An elementary question about a battery's Ah or mAh capacity and its implications.

[navienavnav]

I came across the following explanation on some forum on the internet and it has had me confused :

if you start with a battery that is rated at 3000mAhr, and it's nominal voltage is 1.5 volts, and then you put a load on it that draws 1000mA aka 1 amp, it should run about 3 hours

Now my doubt :

Let's say I have a battery which is rated at 3000mAh and the open terminal voltage at fully charged condition is 4.2 v. Now suppose I put a load on it which draws a current 0f 1000 mA when a potential of nearly 4.2 v is applied across it. Now, here, what does 'mAh' really mean? Does it mean that it can provide a current of 1000mA for three hours? But from experience, the voltage of the battery begins to drop once it is started to be used. Hence, since the voltage begins to drop, the potential across the load will drop and thus, it won't draw a current of 1000mA from the battery anymore and the amount of current drawn will begin to decrease (that's what I am thinking, please correct me if I am wrong.) So, what exactly does it mean when someone tells me that the capacity of the battery is 3000mAh? How am I suppose to relate this information to real life scenarios? :shock:

 

[shahbaz.ele]

thoratically the staement looks true.
but practically you can not say that.
it should show 70% of 3 hours

 

[navienavnav]

it should show 70% of 3 hours

**broken link removed**

 

[shahbaz.ele]

70% of 3hours
(70/100)*3=2.1hours
means more than 2 hours

 

[navienavnav]

But does it mean it will provide a constant current of 1000mA continuously over a period of 2 hours? But that to happen, won't it be necessary that the battery also maintains the terminal voltage at 4.2 V continuously over the time period? Is that possible?

 

[shahbaz.ele]

a good battery should maintain its output

 

[Ratch]

naviennavnav,

Perhaps all your life you have been told that a battery gets "charged", and after it is charged, its capacity is X amp-hours. Well, batteries don't get charged, they get energized. A 3 amp-hr battery has enough chemical energy to pump 3*3600 = 10800 coulombs of charge into some electric circuit. That does not mean that 10800 coulombs of charge are stored in the battery. A practical battery will not be able to sustain its beginning voltage throughout its de-energize period, but even at a lower rate, it should eventually be able to pump its amp-hour capacity through a circuit.

Ratch

 

[enjunear]

Check out batteryuniversity.com for a bunch of good articles on battery composition, usage, runtime, failure modes, etc.

Maybe start with this one, since you are trying to understand run-time and voltage/current impacts.


The terminal voltage of a battery will drop as it's discharged... that's one way to measure/monitor how much energy is left (see also, DOD = depth of discharge). So, you cannot assume that the terminal voltage will remain constant during a discharge test. Also, you should not assume that the open-circuit voltage will be the same as the voltage once a load is applied; it will also decrease.

Batteries are rather non-linear devices, so be careful which assumptions and equations you use to model them.

 

[ahsan_i_h]

(that's what I am thinking, please correct me if I am wrong.)

may be you are thinking that battery is like a capasitor. But it is not. Capasitor voltage increase/decrease during charge/discharge. But battery voltage do not increase/decrease like capasitor.

 

[ALERTLINKS]

Well, batteries don't get charged, they get energized.

SUN rises from the east. Sure!

 

[ALERTLINKS]

And earth revolves around its own axis. (technically speaking).

 

[Tahmid]

But does it mean it will provide a constant current of 1000mA continuously over a period of 2 hours? But that to happen, won't it be necessary that the battery also maintains the terminal voltage at 4.2 V continuously over the time period? Is that possible?

Well, mAh along with voltage give you an indication of the energy. E=VIt. mAh is I * t. So, mAh is indicative of energy stored by the battery. But the output current will depend on the output resistance and potential difference. And the battery energy storage time depends on the output current. You have to also factor in the losses in the internal resistance and other factors contributing to inefficiencies.

And remember that the terminal voltage of a battery does not remain constant. As battery is discharged, the terminal voltage decreases.

Hope this helps.
Tahmid.

 

[Syncopator]

The terminal voltage of a fully, and recently, charged battery will always be rather higher than the battery's normally quoted voltage.

The reading will quickly settle to the nominal, quoted, figure within minutes of having a load connected. The voltage will then stay reasonably constant until it nears the end of its capacity.

You can ignore the slow decrease in voltage during a discharge cycle for most practical purposes.

The capacity of a battery is usually quoted for discharge over 10 or 20 hours, 10 being (I believe) being the most common. The capacity is, as you seem to know, the product of time and current.

So a fully charged 3000mAh battery will, or should, be able to deliver 3A for 10 hours.

At higher discharge rates the capacity is effectively reduced. In other words it will be able to deliver the stated current for less time.

 

[navienavnav]

Well, mAh along with voltage give you an indication of the energy. E=VIt. mAh is I * t. So, mAh is indicative of energy stored by the battery. But the output current will depend on the output resistance and potential difference. And the battery energy storage time depends on the output current. You have to also factor in the losses in the internal resistance and other factors contributing to inefficiencies.

And remember that the terminal voltage of a battery does not remain constant. As battery is discharged, the terminal voltage decreases.

