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In Reply to: RE: Which capacity batterie for cMP HTPC posted by hanssatink on June 20, 2009 at 06:05:23
Hans,
congrats for deciding on batteries! :-)
The proper capacity for listening for 4-5 hours without charging depends I'd say on a number of factors:
- What parts of the PCs power supply are you willing to power with batteries? Only P4, or also P24, or even the "dirty supplies" etc.?
- You write "cMP HTPC" - how close is your system to the "recommended" setup, i.e. what CPU / RAM / PCI cards / software do you run, and which file formats do you use etc.? I assume that the "HT" (=Home Theatre) part in "cMP HTPC" might require additional calculation ontop of a "standard" cMP setup's power consumption.
For a "standard/recommended" cMP setup, you could use two methods of calculation to get the current draw:
- A rather exact one for every line involved, looking at the various measurements that have been made on P4 and P24 consumption. Here you have ~350mA @ 12V on P4 = 4.2Watt, and ~140mA @ 12V plus ~300mA @ 3.3V plus ~4200mA @ 5V on P24 = 1.7 + 1 + 21 Watt = 23.7Watt, or 27,9Watt in total. Once you power all that with 12V batteries (e.g. P24 through a picoPSU), you have a current draw of 27.9Watt / 12V = 2.3A
- In cics' currently recommended setup with parameters set as specified, he says "power consumption will be below 20Watt", so 20Watt / 12V = 1.7A
Now you have to consider the discharge characteristics of the batteries you intend to use. If you e.g. take Panasonics 7.2Ah 12V VRLA model (LC-R127R2P), it will provide a terminal voltage of ~12V for ~60mins / ~90mins at 2.3A / 1.7A . So in case of these batteries you could use 5 or 6 of them in parallel and be quite safe for 4-5hrs, plus reducing the overall internal resistance to acceptable levels.
I myself tend to extreme solutions ;-) so I would recommend to use just 2 of Panasonics 28Ah 12V batteries (price ~100EUR, so still acceptable) - their terminal voltage falls below 12V at ~2A discharge current after ~600mins, so you're really safe plus you get a decent internal resistance - which I would strongly recommend to combine with proper capacitance following the batteries anyway!
Hope that helps
Robert
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