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