Capacity of a Battery

  • Capacity of a cell determines how much energy is available in the cell. To make it simpler, it is the measure of stored energy and is measured in amp hours (Ah) or sometimes in milliamp hours (mAh), where 1Ah is equals to 1000 mAh.
  • Batteries can be from few miliAmphours to well over 100Amphours.
  • Let me show you an example, as u can see in this battery it says 1000 mAh which means it is 1Ah.
  • Occasionally u may even find Watt hours (Wh)  listed on a cell instead of the amp hours, well that’s okay because Watt hours is another unit of energy. Now how do we find amp hours?
  • We would need nominal voltage of a battery. When we divide the watt hours by nominal voltage we get amp hours.
  • Confused? Let’s simplify it with an example!!
  • Let’s take a lithium ion cell with nominal voltage of 3.7 Volts and capacity rating in Watt hours of 11.1 WH; by simply dividing 11.1 WH by 3.7 Volts we get 3amphour which is equals to 3000milliamphour. I hope it simplified.
  • So the formula is Amp hours of cell is equals to capacity rating in Watt hours divide by nominal voltage.
  • Remember capacity rating only tells us how much energy the cell can store and provide.
  • One last note about capacity, you should not expect your cells to achieve their entire rated capacity. As manufacturers usually test the capacity of their cells using two tricks to eke out as much capacity as possible.
  • They treat at an incredible low discharge rate; usually about 0.2 C (we’ll talk about C rating in our future videos).
  • They also discharge the cells all the way down to their absolute minimum rated voltage, often 2.5V for most lithium ion cells. Discharging that low is possible, but it will decrease the life time of the cell if done too often.
  • Most of the commercial products using lithium ion cells discharge down to around 3.0 V , if not higher, in order to get longer life out of the cells.
  • So just because your cell might say it’s rated 2.0 Ah, don’t be surprised to see it perform closer to 1.9 Ah when you test it.
  • So this was about Capacity. I’ll see u guys in the next video which will be going to be on Maximum discharge rate.