Does mAh measure how long a battery would last?

  • I know mAh tells how much milliamperes a battery can deliver in an hour. But does that also tell how many hours the battery would last? Sorry but I don't really get it. If we're talking about a water tank, to my impression, mAh is like how big the faucet is and not how much water there is in the tank. I'm really confused as to why we measure battery capacity in mAh if my understanding about it is correct.

    The mAh of a battery, IS a "general" indicator of how long a battery will last. a 3000 mAh battery will last 3 times more that a 1000 mAh battery (in the same circuit/application). However, neither will last the total "current-time" specified by the manufacturer, due to losses and the minimum requirements of the circuit/application.

    No - it's how much water is in the tank, not how big the faucet is. mA (or Amps) is already a "per second" unit e.g. using your water analogy: gallons per hour. So mAh is equivalent to (gal / h) * h = gallons of water.

  • Phil Frost

    Phil Frost Correct answer

    7 years ago

    mAh (or mA·h) is not how many milliamperes a battery can deliver in an hour. That would be mA/h. Current, measured in amperes, is already a rate of stuff. Specially, one ampere is one coulomb per second. So, if current is like speed, then mA/h is like acceleration, and mAh is like distance.

    Rather, mAh it is a unit of charge. It is what you get when you multiply current by time. By multiplying by time, the "per time" part of the ampere is cancelled, and you get back to charge.

    If an ampere is a coulomb per second, then:

    $$ \require{cancel} 1~\mathrm{mAh} = 1\cdot10^{-3}~\mathrm{\frac{C}{s}h} $$

    and by dimensional analysis:

    $$ \require{cancel} \frac{1\cdot10^{-3}~\mathrm{C\cancel{h}}}{\cancel{\mathrm{s}}} \frac{60\cancel{\mathrm{s}}}{1\cancel{\mathrm{min}}} \frac{60\cancel{\mathrm{min}}}{1\cancel{\mathrm{h}}} = 3.6~\mathrm{C}$$

    For example, if you draw 1 mA for 1 hour from a battery, you have used 1 mA · 1 h = 1 mAh of charge. If you draw 2 mA for 5 hours, you have used 2 mA · 5 h = 10 mAh.

    You can approximate how long a battery will last by dividing its total charge (in mAh) by your nominal load current (in mA). Say you have a 1800 mAh battery, and you connect it to a 20 mA load:

    $$ \require{cancel} \frac{1800~\mathrm{mA\cdot h}}{20~\mathrm{mA}} = \frac{1800\cancel{\mathrm{mA}}\cdot\mathrm{h}}{20\cancel{\mathrm{mA}}} = 90~\mathrm{h} $$

    This is an approximation because:

    • The charge capacity (the number measured in mAh) is determined by measuring how much charge can be removed from the battery before voltage drops to some arbitrarily selected level where the battery is considered "discharged". This may or may not be the threshold at which your circuit no longer functions. Battery manufacturers, wanting to make their batteries seem as good as possible, typically select a very low threshold voltage.

    • Assuming you are considering charge available only down to some voltage threshold, the actual charge available from the battery depends on temperature, and the rate at which you discharge it. Lower temperatures slow the chemical reaction in the battery, making it harder to extract charge. Higher rates of discharge increase losses in the battery, decreasing the voltage, thus hitting the "discharged" voltage threshold limit sooner.

    • The electric potential difference provided by the chemicals in the battery is actually constant; what makes the voltage decrease is the depletion of the chemicals around the electrodes and degradation of the electrodes and electrolyte. This is why battery voltage can recover after a period without use. So, the point at which the threshold voltage is reached can actually be quite complex to determine.

    If you can find a good datasheet for your battery, it may give some insight into the parameters under which these calculations were made.

    Very well explained. So mAh is like the total distance traveled when the speed is multiplied by time. Same goes for ampere when multiplied by time, we get the total charge generated.

    @supertonsky yes. Except, I would say charge isn't generated; it's just moved around. A battery is essentially a pump for charge (but not a charge pump; that's a different thing).

    You can also say: '/' = 'per', while '.', or 'x', or often '' = 'for an'. So 2000mAh is 2000mA for an hour.

  • Milliamp-hours is a measure of current capacity over time. It is a representation of how much total charge a battery has. If you use the battery to operate something that doesn't require much current, it will last a long time.

