in hours). Having a known resistance in this circuit doesn't help you, because the voltage will drop as the battery discharges, thus any calculation you make with ohm's law will not be accurate.

    For determining the ACTUAL mah of your battery, I would reccomend you fire the battery with your meter in dc current mode, leads in series with the circuit over a 2 ohm resistor, and time it. That would get you close.
    For truly accurate capacity measurement a low resistance bar of known resistance per square mm (also called a shunt) is preferred - however this is not advisable with Lithium-Ion batteries, unless you have a very controlled lab set up.

    Hope that helps.
  7. cskent

    cskent Vaping Master ECF Veteran

    To test the mah rating you'd have to put a fully charged battery under a known load until it is completely drained. An eGo battery, or a protected battery of any type will cut off when the voltage drops to a pre-set level. Therefore, you'd have to remove the protection circuit in order to test this type of cell. That wouldn't be adviseable with a li-ion cell.

    As an example, if you start with a 1000 mah battery at full charge and put a 10 mah drain on it, it should take 100 hours to completely discharge. If it takes 80 hours, well you've got an 800 mah battery instead of a 1000 mah.

    That's the theory anyway. Other factors can influence your reading such as temperature, or age of the cell.
  8. zoiDman

    zoiDman My -0^10 = Nothing at All ECF Veteran

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    Thanks, it does.

    Yeah, that’s where those God Awful Differential Equations come in.

    Since the Voltage Rate of Change from say 4.2v ~ 3.3 volts is Non-Linear, the Dif-Eq will probably be Nasty at Best.

    So If the Same 1,000 mAh Battery was tested at 20 mAh, would the Discharge time be Exactly one half the time of what it was when tested at 10 mAh?

    If it does then I think I'm getting this.

    I guess I should have paid more Attention in those EE classes instead of checking out the Undergrad next to me in the Short Skirts back in the 70’s.
  9. Duekster

    Duekster Senior Member ECF Veteran

    There is the aspect of the chemical power source dropping off the E as it discharges but other wise it is straight math with a known load and time.
  10. zoiDman

    zoiDman My -0^10 = Nothing at All ECF Veteran

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    If I'm understanding this correctly.

    The Straight math part is put a Constant Load on the Battery and then Time how long it takes the Battery to go from Full Charge to Full Discharge.

    Then Multiply the mAh times the number of Hours

  11. stephpd

    stephpd Super Member ECF Veteran

    Works the same way from fully discharged to fully charged too.
    Charging there's a constant voltage and current up until the very end, when the light starts blinking.
    But at that point the battery is close to 95% charged. (or somewhere around there.)
  12. Duekster

    Duekster Senior Member ECF Veteran

    1000 mAh = 1000 hours X 1 mAh load or 1 hour X 1000 mAh load.

    Using the charge method would likely be the easiest way as suggested I suspect you will put in more charge than you get out just because nothing is 100% efficient.

    Also batteries do decline with increased cycles and the number of cycles they can handle is determined by chemistry and how hot the charger ( rapid charging) is refilling.
  13. zoiDman

    zoiDman My -0^10 = Nothing at All ECF Veteran

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    Yes, this is true. As a Battery is used more and more, it's mAh will decline.

    BTW - I didn't want to imply that the Math was Difficult in Determining the Total mAh a Battery may have. Multiplying a mAh times a given amount of time seems pretty Straight Forward.

    But what if you would like to Determine when a Battery Reaches 75% of its Maximum Capacity? Or it output Voltage at Any given time between Fully Charged and Fully Discharged?
  14. Duekster

    Duekster Senior Member ECF Veteran

    I am going to know the battery is in decline when I have to swap them more but my juice use is the same.
  15. zoiDman

    zoiDman My -0^10 = Nothing at All ECF Veteran

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    That is one way to know.
  16. DarkAynjil

    DarkAynjil Senior Member ECF Veteran

    Yes, you are correct. It's always important to remember when dealing with batteries that voltage is roughly indicative of charge level. Since batteries don't have built-in current limiting other than internal resistance, it's also best to be very careful when shunting them (with a load-tester on your car battery, for example).

    Me too. I'd be able to explain it all better if I could remember all the math.
  17. zoiDman

    zoiDman My -0^10 = Nothing at All ECF Veteran

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    I hear you.

    I was looking thru an old Dif-Eq book last night that had a lot of Circuit Problems in it.

    The Concepts came back Very Fast but doing the Calculus wasn't as easy as it was 20 years ago.
  18. stephpd

    stephpd Super Member ECF Veteran

    And then there's still the problem of Ego type batteries not fully discharging.
    When voltage gets to a certain point they stop working and the light flashes.
  19. DarkAynjil

    DarkAynjil Senior Member ECF Veteran

    True. However, almost every lithium-ion battery has this built in circuit to shut the battery off at a threshold voltage, for safety, and to preserve the battery from reaching an "unchargeable" state.

    AW IMR's seem to be an exception - I have drained them to around 2.0 volts accidentally, they would have kept supplying current if I had kept hitting the fire button. IMR's typically don't have the circuit protection built-in, and are often sold as "unprotected".
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