> I've been using the Sony Li-ion batteries for a couple of weeks, and > have been disappointed with their capacity. Generally, its been 60% - > 70% of what I expected. But I think I've found a potential problem with > Lithium Ion batteries. I went out browsing for more info and found a > page that compares battery capacity to discharge rate: > > http://www.pcim.com/arc/art0006/freeman1.htm > > My wearable now uses 5-6 watts per hour and at that rate I am drawing > 1-1.5amps from a single Li-ion battery (I've been only using one so > far). Based on the graph in the above page, @1.5A the Li-ions have only > about 85% of the capacity they are rated at. Compounded with the fact > that I found out the DC-DC isolator I have been using from Datel is only > 75% efficient gives me roughly 13.5 watts from a 21watt battery. :-\ since your current drain is high, consider using 2 batteries as this will reduce the current draw, and hence more than double lifetime, as it moves down the curve toward optimal drain rate. (batteries meant for camcorders, less drain). > I think the way to fix this is to switch to a DC-DC converter with 14v > (2 Li-ion in series) battery side and get a more efficent converter. > This will reduce the current from 1.5A to 0.75A which raises the > capacity of the batteries from 85% to 95%, and Datel has a new DC-DC > converter that is >90% efficient (although I'm still waiting for someone > to call me back about availability and pricing). consider the powertrends converter described in http://wearcam.org/wearhow/node11.html if you find something better, please let me know so i can test it out and then update the http://wearcam.org/wearhow/index.html site. > Also, I have been surpised by the wide range of voltages these Li-ions > run at. They start fully charged around 8.5V and slowly drop to 6V, > which makes managing power a little crazy. that's why the regulators are recommended in the wearcomp design. > -Paul > -- > R. Paul McCarty / DARS Coordinator // x52059 > 317 Lattimore Hall, University of Rochester, Rochester, NY 14627 > Computers don't make mistakes;what they do,they do on purpose.-Dale/KOTH
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