    Be aware that batteries (cells, really) have a nonlinear depletion characteristic. Even though milliamp-hours is a finite amount of charge, you must realize that not all of it will be usable by a given load at a given voltage, and that the value given by the manufacturer is generally for the case where the cell is powering something with low current demands. In that situation, you get almost all of the available energy. However when you power something that requires more current, you won't actually get the full capacity.

    Technical edit, per Phil's comments: By saying " won't actually get the full capacity." I mean "You won't actually get the full capacity given the same load which requires a certain voltage to operate." The cell's voltage will drop and become insufficient for the load, at which point the charge is still in the cell, but it's not necessarily usable.

    Consider the datasheet for Energizer AA cells. A chart is provided which shows you the various milliamp-hour capacities at various loads:

    AA Battery mAh Depletion Chart

    If you continually power a device with 25 mA, the cell will have approximately 2750 mAh. If you divide this current into the capacity, 2750/25, you get the number of hours that the battery can sustain it: 110. If the load is 500mA, the cell's usable capacity actually drops to approximately 1500 mAh, and 1500/500 is only 3 hours.

    Devices like remote controls do not use continuous power. They spend most of their time in an idle or "sleep" state, and consume power only when you press a button. In those cases, cells will continue to be viable and power the device for a very long time. The milliamp-hour capacity chart is based on usage, not on idle time.

    Environmental effects and physics will erode the chemistry in a cell even when it isn't being used. The datasheet assumes you are working with fresh cells and within certain environmental conditions.

    "Even though milliamp-hours is often treated like a finite amount of energy" suggests that maybe \$mAh\$ is a unit of energy, but it's not. Also, at higher rates of discharge, you *do* get the full capacity, but you get it at a lower voltage. Note that the graph says "discharge *to 0.8 volts*".

    I should probably change "energy" to "charge" for accuracy. I'm not sure I understand/agree that the full mAh charge is available at higher discharge rates. The cell discharges and its voltage lowers at any discharge rate, but it's a matter of how soon that occurs. Isn't it true that a higher discharge rate decreases the *efficiency* of the cell and therefore total mAh capacity is reduced?

    Not really. There is so much charge separation in the battery, defined by the chemicals in it and the quantity of those chemicals, and the charge can't go anywhere else. At a higher discharge rate, the voltage at the terminals decreases, and can drive the charges less quickly, but they are still separated. You just might have to wait longer for them to un-separate, and you will have less voltage available at the terminals, and thus less electrical energy.

    Also: Charge conservation. If you also consider that the battery has no self-discharge (valid only for some battery types, I suppose) this is how you conclude all the charge must go through the load. If these is self-discharge, then there's a point at which lower discharge rates are less efficient, because most of the energy is lost to self-discharge. For alkaline batteries however, self-discharge is very low.

    @Phil Thanks for the clarifications. I've added some more to my answer per your comments.

    @PhilFrost Your August 2013 comments here state that you still get "full capacity " from a battery at increasing discharge rates. This is not the case for essentially all practical battery chemistries and batteries. The gross amp-hour capacity decreases with increasing current and the Watt-hour capacity also decreases. eg a battery rated at say 10Ah at C/10 rate will give progressively lower Ah capacities at higher currents **AND** the same applies for gross delivered Watt-hours whether at constant or variable Watts - higher rates give lower capacities. ....

    .... While this is slightly complicated by eg constant current loads or constant power loads or constant resistance of whatever, the result is consistent - a load which increases rate of depletion also decreases available resources. | Mechanisms vary and interact. Internal resistance causes higher % internal losses at increasing I and may also increase more rapidly due to chemical-mechanical effects. Secondary reactions may occur at greater rates. | A 'confounding' effect is that at very low rates "self discharge" will use increasing amounts of available energy, but this is a secondary factor.

  • As a very rough guide it will give you a ball park figure for current/timing. So a 100mAh capacity battery would be expected to give 10hr @ 10mA or 1Hr at 100mA. In reality you get less. This will depend on the battery type, its age, condition, temperature etc.

  • Current is a vector of charge over time across a load, ie. It is the rate of coulombs per second. So to get a measurement of charge, then we multiply the rate by the time. For example, we got a one hour marathon sprint.. if i could sprint at 10 km/h for a whole hour then I'd have run 10 KMs. Speed in km/h is already referenced as a rate over time so its more straight forward to work out. If we had a specific unit for measuring speed, lets say if:

    1 Gonzales = 1 km/hr

    Then my rating for being able to sustain a speed of 10km/hr for one hour would be 10GH (Gonzales Hours).

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Content dated before 6/26/2020 9:53 AM