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# Battery Question_2

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#### khatus

##### Member level 3

Hello guys the meaning of underlined lines is not clear to me. What I understand is that a battery car with a high energy density will cover long distances at low speed. Conversely, a battery car with a high relative capacity will cover short distances at high speed. Can anyone tell me whether I am right or not? What is the reason for batteries with high specific power usually have low energy density?

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Hello guys the meaning of underlined lines is not clear to me. What I understand is that a battery car with a high energy density will cover long distances at low speed. Conversely, a battery car with a high relative capacity will cover short distances at high speed. Can anyone tell me whether I am right or not? What is the reason for batteries with high specific power usually have low energy density?
You have understood right.

One term deals with energy and the other aspect is how the energy is delivered per unit time (power).

Usually batteries are rated by the total energy they can store. But the rate at which that energy (power) can be delivered is also important.

And specific usually means per unit mass; that helps relative comparison. For example a D-size cell does store more energy compared to a AAA; but the effect is mainly due to the larger size, Hence the specific energy density will be the same.

But if you need lots of energy delivered very quickly (you want a high speed for the car), you need high power. To compare different chemistry (or even brands) you need to look up their specific power.

But why a battery cannot deliver all the energy very fast? The reason lies in the chemistry (the reaction processes within the cell) and the internal resistance (causes a reduction of voltage when you take high current) irreversibility of the electrochemical process..

So batterery energy capacity is often specified at a given discharge rate. Higher the discharge rate, the greater is the waste of energy within the cell.

think of this (battery) as a water tank.
the faster the speed of water flows out of the tank, (with a wider hose pipe, the hose pipe being current thus voltage), the lesser time it takes for the tank to become empty and the lesser water the tank hold as a result of fast discharge of water; and vice versa.
so the current i.e voltage is the pipe, the amount of water coming out, or going in to the tank, is the charge, i.e Ah and the tank itself is the battery, i.e Kwh.
Thus V x Ah = watts or Kwh.

Hope that helps.

think of this (battery) as a water tank.
the faster the speed of water flows out of the tank, (with a wider hose pipe, the hose pipe being current thus voltage), the lesser time it takes for the tank to become empty and the lesser water the tank hold as a result of fast discharge of water; and vice versa.
so the current i.e voltage is the pipe, the amount of water coming out, or going in to the tank, is the charge, i.e Ah and the tank itself is the battery, i.e Kwh.
Thus V x Ah = watts or Kwh.

Hope that helps.
Confused?

Current and voltages are completely independent quantity. A given battery can have any voltage. This voltage is independent of the current capacity (deliverable) of the battery.

I had a mercury cadmium cell (see for details: https://en.wikipedia.org/wiki/Weston_cell) that has a voltage of 1.018V but cannot deliver more than a few microamp of current (but that is because they have soldered a 10k resistor within the package). I used it (more accurately, used to use it) to test my digital multimeter (good old days).

George Simon Ohm found a law that relates the flow of charge (current) in a conductor with the electrical force (the electric potential across the conductor) across it. This law defines a new property of the conductor (called resistance) that uses two earlier defined quantities: voltage and current.

You are also confused between current and charge. Current is the rate of flow (dQ/dt; rate always refers a derivative wrs to time) of charge. Charge flows, produces current.

Watts is power, rate of energy delivered (dE/dt); hence watt-hour is energy (60*60 joules; 1J= 1 Watt-sec).

Voltage is a force, the causative agent for the movement of charge. It is produced by an electric field. Every point in the electric field is associated with a potential (or voltage; voltage is more commonly used for a potential difference).

Still confused?

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