[sf-lug] will increasing RAM increase power draw during suspend-to-RAM?

Michael Paoli Michael.Paoli at cal.berkeley.edu
Fri Sep 5 06:11:42 PDT 2008

Well, doubling RAM will generally less - and potentially quite a bit
less - than double power drain when in suspend-to-RAM/"sleep" (not to
be confused with suspend-to-disk/"hibernate") mode.  *However*, power
drain and time available on battery power may not be particularly
linear.  Most notably, if, theoretically, the current draw from the
battery is precisely doubled, the available time on battery is not cut
in half, but is cut by *more* than half.  Why?  First of all, laptops
will generally cut off from battery when the battery reaches some
minimum voltage threshold.  For a given charge state of battery, the
greater the current draw, the lower the battery output voltage - so
even with the same amount of charge remaining in the battery, at higher
current draws, the laptop will shut off sooner.  Secondly, there's some
basic physics going on too.  Batteries have a certain amount of
internal resistance.  Drawing more current reduces the available
voltage, and the faster that energy is drained from the battery, the
less energy that can be extracted as electricity from the battery, as
more of the battery's energy is used to warm itself up overcoming its
own internal resistance, as opposed to producing externally usable
electrical power.

So, ... in short, doubling RAM likely less than double the current
draw from the battery, but will cut the time available running from
battery by ratio greater than the increase in the current draw.

> Date: Thu, 4 Sep 2008 10:13:14 -0700
> From: "Alden Meneses" <aldenm at gmail.com>
> Subject: Re: [sf-lug] will increasing RAM increase power draw during
>       suspend-to-RAM?
> Could it be other factors like the age of the battery? I have not changed
> anything on my laptop in regards to the hardware and the battery life is not
> what it used to be.
> On 9/4/08, Mark K. Zanfardino <mzanfardino at gmail.com> wrote:
>> John,
>> This is a very good question and it brought to mind the fact that my
>> laptop does not maintain it's battery life nearly as long as it had
>> prior to my adding an addition 1GB SODIMM to the existing 512MB base RAM
>> installed.  Prior to the expansion I could safely leave my laptop in
>> sleep mode for nearly 24 hours; now it doesn't last 12.  I haven't run
>> any conclusive tests to see how long it will last, but clearly there has
>> been a dramatic decrease.
>> NOTE: I wasn't looking at the RAM initial for this decrease as I've made
>> a number of changes including installing a larger HD (moved from a 40GB
>> 4400rpm drive to a 120GB 5400rpm drive) and installing a newer release
>> of Kubuntu (I was running 7.04 and am now running 8.04.1).  It seems
>> unlikely that the drive would be the issue, as it should be parked while
>> sleeping.  The OS might be an issue, I guess, but I can't be sure.
>> However it seems the RAM the likely suspect.
>> I'd be interested in reading any other's thoughts on this issue.
>> Mark
>> John Magolske wrote:
>> > When suspended to RAM, a laptop's battery is slowly drained.
>> > My understanding is that this power is going towards keeping
>> > the RAM active. I'm wondering how much of the total power draw
>> > is is being consumed by the RAM chips. Would doubling the
>> > amount of RAM from say 512MB to 1GB have the effect of roughly
>> > doubling the power consumption while suspended?

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