From: Lawrence H. <gumstix@HariTech.com> - 2008-05-06 21:01:52
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There are a couple of things to be aware of with NiMH (and NiCd to a slightly lesser extent). During discharge in parallel one cell will provide all the current until it's voltage drops below the other then they will swap so you can get overall longer running times but not increase current draw by paralleling cells. During charge the normal algorithms involve looking for a 'peak' in the charging voltage to determine when the cell is full. This is caused by a change from absorption of charge to pure heating. In NiMH this is a very small bump. When you put two cells in parallel they can often mask each other and the charger will never see the peak and wont shut off. You then get thermal runaway and destroy the cells if not the product. The solution is a very slow trickle charger or separate the cells for charging. Similarly with the cells in series often the first cell to 'peak' stops the charger. This leaves all the other cells not quite full. Each cycle causes this problem to get worse, running time gets shorter until the battery looks like it's dead. A full slow deep discharge of the whole pack and then slow charge will revive the pack. This problem is less of an issue than the parallel situation and manufacturers seem to not worry about it. Usually means they sell more battery packs but don't have to deal melted plastic cases. Of course none of the charging issues are an immediate problem if you take the batteries out and charge them in wall wart style charger as they almost universally charge each cell separately, though there are good ones that baby the cells and bad ones that just cook them. The ideal charger does each cell individually, monitors for the peak, has a timer to cut off charging if the peak is missed and a thermistor to monitor the temperature if the cell starts to go into thermal runaway. Lawrence Harris adrian cockcroft wrote: > I wanted capacity in a lower profile, so was hoping to use four AAA in > series/parallel rather than two AA in series. The AA are also longer > which makes the power bulge harder to fit. Are there any charging > issues with series/parallel NiMH? > > Adrian > > On Mon, May 5, 2008 at 7:45 PM, Gordon Kruberg <go...@gu...> wrote: > >> The goliath board you have takes, out-of-the-box, 2 x NiMH for the charger. >> Gordon >> >> >> >> > I have a beta Goliath board, the issues are mostly just getting the >> > design right, there is a lot of complex stuff on the board. I've been >> > working on a case for it, which could be adapted by anyone else using >> > the 4.3" LCD. Latest update is that I added a power bulge/battery >> > holders to the rear cover of the design. It takes four AAA 1000mAh >> > NiCd. You can see it here: >> > >> > http://wiki.hbmobile.org/index.php?title=MyPhone_Landscape_Case >> > >> > I use the free version of Alibre to design this, (www.alibre.com) and >> > I'm still working on the details of the battery holder. If anyone >> > wants copies of the design files, drop me an email, and I'll send you >> > the latest. When it stabilizes, the design files and STL files for 3D >> > printing will be on the wiki. >> > >> > Adrian >> > >> > On Sun, May 4, 2008 at 11:35 AM, Jeff Sadowski <jef...@gm...> wrote: >> > >> >> I am curious about the status of this product? >> >> >> >> I'm guessing since it was having issues in reliability it was pulled? >> >> >> >> or was it that no cell phone service would support it being on their networks? >> >> >> >> Are there any other open source phone projects? >> >> >> >> ------------------------------------------------------------------------- >> >> This SF.net email is sponsored by the 2008 JavaOne(SM) Conference >> >> Don't miss this year's exciting event. 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There's still time to save $100. >> Use priority code J8TL2D2. >> http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone >> _______________________________________________ >> gumstix-users mailing list >> gum...@li... >> https://lists.sourceforge.net/lists/listinfo/gumstix-users >> >> > > ------------------------------------------------------------------------- > This SF.net email is sponsored by the 2008 JavaOne(SM) Conference > Don't miss this year's exciting event. There's still time to save $100. > Use priority code J8TL2D2. > http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone > _______________________________________________ > gumstix-users mailing list > gum...@li... > https://lists.sourceforge.net/lists/listinfo/gumstix-users > |