Thanks, Phil. That's pretty interesting, although I still do wonder about keeping within spec. I especially wonder about Ag/Zn where there is that chemistry change in the middle of the charge cycle; what is the result of perhaps never crossing chemistry change threshold with shallow cycles? I certainly don't know.
One of the ARRISat papers I read talked about Ag/Zn being safe when shorted. I suppose that 'safe' is for the battery itself. You would still have a lot of current going through whatever caused the short. I was also intrigued when you mentioned super-caps in a previous posting. The enormous cycle life is really useful...it seems like just keeping the IHU ticking over would be really useful, even if we had no up/downlink capability during eclipse. And of course we could think about having them as a backup after the 'standard' battery died. Interesting to think about... BTW, you may not remember, but we exchanged email many years ago; we were both on one of the USENET boards (sci.space.columbia?) I asked if I could join AMSAT before I got my ham ticket. You were kind enough to encourage me. Well, I did join AMSAT a year or 2 ago, and just this spring got my general ticket too. So thanks for the encouragement even if it was a few decades before I acted on it :-) Burns, W2BFJ On Sun, Aug 14, 2011 at 8:17 PM, Phil Karn <[email protected]> wrote: > On 8/14/11 11:31 AM, Burns Fisher wrote: > > > What about another strategy? Suppose we did exactly what the battery was > > spec'ed for: Deep cycles. > > Almost every type of battery does better with shallow cycles than deep > cycles. So this is almost certainly true for Ag/Zn as well. These > batteries just aren't designed for many cycles. > > There is a company (Z-power) working on a Ag/Zn battery with a decent > cycle life, intended for use in laptops. They claim the price of silver > isn't an impediment because laptop li-ion batteries are also expensive > and the silver in their batteries can be reclaimed and reused > indefinitely. One advantage they keep citing is the increased safety of > a battery with a water-based electrolyte. But newer li-ion batteries > have also been developed with different positive plate materials that > greatly reduce the inherent fire hazard of these batteries. > > I don't know what the recent run-up in precious metal prices has done to > their business plans. > > I would like to investigate super/ultracaps for spacecraft use. Their > energy densities are low (a Maxwell ultracap the size of a D-cell > battery holds about 0.35 watt-hour) but their cycle lives are extremely > high: about 500,000 for a 20% degradation in capacity. These could still > run a satellite computer through a LEO eclipse, eliminating one of the > major problems with a modern satellite whose battery has failed. > > If a few of these caps could be flown, it might also be possible to run > a low power, efficiently coded telemetry beacon through eclipse. > > But of course the caps would have to be safety rated. They do contain > small amounts of potentially hazardous materials, but then again so do > Ag/Zn batteries: their electrolytes consist of a strong lye (KOH) > solution. It's hard to understand why these batteries would be approved > but not NiCd and NiMH as they also use KOH in H2O as their electrolytes. > If stored energy is considered a hazard, they could always be launched > discharged and charged after deployment. > > -Phil > > > > > _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
