battery power

When recently confronted with a potential need to run a LAPTOP at a conference for 8-10 hours,  then an exercise was undertaken to explore external batteries on the market allegedly providing considerable extra power…..  but not so.  Read the fine print on the calculations!   Some of these ‘boost’ batteries are in fact capable of jump starting a diesel 4WD – and that’s really the short-burst power capability of these small devices.  They are NOT for extended consumption by a powerful laptop.  But the exercise certainly gave rise to a better understanding of what power usage is all about..


FYI: It’s the old story. If you want 1TB running RAID, plus another 1TB of storage, 8 processors and max memory running a 1080, with 5 operating systems running in virtualisation mode – you have to give up battery life as a compromise – welcome to my life. see:

Battery Myths

Whilst the label says:  16,800mAh  – but what can it do?   What voltage is it rated at?
Just what can you you do with this alleged 16800mAh of capacity?

the number more likely represents  3.7v  *  16800mA  being  62.16Wh  (but only as a PEAK!)

Stated Purpose
Let’s consider these small external JUMP-START batteries offering 250-300CCA (500-600 peak CCA) for starting a car’s flat battery.
Their design is based on short (3 second peak, and no more that 30 seconds)  connection for a short COLD START of a medium sized engine.

They do NOT offer holding power for sustained output.

Don’t fall for the 16800mAh versus  your internal battery of  4025mAh and dream that it will run for 4 times the normal use of your laptop.

The discharging process bears no relationship to such poor mathematical expectations

10 observations

1.   It is a Lithium Battery – but is it ION or Polymer
Of concern, was that the INPUT CHARGER was a light-weight humble  240v – 12v adapter only providing 1A output!
That’s an mere 12W charger trying fill up an alleged 16800mAh device!!   That will take 17 hours assuming no loss of current!

2.   Batteries cannot transfer power to another battery at a consistent energy RATE.
If they contain a 3.7 Polymer battery,
and if they have transformers that permit 5v   12v   16v   19v  output,  it is likely to be a a different Amperage output rate.
ie:  the test unit states:   OUTPUT of:   19v at 3.5A

3.   But manufacturers of course,  never state for HOW LONG that output can be sustained.   The reality is,  it is NEVER sustained.
The UNWRITTEN REALITY is that it is a maximum capability statement,  for initial output only

4.   External Battery Output  is 19v at 3.5A  suggests output of  (w = v * a)   of    66.5W (watts)     – a statement of maximum output capability….

5.   The laptop appliance will PULL (draw) the power that it requires (as a demand system),  based on the laptop VOLTAGE and Amperage needed

6.   EXAMPLE:   Gary’s Venom 13″ laptop with Debian Jessie – i7 CPU, 16GB ram 2x mSATA 512GB running raid-1 PLUS another 1TB SSD drive
1080 Hi-Res screen, backlit keyboard,  Intel GPU (and an Nvidia GPU, disabled)
has an INTERNAL  19.v   5600mAh battery.    This is 2 years old, and degraded to 72%  at 4054mAh  according to #upower -d     and
root@absvenom:/home/gaz# acpi -bV
battery 0: design capacity 5600 mAh, last full capacity 4054 mAh = 72%
So, that power consumed for a realistic 83% usage of that degraded available energy is say:
83% of 4050mA   in 70 mins  =      2881mA per hour  but only for  70 mins  until DEAD.

7.  So, the LAPTOP is drawing power for USAGE, during that one hour of operation is  2.881A at 19.5v  =    56.18W  constant demand

8.  When connecting the external battery to the LAPTOP (fully charged to start with),  what transpires is:
a)   The Laptop declares it is being CHARGED from AC (but really from the external battery source)
b)   The internal battery level indicator declares it is 100% full until the external battery becomes depleted and stops
c)   The external battery lasted ONLY  35 minutes
d)   Then the internal battery starts to discharge,   and in the test case,   reduced to 62% (down 38%)  in a mere 20 minutes!
           (of interest to notes, was a 12% loss in a 70 second interval!)…. 
Previous tests have show a  1.18% loss of power every minute,   which is the typical expected.
e)   It was decided to abandon the testing,m and recharge the internal battery from 240v / 19.5v 6.15A (120W) external ADAPTER.
This took from 15:21PM (at 62%)  to    16:50PM (100%)     ie:  a little more than an  90 minutes to top up 38%….   (using a 120W supply!)

9.  Meantime the external unit is now awaiting a long  night to recover from its 35 minutes of enjoyment!

10.  Do they work?   For a quick boost for 35 minutes – yes,   but for sustained usage:   **NO**


The good news

The conference where power for laptops was needed, saw it prudent to supply 4-way power boards under each table throughout the venue.  Nothing like 240v to run a laptop all day!


If you want a jump start for your car,  here’s an example of what was tested