Battery ratings


1. Battery basics.

Contents:
1.Voltage.
2.Current.
3.Capacity.
4.C-rating.
5.Watt-hour.



1.Voltage.

             The voltage that we see on a cell is nothing but Nominal voltage. Nominal voltage is also referred to as average voltage. Battery manufacturers always mention nominal voltage on the cell/battery. Let's take an example of a Li-ion cell, it's nominal voltage is 3.7v. But it's maximum voltage is 4.2v when fully charged. The minimum voltage is 3v.
Warning: Don't discharge the cell/battery below its minimum operating voltage that leads to battery damage. In the case of the Li-ion cell, it is 3v.

*All of this information you can get in the battery/cell datasheet.

If we are using a cell/battery directly to the load, then the load will not get constant voltage. Voltage keeps on decreasing. That is when the voltage regulators come into the play to provide a constant voltage.

2. Current.

               On every battery/cell you get to see the mAh rating. mAh stands for Milliamps hour. 
Let's take again Li-ion 18650 cell, it has got 2600 mAh rating. That means this cell can deliver 2.6A of current for 1 hour continuously.
If the load connected to the cell is drawing 2.6A of current then for 1 hour the cell can drive the load.
If the load connected to the cell is drawing 1.3A of current then for 2 hours the cell can drive the load.
If the load connected to the cell is drawing 5.2A of current then for (1/2) half an hour the cell can drive the load.   (There is a reason below for why this sentence is in red).

3.Capacity. 



                     Batter/cell capacity is nothing but the mAh rating. 

                     Battery life = Battery capacity(Ah) / load(A).

  • If the load is drawing 2A,  Battery life = 2.6/2 = 1.3h.
  • If the load is drawing 4A,  Battery life = 2.6/4 = 0.65h.
  • If the load is drawing 2.6A,  Battery life = 2.6/2.6 = 1h.
  • If the load is drawing 1A,  Battery life = 2.6/1 = 1.6h.

  • If the load is drawing 3.4A,  Battery life = 2.6/3.4 = 0.76h.

4. C-rating.

                      Take a look at the above two red-colored statements. Can you guess what's wrong with them? The very first question that I get when I read those statements is, Does battery provide any amount of current that load demands? And the second question is, Is there something called a maximum current that a battery can give out?   

Does battery provide any amount of current that load demands? 
Ans: No. 

Is there something called a maximum current that battery can give out?   
Ans: Yes.   

The maximum current that a battery can deliver is determined by C-rating.

Let's take some examples:

1. Li-ion cell is rated as 3.7v,2600mAh,1C.
max. current = 2.6A*C-rating = 2.6*1 = 2.6A

2. Cell/Battery is rated as 3.7v,2600mAh,2C.

max.current = 2.6 x 2 = 5.2 A.

3. Cell/Battery is rated as 3500mAh,30C.

max.current = 3.5 x 30 = 105A. 


So.

If the Li-ion cell that we have taken above as an example is having a 1C rating. 

max. current = 2.6 x 1= 2.6A.
  • If the load is drawing 4A,  Battery life = 2.6/4 = 0.65h.           wrong, 4A 🤣not possible  
  • If the load is drawing 3.4A,  Battery life = 2.6/3.4 = 0.76h.     wrong, 3.4A not possible🤣

If the Li-ion cell that we have taken above as an example is having a 2C rating. 

max. current = 2.6 x 2 = 5.2A.     
  • If the load is drawing 4A,  Battery life = 2.6/4 = 0.65h.           Correct 4A possible 👍.
  • If the load is drawing 3.4A,  Battery life = 2.6/3.4 = 0.76h.     Correct 3.4A possible 👍.
Why there is a minimum current that a battery/cell can safely deliver?

Ans:  Internal resistance.


 * Internal impedance limits the amount of current that the battery can deliver.
* In reality, the internal impedance is just a limitation of the battery chemical reaction and electrodes.






5. watt-hour(wh).

         
3.7v, 2200mAh



11.1v, 2200mAh








                 

We defined the capacity of the battery as mAh.  The above cell and a battery are having the same mAh rating.   So, Do they have the same capacity? 

Now let's define another term called watt-hour,

watt-hour = V x Ah.

Watt-hour tells the amount of energy that a battery contains.

So,   1st cell has 3.7 x 2200mAh = 8.14wh of energy,   The batter on the right side has 11.1 x 2200 = 24.42wh.
Therefore 11.1v,2200mAh has more capacity.

Example:
             
 

  Nominal voltage = 7.4v
  capacity in mAh = 5000mAh, it can deliver 5A for 1 hour continuously.
  C-rating  =  50C

Max current = 50 x 5 = 250A,       5Ah/250A = 1.2 minutes , Just for 1.2 minutes battery can deliver                                                                                                                                                           250A.
Watt-hour/ energy = 5 x 7.4 =37wh.  Can handle 37w load for 1 hour.


Post #2: Battery pack, BMS, Types of battery, battery charging.

                      






     
      



Comments

  1. ELECTROBOOSTMay 21, 2020 at 10:50 AM

    Wonderfully explained!

    ReplyDelete
  2. UnknownAugust 19, 2020 at 5:16 PM

    Hi,
    You mistyped this row: "If the load is drawing 1A, Battery life = 2.6/1 = 1.6h."

    ReplyDelete
    Replies
    1. MADHUKUMAR.S VITianNovember 10, 2020 at 1:11 PM

      Yes , I'm sorry , I will correct it, Thank you for pointing it out

      Delete
  3. UnknownAugust 19, 2020 at 5:28 PM

    I am expecting only one more logical step after the
    "Can handle 37w load for 1 hour." example:
    Show with C rate.
    Can handle 37w *50 = 1850 W load for 1 hour/50 =1.2 minutes."
    This is the max Watt, that the example battery can provide.

    ReplyDelete
  4. UnknownAugust 19, 2020 at 5:31 PM

    In this part:
    "If the load is drawing 4A, Battery life = 2.6/4 = 0.65h. Correct 4A possible 👍.
    If the load is drawing 3.4A, Battery life = 2.6/3.4 = 0.76h. Correct 3.4A possible 👍
    "

    All can be green.

    ReplyDelete
  5. UnknownAugust 19, 2020 at 5:34 PM

    Good article. Grat.

    ReplyDelete

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