Each Fox-1 satellite uses 6 Sanyo KR-1400AE NiCd A cells.
Obviously we all hoped that the batteries would last a lot longer. As discussed during the Symposium, the likely cause of battery failure is a higher than planned depth of discharge. Note that AO-92's batteries failed more quickly than AO-91. Since AO-92's evening passes were generally between 9 and 11 pm, AO-92 saw lots of use while in ecilpse. Contrast that to AO-91, where the evening passes were generally after midnight, so it saw less use while in eclipse.
I'm not sure there's much to be learned from the experience with these batteries as NiCd battery technology is now certainly obsolete and will not be used in future AMSAT satellites. AMSAT's GOLF series of satellites will use lithium ion batteries. More information about the progress of GOLF, including the GOLF-TEE status paper from the 2020 Symposium Proceedings, can be found at https://www.amsat.org/greater-orbit-larger-footprint-an-introduction-to-the-...
73,
Paul, N8HM
On Mon, Dec 7, 2020 at 2:41 PM John [email protected] wrote:
Hi John,
My gut feeling is that it'll be related to the battery chemistry (assuming your statement is correct that the batteries used are NiCd batteries) - not fully discharging a NiCd before recharging it again creates a 'memory effect' (https://en.wikipedia.org/wiki/Memory_effect) of sorts in the battery, and over time reduces the reasonable use life of the battery significantly. I have memories of my father meticulously discharging all his NiCd batteries in a proper discharge unit in order to avoid this, when we used to use them for our radio control toys etc when I was a child.
In contrast, when he picked up some "Rechargeable Alkalines" as he called them, he made the point of explaining to my brother and I that actually these were designed to be recharged after only partial discharge, and quite the opposite was true - one should avoid fully discharging these batteries in order to avoid reducing their lifespan (see https://en.wikipedia.org/wiki/Rechargeable_alkaline_battery). It would seem that this would be a better chemistry type to use for something like a satellite, where whilst in sunlight the batteries can be floated/topped up, than a chemistry such as NiCd where recharging before they hit their minimum threshold causes issues, however the number of cycles required to bring them back to full condition from a deep discharge could also be problematic.
I'm aware we aren't talking about run-of-the-mill AA cells, and are likely talking about some kind of custom size/shape cells instead, but it would seem to me that the chemistry is the cause of the 'early' failures, especially when coupled with the discharge/recharge cycles that the batteries would normally experience when moving between eclipse and sunlight. Of course this doesn't account for any clever software managing the charge cycle, and instead assumes that sunlight = charging and eclipse = discharging, so there's likely to be a lot more at stake than just the chemistry. Still food for thought, though, assuming the initial premise of 'they use NiCd cells' is correct.
73, John (M5ET)
-----Original Message----- From: John Brier [email protected] Sent: 07 December 2020 19:28 To: AMSAT BB [email protected] Subject: [AMSAT-BB] How to determine why Fox satellite batteries failed before projected 5 year lifetime
I hope for this to be a purely engineering based discussion that could shed some light on this question.
Please only respond from an engineering perspective in this thread.
Data:
According to the "AMSAT Fox-1 Systems Engineering Documentation" the satellites were expected to last at least five years. [1]
For each satellite, before there were any issues with the batteries, they were able to power the transponder continuously without going into safe mode. However, once battery issues were encountered, the transponder would turn off unexpectedly while the satellite went into safe mode. Using that cutoff my back of the hand calculations show the following [2]:
AO-92: batteries lasted 35% of their expected lifetime AO-91: batteries lasted 60% of their expected lifetime. AO-85: batteries lasted 87% of their expected lifetime.
I know the batteries used were Nicad. I know they were matched. I believe the issue so far has been that one cell in the set has failed such that the minimum voltage required to run the transponder is not met. Is that last point correct?
So that is the data that I am currently aware of.
I think the next data to procure would be to understand what information was used to project a five year lifetime. Then you could look at the available data about the real satellites (from telemetry for example) to find out if there are any discrepancies that would explain the reduced lifetime.
From my scanning of the "AMSAT Fox-1 Systems Engineering Documentation" I didn't see the battery chemistry mentioned.
However, in addition to the "AMSAT Fox-1 Systems Engineering Documentation" there is a lot of information in the Fox engineering material from past Symposium Proceedings. These are available as PDFs to AMSAT members.
Do people agree that the next steps would be to understand what information was used to project a lifetime of five years?
If so, can anyone tell me, or can anyone go through the past Symposium Proceedings, and lay out what information was used to project a five year lifetime?
If not, what are the better next steps in understanding the reduced battery lifetime?
73, John Brier KG4AKV
- 2.3 Reliability Requirements
2.3.1 The satellite shall be designed for a minimum 5-year, on-orbit lifetime. "
https://www.amsat.org/wordpress/wp-content/uploads/2018/06/AMSAT-Fox-Documen tation.pdf
- AO-92 was launched/commissioned in January 2018. AO-92 has had battery
issues since this fall such that it can't be used. AO-91 was launched/commissioned in November 2017 and now it too seems to be having the same battery issues. AO-85 was launched October 2015, declared EOM February 2020.
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