# Confused about battery's specific power and energy

Hello there,

I am so confused about the term battery specific energy (in Wh/kg) and specific power (in W/kg)

Actually, I know what it means by power and energy, of course P = E/t

But looking at the text, which quotes like "Ni-Cd batteries have high specific energy (than Ld-acid) but the disadtantage is they have low specific power"

and the table says,

Lead-acid 40 Wh/kg 180 W/kg
Nickel-cadmium 50 Wh/kg 120 W/kg
Nickel-metal hydride 70 Wh/kg 200 W/kg
Lithium-ion 130 Wh/kg 430 W/kg

What causes this to happen? How come, for the Ni-Cd which has higher Wh/kg has a lower W/kg compared to Ld-acid?

Isn't power derived from energy divided by time?

Thank you

## Answers and Replies

Maybe an analogy will help:

The energy density of a battery is comparable to how many people a room can hold (i.e. the size of the room), while the power density is comparable to how quickly the people in that room are able to escape/leave the room (i.e. the number/size of exit doors). A high energy-density battery with a low power-density would then be equivalent to a large room full of people, but with only a single small exit door.

Typically, the electrical equivalent of the small door would be a high internal (current-limiting) resistance.

Maybe an analogy will help:

The energy density of a battery is comparable to how many people a room can hold (i.e. the size of the room), while the power density is comparable to how quickly the people in that room are able to escape/leave the room (i.e. the number/size of exit doors). A high energy-density battery with a low power-density would then be equivalent to a large room full of people, but with only a single small exit door.

Typically, the electrical equivalent of the small door would be a high internal (current-limiting) resistance.

Hi gnurf,

This is so clear. I not get it.

Thank you so much

sophiecentaur