[Ground-station] Yes, you can buy a cubesat, but...

[Mark Whittington]

On Tue, Apr 24, 2018 at 2:44 AM, Phil Karn via Ground-Station <
ground-station at lists.openresearch.institute> wrote:


> Lithium-ion is both better and worse here. It's better in that coulombic
> efficiency is 100%; there's no water in the electrolyte to steal
> electrons. This makes it difficult for the cells to get out of balance.
> But they're worse in that if they ever do, or if changes in individual
> cell capacities cause an individual cell voltage to exceed allowable
> limits, the results are much more catastrophic. Discharge a Li-ion cell
> below the minimum voltage and you'll ruin it. Charge it above the
> allowable maximum and it's quite likely to catch fire or explode.


This may be old news to this group, but I thought I'd mention it just in
case.

I've been looking at lithium iron phosphate (LiFePo4) chemistry batteries
for a few applications.  As compared to other li-ion chemistry batteries,
they have a flatter discharge curve, a longer cycle life (up to 2000 cycles
as compared to 400 cycles for LiCoO2), and are much less likely to catch
fire if you overcharge or short circuit them.  They are still prone to
dying if you over-discharge them and they are a bit less energy dense than
LiCoO2 cells (86% of LiCoO2), but overall they're pretty robust.

Nominal cell discharge voltage is 3.2V, peak voltage is 3.65V and minimum
discharge voltage is 2.0V.  Specific capacity is 145Ah/kg, gravimetric
energy density is ~130Wh/kg.  Ideal charging temperature range is 0°C to 40°
C, discharge temperature range is -10°C to 60°C.

I did a quick search and found the results of a final thesis project from a
university in Barcelona that aimed to study and test LiFePo4 cells for
satellite applications.  I'll link the paper and some other references
below, but the results seem encouraging.

Paper -
https://upcommons.upc.edu/bitstream/handle/2099.1/26410/David_Balaguer.pdf
Charge/discharge technical data: https://www.powerstream.com/LLLF.htm

73
-Mark
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