2009/12/15 Pavel Machek <pa...@ucw.cz>: > Hi! > >> > > I'm not sure how familiar you are with the issues surrounding trying to >> > > do a voltage to charge mapping for a battery but it's much more complex >> > > than a simple table if you want to get it accurate. There's a lot >> > > of >> >> > Well... current zaurus kernels use _huge_ table that maps voltage to >> > battery %... and that table is linear function :-(. >> >> > Do you have some papers on that? >> >> Something like "Measure Battery Capacity Precisely in Medical Design" >> by Bernd Krafthoefer in Power Electronics Technology Jan 2005 might be >> useful here. > > Well, that would really require extensive hardware modifications: it > needs _way_ more accurate ADCs on battery for a start, and a way to > generate huge load.
I've worked on several battery powered devices and battery monitoring is very hard to achieve without a deep understanding of the chemistry, circuitry and the algorithms. Large current loads can cause the battery voltage to drop and therefore there can be a significant voltage drop between measuring the quiescent and the loaded battery voltages. I once did try to implement an algorithm which took into account the different battery voltage during heavy current drain but it's hard to know how to interpret that data. One of the RFID systems I worked on could run the user interface for days on a low battery but a single RFID read would cause the system to die due to the internal resistance current drop ... One of the easiest ways to monitor the battery is with a coulomb counting method (accumulate current going into and out of the battery) e.g. "LTC4150 - Coulomb Counter/Battery Gas Gauge" http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1037,C1134,P2354 As long as you aren't draining very large amounts of current from the battery, the accumulated current measurement should be almost equal to the amp-hour rating of the battery. Unfortunately the simple coulomb counting chips have the disadvantage that the CPU has to be running to accumulate the pulses. Of course, the pulses could wake the CPU from a suspend mode, but I'd rather not do that just to add "one" to a counter ... Finally we decided on a fully integrated "smart" battery monitor device like the DS2782 http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4779. This chip has Linux device driver support, I got the driver from Ryan Mallon at Bluewater Systems. It was fairly complex to set up and calibrate but we now use it in a commercial product. I recommend having a look at the datasheet for the DS2782 to see what methods they use for determining the remaining charge in a battery and to get an idea of the complexity of the problem. Good luck! Aras Vaichas -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
