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What does 53Whr mean in terms of battery life for a new laptop?

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2014-05-05 16:21:29

"Whr" is an invalid unit. The correct unit is "Wh" which is the

SI unit of power multiplied by time, typically hours, but minutes

and seconds are equally valid as in "Wattminutes" and

"Wattseconds."

A simple answer is that a 53Wh battery will last half as long as

a 106Wh battery. But what does that really mean?

To answer the question more fully, batteries are typically rated

in Ah or Amphours which means a battery with a rating of 50

Amphours (like a typical car battery) can deliver 50 Amps for one

hour, 10 Amps for 5 hours, etc. Note that this is strictly a

current over time rating without regard to the battery voltage.

However, Ah is NOT the amount of power a battery can supply.

Power = Amps x Volts = Watts. However, Watts are a RATE of power

being supplied, not how much in total. To signify an AMOUNT of

power, you need to add time. Thus, Watts x time = total amount of

power, or more correctly, energy supplied.

So, taking your question of "What does 53Whr mean in terms of

battery life for a new laptop?", we can answer that in terms of how

long the battery charge will last because all batteries

fundamentally suppy x amount of current for x amount of time

regardless of what the battery voltage is. But we need to know the

voltage in order to determine how much energy a battery can

supply.

If we work backwards, and take 53Wh, we are starting with the

total amount of energy the battery can supply. But that does not

give any real idea as to HOW LONG the battery will run the

computer.

To know how long the battery will last, we need to know just a

few of several possible things: the battery's Amphour rating and

the current drawn by the computer or, it's Wattage (Watts drawn)

and the battery's Wh rating. The latter one you know. Or, it's

voltage and Amphour ratings. From these we can convert back and

forth to any other unit(s).

Since you only have the battery's Wh rating, to figure out the

battery's Amphour rating, one must know the battery voltage because

Watts = Amps x Volts.

There are multiple ways to approach this question depending on

what units you are given but we can really only make an estimate as

the amount of power modern computers use depends on what you are

doing. Like is the backlight on high or low? Or, are you typing a

note, or playing a high-power game, or watching a DVD etc. etc...

.

The only way to mostly answer your question verbatim is to make

some assumptions, get an answer, and show you how one calculates

the run time. Then, you would have to use actual values to get a

"real" answer.

Here are three paths leading to the same answer:

First, let's assume that, Battery rating = 53Wh, and Laptop

nameplate power is 35 Watts.

We know the laptop will use anywhere from 35W to maybe 1W so we

take the worst case of 35W.

Very simply, Wh = Watts x hours. Thus, hours = Wh ÷ Watts.

So 53Wh ÷ 35W = 1.51 hours or 91 minutes. That's the simplest

way.

But, what if instead the laptop's label says 2.92 Amps @ 12

Volts?

In this case, we know the battery voltage, and the laptop's

current comsumption, but we need to convert that into energy which

is what the battery provides, and is what Watthours are.

Using Ohm's law (sorry if this is getting too technical), Amps x

Volts = Watts.

Using our assumed numbers, 12 Volts x 2.92 Amps = 35W.

Appying the first scenario, the run time would then be the

battery's 53Wh ÷ 35W = 1.51 hours, the same time we calculated

above.

As long as you know the battery voltage, and either Watts

consumed, or the current (Amps) drawn, you can convert into Wh and

determine how long the battery should run.

Finally, a third way to get the same answer: Again, assuming the

battery is 12 Volts, and is rated at 53Wh, we can say 53Wh ÷ 12V =

4.42 Amphours (Ah). Assuming the same current draw of the laptop of

2.92 Amps, we can calculate 4.42Ah ÷ 2.92Amps = 1.51 hours, again,

the same time.

Interestingly, you asked about a "new" laptop. As the battery

ages, it's Amphour and thus Watthour rating will slowly decline

until it's energy capacity becomes low enough to become a pain in

the butt. Then, it's time for a new battery!


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