Battery Fuel Gauge [RE-wrenches]

[Joel Davidson]

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Wrenches,

The Sept/Oct 2006 issue of Battery Power Products & Technology had an 
article (not online but see http://www.batterypoweronline.com/bppt_back.htm) 
"Battery Fuel Gauges: Accuracy is the Game" by Wagner at www.maxim-ic.com 
that was written primarily for the booming lithium-ion battery market, but 
had some interesting comments applicable to PV/battery systems. "To 
determine the available charge in a battery, simple monitoring methods are 
preferred. They should consume little energy, and should (ideally) allow one 
to deduce the charge level from battery voltage. Such a voltage-only method 
can produce unreliable outcomes, however, because no unique correlation 
exists between the voltage and the available charge. Battery voltage also 
depends on temperature, and dynamic relaxation effects can cause a slow 
increase in the terminal voltage after a reduction in load current. Thus, 
pure voltage-based monitoring is unlikely to provide charge level accuracies 
better than 25%."

The author goes on to write, "The relative charge level, often called state 
of charge (SOC), is defined as the ratio of residual charge to the battery's 
charge capacity. Hence charge flow must be measure and monitored - a 
procedure called 'coulomb counting.' In practice, coulomb counting is 
accomplished by integrating the currents flowing into and out of the cell. 
To measure these currents with a high-resolution ADC, a small resistor is 
typically connected in series with the anode."

Then the author lists 3 ICs: DS2761 coulomb counter; DS2780 fuel gauge; 
MAX1781 programmable fuel gauge (obviously a pitch for Maxim's ICs).

So where's this going? Last Sunday afternoon, I experienced a Southern 
California Edison (SCE) power outage followed by 3 evenings and counting of 
grid power brownouts (101 VAC). My SW4048 line-tie inverter saw the low grid 
voltage and went to stand-alone mode, but my four old Johnson Controls 
GC12V100B batteries did not last long and the inverter did its low-voltage 
shutdown. So I flipped the inverter bypass switch and put up with low grid 
voltage (thank you SCE for shortening the life of my home equipment). 
Trouble-shooting and flipping switches by flashlight and re-programming the 
inverter the day may be interesting for techno-guys, but the average on- or 
off-grid PV system owner (our customers) would be frustrated and angry if 
their battery bank ran out of juice leaving them in the dark.

Finally, the question: What "battery fuel gauge" do you think is the most 
non-technical customer-friendly (easy to use, intuitive, reliable, etc.)?

Joel Davidson 


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