Subject: Re: [linux-audio-dev] OT: Electronic advice for PC.
From: Lamar Owen (lamar.owen_AT_wgcr.org)
Date: Mon Apr 29 2002 - 13:46:39 EEST
On Monday 29 April 2002 12:16 pm, Patrick Shirkey wrote:
> I am wondering if anyone can tell me how I can deduce the correct power
> rating for this machine so that I can design a battery pack that will power
> it efficiently.
Ok, there are several criteria to consider for sizing batteries:
1.) Required voltage. What voltage is needed? If this is going to be driving
an inverter, 12V is typical.
2.) Required current. At the required voltage, how much current does the PC
draw? This is necessary for the next criterion:
3.) Required battery run time. How long do you want the battery to provide
required voltage at the required current? Battery capacity is rated in
ampere-hours (AH) -- suppose your PC draws 5A at 12V -- and you want to run
10 hours. 5x10=50AH. That is a little larger than a motorcycle battery. My
Sony Vaio notebook has twin 3000mAH Li-Ion batteries. It runs for about 2.5
hours. This means it is drawing 6000mAH/2.5H= 2400mA=2.4A. Of course, the
Li-Ion discharge curve being what it is, this is approximate at best.
4.) Charger ratings. The larger the battery in AH, the beefier the charger
needs to be to charge it is a reasonable amount of time.
I designed a battery system for our studio here. The requirements were:
1.) 12V for inverters (StatPower ProSine 1800 x2)
2.) 24 hour run time.
Experiments concluded that the Prosine 1800's could reliably start and run
about a 1000W load (monitor turn-on current surges will cause the inverter to
drop out -- I measured the surge load of this 1KW average load as being about
2700W, which is within the 1800's 2900W peak rating). With a 1000W load and
12V, they draw right around 90A.
To get 24 hour runtime, we need 90x24=2160AH of capacity. Fortunately, the
1000W is the designed largest load -- they typically run about 70A.
Now, the local telco was needing a place to off-load some large flooded
lead-acid cells rated 2320AH. We were able to get them donated. With the
voltage drops in the wiring taken into account, we have about 24.2 hours
runtime at an 80A load -- which is a little less than what I wanted, but good
enough at this point in time. I just have to keep my inverter loads around
850W or so. At which load the Prosine 1800's are very happy.
Now the chargers became a major problem. At 2320AH, you need 232A to recharge
in 10 hours, which is the recommended rate for these cells. Well, I couldn't
afford the real telco chargers necessary for such. So I compromised -- I get
a 36 hour recharge rate by using a pair of 100A rackmount 13.8V regulated
power supplies made by Samlex America. These supplies, with the resistance
of the wire to the batteries, level off at 100A fo the first couple of hours,
and then begin to taper the current down afterwards. A full equalizing
charge requires 150 hours, but that's OK.
Hope that helps.
-- Lamar Owen WGCR Internet Radio 1 Peter 4:11
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