Hope this helps.
Tahmid.

So, what I could gather from my own thinking and reason, as well as the discussion on here, 1400mAh gives no information about how much current will be "delivered" for what time period but it simply tells us that suppose there is a conductor (doesn't matter what the resistance,length,resistivity is) that has a current of 1400 mA flowing through it, then the amount of charge that has passed through the conductor in that hour is the amount of charge stored in the battery. That is, it is simply an indirect way of stating the amount of charge stored in the battery. Is this correct?

 

[Ratch]

navienavnav,

So, what I could gather from my own thinking and reason, as well as the discussion on here, 1400mAh gives no information about how much current will be "delivered" for what time period but it simply tells us that suppose there is a conductor (doesn't matter what the resistance,length,resistivity is) that has a current of 1400 mA flowing through it, then the amount of charge that has passed through the conductor in that hour is the amount of charge stored in the battery. That is, it is simply an indirect way of stating the amount of charge stored in the battery. Is this correct?

No, it is not. Evidently you have not read or comprehended what I posted on this thread back in May in post #7. Batteries do not store charge. They move or "pump" charge around at a specific voltage. Batteries with a higher amp-hour rating contain more energy, not charge.

Ratch

 

[Tahmid]

Batteries store energy - chemical energy - which is converted to electrical energy by the flow of charge.

 

[ALERTLINKS]

I have a video camera. It has different modes of operation. It displays remaining battery time in minutes. While recording it showes nearly 100 minutes when battery fully charged. Many times i have used it till battery fully drained and found the timing surprisingly accurate both in full span and to the end. that means if it shoes seven minutes left, it is so.

 

[Syncopator]

So, what I could gather from my own thinking and reason, as well as the discussion on here, 1400mAh gives no information about how much current will be "delivered" for what time period ..... Is this correct?

No, it is incorrect. It most certainly does tell us the current which it can deliver versus time. That's the whole purpose and meaning of the ampere/hour capacity.

You obviously do not understand what I wrote in my post #13.

To recapitulate:

The ampere/hour capacity of a cell or battery is the product of current and time. But note that the figure is usually given for a discharge time of 10 hours.

Over shorter times the capacity is rather less because the implied higher current generates greater heat, but that is a detail which needn't bother us for this explanantion.

A 1400mAh cell or battery should be able to supply 1.4A for 1 hour, 14A for 6 minutes (0.1 hour), 140mA for 10 hours, 14mA for 100 hours, 700mA for 2 hours, 350mA for 4 hours, 1A for 1.4 hours, etc etc, ad infinitum.

 

[navienavnav]

No, it is incorrect. It most certainly does tell us the current which it can deliver versus time. That's the whole purpose and meaning of the ampere/hour capacity.

You obviously do not understand what I wrote in my post #13.

To recapitulate:

The ampere/hour capacity of a cell or battery is the product of current and time. But note that the figure is usually given for a discharge time of 10 hours.

Over shorter times the capacity is rather less because the implied higher current generates greater heat, but that is a detail which needn't bother us for this explanantion.

A 1400mAh cell or battery should be able to supply 1.4A for 1 hour, 14A for 6 minutes (0.1 hour), 140mA for 10 hours, 14mA for 100 hours, 700mA for 2 hours, 350mA for 4 hours, 1A for 1.4 hours, etc etc, ad infinitum.

I still can't seem to buy that. What does the statement "A 1400mAh cell or battery should be able to supply 1.4A for 1 hour" even mean? For a battery of 7.2Ah, a load which demands 7.2A from the battery if connected to it will run for one hour and then suddenly stopping after an hour? It would imply that the battery would have to keep supplying 7.2A to the circuit for one hour (or nearly the same) but that just doesn't seem right from experience!

 

[enjunear]

I still can't seem to buy that. What does the statement "A 1400mAh cell or battery should be able to supply 1.4A for 1 hour" even mean? For a battery of 7.2Ah, a load which demands 7.2A from the battery if connected to it will run for one hour and then suddenly stopping after an hour? It would imply that the battery would have to keep supplying 7.2A to the circuit for one hour (or nearly the same) but that just doesn't seem right from experience!

You need to understand two key points.
1. Battery capacity is computed when the battery is operating within it's designed operating range.
This means that you should not run it outside of it's target operating voltage range. Once you reach the voltage that represents 100% DOD (depth of discharge), you should recharge the battery. Otherwise you risk permanent damage. The battery will continue to source current until all of the charge is exhausted, but that will generally leave you will a battery that will never fully recover. A well maintained battery can run near it's designed capacity for many cycles.

2. Battery capacity is a way of nominally quantifying how much potential energy exists inside a battery. Generally speaking, capacity is given at a discharge rate of C/10 (or t=20 hrs, for some manufacturers... read the datasheet for your particular battery). For example, if you have a 7.2Ah battery, it should reach 100% DOD in 10 hours if discharged at (7.2 / 10 = ) 0.72 amps. If you draw more current than that, the run-time to 100% DOD will be reduced. Read up on Peukert's Law, and check out this short lesson at **broken link removed** to see how the rate of discharge impacts the capacity [when you stop at the specified 100% DOD voltage].