Dear Friends,
This is speculation but it looks to me like we have had a bit of good luck regarding the battery.
Looking at the battery voltage from deployment on...
Up until Aug 11, the battery seems to be deteriorating "normally" with a slight downtrend in the max voltage as expected. But on Aug 11, the battery voltage suddenly rises up to 36 V max and the satellite has started resetting in eclipse.
I think the explanation is that the battery experienced a significant event on Aug 11 where it lost the electrolyte in one or more cells. If this is true, the bad news is that it will no longer hold a charge and will not operate in eclipse any more.
But the good news is that without electrolyte, it would also stop dendrite growth that causes the eventual battery short circuit.
In our ground testing, our test battery failed in the usual way with the battery load increasing until the solar panels could not drive the power bus high enough to run the satellite. But interestingly, several cells also cracked and dumped their electrolyte during this testing.
If a cell on the flight battery cracked and dumped its electrolyte BEFORE the shorts were formed, it should stay that way and the satellite may very well continue to operate in the sun until it starts to re-enter. We just need some luck to avoid a bad solar angle that would cause a reset in sunlight.
Keep your fingers crossed! :)
73, Tony AA2TX
On 8/14/11 3:18 PM, Anthony Monteiro wrote:
I think the explanation is that the battery experienced a significant event on Aug 11 where it lost the electrolyte in one or more cells. If this is true, the bad news is that it will no longer hold a charge and will not operate in eclipse any more.
I agree with your analysis in that one or more cells seems to have gone open-circuit, or at least greatly reduced in capacity. If you know these batteries and their specific failure mechanisms, then I'd say your lost electrolyte theory is a pretty good one.
I never have understood why the dendrites would not be melted by high current as soon as they crossed the gap between the two plates. I assume they grow on the zinc plates as the battery is charged?
-Phil
Hi Phil,
I think the initial dendrites DO just spark and open up but eventually the spacer is damaged and the short is enough to bring the cell voltage down.
Yes, Zinc dendrites will form also if the battery is overcharged but we should not be able to do that on ARISSat-1 so only the silver ones should be an issue.
73, Tony AA2TX ---
On 8/14/2011 6:56 PM, Phil Karn wrote:
On 8/14/11 3:18 PM, Anthony Monteiro wrote:
I think the explanation is that the battery experienced a significant event on Aug 11 where it lost the electrolyte in one or more cells. If this is true, the bad news is that it will no longer hold a charge and will not operate in eclipse any more.
I agree with your analysis in that one or more cells seems to have gone open-circuit, or at least greatly reduced in capacity. If you know these batteries and their specific failure mechanisms, then I'd say your lost electrolyte theory is a pretty good one.
I never have understood why the dendrites would not be melted by high current as soon as they crossed the gap between the two plates. I assume they grow on the zinc plates as the battery is charged?
-Phil
On 8/14/11 6:11 PM, Anthony Monteiro wrote:
Yes, Zinc dendrites will form also if the battery is overcharged but we should not be able to do that on ARISSat-1 so only the silver ones should be an issue.
Oh, so they form on both plates. Interesting. Charging the battery plates out zinc metal on the negative plates, but any metallic silver on the positive plates should be oxidized back to silver oxide. Silver dendrites should form only during discharge, right?
-Phil
Hi Phil,
Yes, that is correct.
73, Tony AA2TX ---
On 8/15/2011 1:26 AM, Phil Karn wrote:
On 8/14/11 6:11 PM, Anthony Monteiro wrote:
Yes, Zinc dendrites will form also if the battery is overcharged but we should not be able to do that on ARISSat-1 so only the silver ones should be an issue.
Oh, so they form on both plates. Interesting. Charging the battery plates out zinc metal on the negative plates, but any metallic silver on the positive plates should be oxidized back to silver oxide. Silver dendrites should form only during discharge, right?
-Phil
Kind of a 2011 version of AO-7, but without the stable orbit!
73, Jim KQ6EA
On 08/14/2011 10:18 PM, Anthony Monteiro wrote:
Dear Friends,
This is speculation but it looks to me like we have had a bit of good luck regarding the battery.
Looking at the battery voltage from deployment on...
Up until Aug 11, the battery seems to be deteriorating "normally" with a slight downtrend in the max voltage as expected. But on Aug 11, the battery voltage suddenly rises up to 36 V max and the satellite has started resetting in eclipse.
I think the explanation is that the battery experienced a significant event on Aug 11 where it lost the electrolyte in one or more cells. If this is true, the bad news is that it will no longer hold a charge and will not operate in eclipse any more.
But the good news is that without electrolyte, it would also stop dendrite growth that causes the eventual battery short circuit.
In our ground testing, our test battery failed in the usual way with the battery load increasing until the solar panels could not drive the power bus high enough to run the satellite. But interestingly, several cells also cracked and dumped their electrolyte during this testing.
If a cell on the flight battery cracked and dumped its electrolyte BEFORE the shorts were formed, it should stay that way and the satellite may very well continue to operate in the sun until it starts to re-enter. We just need some luck to avoid a bad solar angle that would cause a reset in sunlight.
Keep your fingers crossed! :)
73, Tony AA2TX
Sent via AMSAT-BB@amsat.org. 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
participants (3)
-
Anthony Monteiro -
Jim Jerzycke -
Phil Karn