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Re: batteries for satellite use



Graham - G3VZV said (in part):

> This is a general question about batteries!

> I have just spent 5 days away from home with mobile phone switched on
> all the time and still the battery is not "flat"

> The battery pack appears to weigh next to nothing and is shown as
> Li-polymer type.

Yep, the lithium batteries are very light and store a quite large amount of
energy.

> I can understand that the current consumption of modern cell phones
> (GSM 800/1800/1900 MHz) is much less than previous generations but
> this performance seems to be a big leap forward.

That is also true.

> Now the question:

> We have heard about power limitations on microsats due to weight and
> battery capacities,and Nicads having limited lives if fully discharged
> (and then they go short circuit!)

> But I have not heard of the use or intended future use of these more
> modern battery technologies in our satellites. Why? temperature
> constraints?, outgassing? or?

Graham,
      The perfect battery is small, lightweight, has a huge power and
energy capability, lasts forever and is cheap, (and for the sake of the
environmentalists, is environmentally benign to build, use and dispose of).
Unfortunately it has not been invented.  Most batteries are some sort of
compromise of the above characteristics.  The lithium batteries (there are
several flavors of them) have a large energy capacity for their size and
they are lightweight.  They are not overly cheap, the power capacity is
fairly low, and their lifetime is fairly short.  You are doing OK to get a
year out of the Li battery on a daily used cellphone and if you get 2
years, you really are treating the battery carefully - or are lucky.  Not a
desirable characteristic for a satellite.  They also require an intelligent
charger - frequently built into the battery these days.  NiMH are not as
good as Li for energy capacity, but the lifetime is better, and they are
more tolerant of charging abuse.  NiCds are far more tolerant of charging
abuse and have a longer lifetime, but have even less energy capacity.  Some
battery types tolerate temperature changes or limits better than others.  I
could go on, but you should be getting the point.
      The summary is that a battery that is great in one respect may not be
the best in some other characteristic.   You have to balance the
requirements of the electrical supply and load, environmental factors (a
flooded cell Lead acid battery would not work well in space for example),
desired lifetime, size, weight, and a few other factors that I have not
even mentioned.  At this point, NiCds are likely a good compromise for a
satellite battery.  That does not mean that better battery wont come along.
It also means that depending on the mission, one battery may be better than
some other battery, and with a different mission, the opposite may be the
case.

73
-----------------------
Jim Walls - K6CCC
k6ccc@amsat.org
Mobile Radio Operations
Southern California Edison Co.
Ofc:   626-302-8515   -   PAX   28-515
FAX:   626-302-7501   -   PAX   27-501



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