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Re: Re: NiCd Battery Experts?




>Bob:
>
>Yes, we matched the cells......

OK, lets assume then that you have a random selection of cells that did not 
exhibit gross failure indicators (mechanical and electrical) at the time of 
selection.  This condition is an important hgh risk factor if you were to 
take the decision to force the battery down by repeated ON commands, see below.

>  There is of course another solution to your problem.  Pick a few good hams
> > spread around the world and just slam PCsat with OFF commands,

since preparing the first memo last night I did some additional 
thinking.  You might want to consider slaming the PCsat ON every chance you 
get pushing the cells down as hard as you can.  This is a valid option IF 
the cells in the battery were matched as was done for OSCAR 11, in view of 
the above this has additional risk issues over a shorter life cycle interval.

>But remember, we only have two options.
>Our Sat has NO PC on board, no program, no timed events, nothing
>that wouild be called a contrl system.  But we can turn on and off about
>40% of our fixed load to change how much solar power is going into the
>batteries by about 40%.

My first and abiding thought is WHY?  I could give you a programmed PIC 
16F628 that would at least help to keep PCsat alive, stuff kept OFF or ON 
as required.  This CPU and peripherals would only be the size of a penny 
paper match (that now costs 5 cents) but that is for discussion maybe at 
another time if your interested.....  I am doing software here for a CPU 
for use in birds and small animal radio tag systems to extend battery life 
(some really small batteries hi) and add experimental functionality to 
researchers who are doing experiments.

>So GIVEN that we may possibly be deep discharging to near zero every
>eclipse for the next 3 weeks and that we only have two command states that
>have anything uselful to do with power budget, then the question remains:
>
>Given that we have two charge rates A and B and A is only 40% higher than
>B, and NEITHER is sufficient to keep the battery above 1.0 volts even half
>of the way through eclipse, then is it better to charge at A and cycle a
>greater chemical process or charge B, a lesser chemical process, given
>that we are going to end up at the same deep discharge state towards the
>end of eclipse anyway?

I have read this description, the previous ones, and messed up several 
sheets of paper.  I have a strong feeling that these "options" are maybe at 
best something that you can only influence to a certain degree, ie there 
might be an ability to have a propensity to focus on one condition or the 
other, I sense that they are in reality as to battery management not all 
that much exclusive or (at the end of the day) different of each 
other.  The issue is do you know what the temperature of the battery 
is?  If you do you should always focus on doing what it takes to keep the 
battery as cold as possible.  NiCd cells have very limited enthusiasm to do 
anything when they are cold, they especially lack the ability to do 
anything exciting when they are cold.  The real issue is what is the 
temperature of the battery in PCsat please?

[some ideas of temperatures, cold for a NiCd in my terms is 5c and below, 
hot is above 25c.  At greater than 35c you should consider most NiCd cells 
in space as potentially venting and about the same potential as a grenade 
with no locking pin in place.  At greater than 45c one should be speaking 
in the past tense for the cell after a very short period of time.]

>I doubt the only 40% difference we can
>make will make that much of a difference, but anyway, that is why we
>asked.

Yes, I tried to say that above, it wont make much difference which option 
you choose at this point.  You already know your not in a good position so 
my recommendation is to focus on minimizing the range of voltage change and 
do what ever it takes to keep the battery as cold as possible.  It is 
little known but you can keep NiCd cells at zero voltage, ie shorted, and 
in cold storage refrigerator or freezer for decades with no measurable 
degradation in performance.

I have received a flood of emails about my earlier reference to the cell 
selection process we used for the OSCAR 11/UOSAT 2 battery.  This goes back 
to the CPM-80 operating system in use at that time (8080/Z80 era).  I have 
looked through my on line documentation here and I do not appear to have a 
copy.  One of our team, no longer an amateur and his health is now fragile, 
is however a packrat of enormous quantities of data files, I will email him 
and ask him to search for the write up.  We did send this off to AMSAT some 
years ago, maybe someone in AMSAT might have some copies of the different 
documentation and data files.  It is important to note that what we did was 
based on a hypothesis I developed that can be written in a page or two.  I 
sought for OSCAR 11 to seek an explicit solution for that satellite, we had 
no time to look for optimization of the process that implemented the 
hypothesis nor did we proceed further once I realized that the OSCAR 11 
battery was such a success I considered the problem of satellite batteries 
for the general case solved and I moved on to other things.  As I am 
preparing this, I have a sense that UOSAT at Surrey in England should also 
have copies of the documentation and data files but I have had no direct 
contact with them for many years now.  I do remember that at one time 
Surrey had a visiting professor type from JA who did extensive work on NiCd 
battery selection for them, I assume his work was published, Surrey being a 
University and that should be available with some searching.  Last and not 
least I distinctly remember some early write ups and data was transferred 
to AMSAT during the MICROSAT project some 12 years ago, I had extensive 
communications with a chap from South America about  6 to 8 years ago who 
had that data, they had received it with the MICROSAT package they had 
obtained from AMSAT.

Bob, in closing, you may in fact already realize that you are facing a 
binary decision that must be made with a distinct lack (i.e. a minimum) of 
perfect information.  You alone have to cut this one, I offer that keeping 
things cold and minimizing the range of voltage swings are the things to 
focus on.  Good Luck to you in your deliberations. I regret of course that 
I am not able to offer you a magic solution, in fact maybe just recognizing 
and stating the obvious, the binary decision to be taken with imperfect 
information, might be the greatest value I can offer.

73

Larry
VA3LK


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