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

[Phil Karn]

On 4/24/18 00:20, Mark Whittington wrote:

> I've been looking at lithium iron phosphate (LiFePo4) chemistry
> batteries for a few applications.

Yeah, the number of cathode chemistries is daunting. I know that LiFePO4
is one of the major alternatives to LiCoO2, maybe the most important one.


> 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.

How can that be 145 amp-hour/kg and only 130 Wh/kg? At 3.2V?

If you're a battery guy, here's a question. During a presentation on the
power system in Fox-1 at the AMSAT-NA symposium several years ago, the
speaker showed how they'd wrapped blankets and electrical heaters around
the battery. I suggested on the fly to do away with that mass and volume
by simply running some low current AC through the cell when necessary to
warm it up. That shouldn't take much, you might do it just by modifying
the charge regulator. (You'd still need to measure cell temperature.)

Since I'd just come up with the idea I didn't actually know if it would
work, so I did some googling. I quickly found that it has already been
used to warm hybrid car packs in cold climates. But those were probably
NiMH chemistry, not li-ion.

Obviously running AC through a Li-ion cell would warm it up too, but
would it be safe?

Phil
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