Laptop Battery Life: internal vs external battery

September 1, 2007 at 2:25 pm 3 comments

My Acer 3680 only last about 2 hours on a charge. Should I buy another internal battery or buy an external battery?

First, let’s examine the laptop’s battery and AC adapter:

  • Battery: 4000 mAh, 11.1 V, 44.4 Wh Li-ion.
  • AC Adapter: 19 V, 3.42 A, 65 W

The best way to measure a battery’s capabilities is Watt hour. The formula for Watt Hour is:

Watt Hour (wh) = Voltage x Amp-Hour

For the Acer battery, the Watt hour would be 4 A x 11.1 V = 44.4 Wh. So if you buy an external battery taht’s 44.4 Wh, you’ll get the same battery life as your internal? That depends on your internal voltage vs external voltage. If you look at the Acer battery specification, it list a voltage of 11.1 V, but if you look at the AC adapter, it list 19V. To get the same battery life on an external battery, you would need a battery with 4 A x 19 V = 76 Wh. Essentially, you need a much bigger battery because of the higher voltage.

Let’s compare the battery life with what is typically available on the market:

Capacity (Wh) Percentage from Baseline Est. Battery Life (Hours)
40 Wh 53% 1:04
50 Wh 66% 1:19
60 Wh 79% 1:30
76 Wh (baseline) 100% 2:00
80 Wh 105% 2:06
100 Wh 132% 2:38
120 Wh 158% 3:10
140 Wh 184% 3:41

Internally, the Acer has two battery options:

  • 4000 mAh, 11.1 V, 44.4 Wh Li-ion.
  • 7200 mAH, 11.1 V, 79.9 Wh Li-ion

So the battery life would be:

Capacity (Wh) Percentage from Baseline Est. Battery Life (Hours)
44.4 Wh 100% 2:00
79.9 Wh 180% 3:36

Amazon currently sale an 80 Wh external battery at around $205, which will give me another 2:06 of runtime. I could get the 44.4 Wh internal battery for $160 for the same amount of battery life. In the end, the internal battery almost always seem to be a better deal.


Entry filed under: Hardware, Laptop.

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3 Comments Add your own

  • 1. Andy  |  September 4, 2007 at 8:28 am

    Great entry. Lots of good info on the topic with a minimum of fluff.
    Thanks for posting this.

  • 2. external laptop battery  |  September 14, 2007 at 10:42 pm

    This little hack was sent in by [andrew], he built it specifically for the long flight from Australia to the US. The idea is simple, but I wouldn’t have considered it given the cost of decent D Cells these days. H put together a 20 D-cell p0wer pack, a regulator to keep it at 24v, and an eighth inch plug External laptop battery to power up his old ib00k.

    Update: I goofed and left out [andrew]’s name.

  • 3. snowyowl  |  October 3, 2007 at 4:52 pm

    keep in mind that if you use external batteries allow continual use while having multiple batteries are cheaper you will need to shut down or suspend to memory assuming that it can hold the charge long enough for you to swap batteries.


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