is undercharging more efficient? SOLAR.KnowledgePublications.com

is undercharging more efficient?
30 aug 1999
dr. sudan misra (smisra@cdtechno.com), director of battery engineering at
c & d charter power systems, says the coulombic efficiency of recombinant
lead-acid batteries is about 100% between 20% and 80% states of charge... 

as i understood him, dr. misra says a cell might begin with a specific
gravity of 1.3 at 50% charge and an open circuit voltage of 2.15v. we might
carefully charge it with a pv panel and smart electronics from an initial
terminal voltage of 2.25v which increases to about 2.45 volts at 100%, or
discharge it from 2.0 to 1.8v at 0% at 77 f, at the 20 hour rate.

as the battery charges, cell voltage increases, but the increased voltage
is available as the battery discharges, no? under these conditions, the only
significant energy inefficiency comes from the ohmic (proportional to current)
loss that causes offset between open circuit and terminal voltages during
charging and discharging. with 100% coulombic efficiency between 20% and 80% 
and a 100 mv offset, the average round-trip battery energy efficiency is
about (2.15-0.1)/(2.15+0.1) = 91.1% at the 20-hour (ah/20h) rate, no?

bill berg wrote:

>>...in south texas summer temperatures (90f plus).  summer starts in late
>>april and runs through september.   flooded vehicle batteries must have
>>an equalization charge every couple of weeks.  wracks the heck out of
>>efficiency.

>how much, one wonders?...

say we cycle between 20% and 80% every day, charging with 0.6x2.25c wh of
energy and discharging 0.6x2.05c wh for 13 days, storing a total of 17.55c
wh and recovering 15.99c wh, for an average round-trip efficiency of 91.1%.

on the 14th day, let's charge it to 100%, storing 0.6x2.25c wh up to an 80%
state of charge and an average of 0.2x2.25x1.1c wh from 80% to 100% (with a
90% average coulombic efficiency.) at the end of the 14th day, we discharge
to 20%, recovering 0.8x2.05c wh.

over the 14-day cycle, we've now stored a total of 19.395c wh and recovered
17.63c wh, for a round-trip efficiency of 90.9%, including equalization,
wracking 0.2% out of the previous efficiency, right? 

nick