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Answered 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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