Subject: [amsat-bb] Re: ARISSat-1 battery eclipse voltage decreasing
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On 8/13/11 6:11 PM, Dave Guimont wrote:
> How about you designing circuitry/program to charge “cells” individually
> rather than the “battery”. Or has that been tried??
That’s a great idea; I wish I thought of it first, but it was Lou
McFadden W5DID who published a paper at the AMSAT Symposium a few years
ago on a new modular power system.
His idea was to turn each cell into an intelligent energy storage module
and to connect those modules in parallel to a power bus. There’d be a
DC-DC converter between the cell and the bus so they could operate at
different voltages. If the cell in one module failed, it would
disconnect itself from the bus instead of dragging it down.
He found it a challenge to achieve high efficiency in the DC-DC
converter with low voltage batteries. For the lower voltage chemistries
(e.g., NiMH at 1.2 V) it might be necessary to compromise by using two
or maybe three cells in series per module. A lithium ion module would
need only one cell since they operate at a much higher voltage of 3.6V.
The beauty of his scheme is that not only would this be far more robust
against individual cell failures than a single series string, you could
fly several kinds of batteries to see which functions best.
With the proper command to a module controller you could perform a
controlled discharge of its cell for a capacity test. That’s kinda neat.
Some modules could use supercaps. They have very high cycle lives
(~500,000) but low energy density (0.35 watt-hour for a D-cell sized
cap). You’d always discharge them first, or perhaps keep one in reserve
to keep a computer going. Since they’re capacitors you’d need the DC-DC
regulator to produce a constant voltage as they discharge.
Thinking more about his scheme, you could program each module with a
command like “Keep the power bus at +12V by pumping up to 2A into it
until you’re 50% depleted” or “charge at 1A max unless the bus voltage
falls to 11.5V”. The computer could issue a new command at any time,
such as one to stop all charging when the satellite enters eclipse.
One module might contain just a load resistor to act as a shunt
regulator to keep the bus voltage from going too high.
And one or two modules might contain high-density (e.g., lithium)
primary batteries as emergency fallbacks to keep things going long
enough for the command stations to figure out what’s going on.