Ian Woofenden said the following to Jim Dunmyer on the subject of
SOLAR POWER IN APT? (21 Nov 97  18:31:00)
 IW> On (17 Nov 97) Jim Dunmyer wrote to Alec Cameron...
 > I hope the 50% makes sweet music now! Best "headroom" seems to
 > apply, when the panel open circuit voltage is twice the battery
 > voltage.
 JD> I think that HomePower Magazine had an article some time ago
 JD> that indicated a need for 18 or 19 volt-rated solar panels to
 JD> charge a 12 volt battery. It was significantly higher than the
 JD> nominal battery voltage, but not double. The reason for the
 JD> higher voltage was as you said, to make up for blocking diode
 JD> and regulator losses. You do need that headroom!
 IW> It's more than just that, and I wish I understood it. Voltage
 IW> depends on the state of charge of the battery. A full and equalizing
 IW> 12 volt battery can get up to 15+ volts. Since voltage is just
 IW> "pressure", in order for the current to flow from PV to battery, the
 IW> PV voltage has to be higher than the highest battery voltage. And
 IW> there are bound to be not only diode losses, but line losses too.
 IW> Does that muddy the waters some more?
 IW> Ian
Some but not too much. Cyclically for a 6 cell battery a constant voltage
charge at 14.6 to 15.0 volts with an initial current 40% or less of
rated capacity is recommended. A trickle charge is done at between 13.6
and 13.8 volts. For example cars trickle charge at 13.8 volts. Finally a
constant current charge is done at 10% capacity. These values will vary
with temperature and could require compensation.
However, the PV voltage can be much lower than the battery charging
voltage if one is willing to accept some loss by the use of a switching
power converter. Modern switching power conversion circuitry can be
routinely designed to attain efficiencies averaging between 75 and 95
per cent or better within a target range of input supply emf's.
Even with an input source emf which is fluctuating, a switching power
converter can continue to furnish a load with the needed voltage so long
as the PV system can furnish the power demand.
For example many computer psu's don't even have a 120/240 switch anymore
since its converter can operate efficiently anywhere in a range between
85 to 285 volts.
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