HiTech wrote:
Is it true that:
Specific Gravity = Cell Open Circuit Voltage - 0.845?
Could one average the specific gravity readings across cells in a battery, and use this formula to check the accuracy of the voltmeter and/or hydrometer (based on which one is calibrated better)?
Jim
turning the equation around:
open circuit voltage=SG +0.845
Now apply this to two boundary conditions, fully charged, fully discharged.
Fully discharged SG=1.1ish
voltage = 1.945*6=11.67
even using 1.0 for discharged SG yields a voltage of 11.1V
fully charged. for trojan T-125's the "nominal" fully charged SG = 1.275.
NOTE: Various batteries have noticeable differences in initial SG. Trojan T-125 GC are 1.275, while the "renewable energy T-125" version is 1.26 or so IIRC. Some batteries are in the 1.28+ range some well below 1.275. So one needs to find out the initial SG from the mfg for your battery rather than "assuming" an initial SG.
voltage = (1.275+.845)*6= 12.7
so for the boundary conditions, the voltages are IMHO "in the ballpark" for batteries at "room temperature". graphs I've seen show fully discharged batteries near 10.8V and fully charged 12.6-12.7. However the link below gives 11.7V for the fully discharged case for the battery electrolyte concentration they give.
so, I suspect the formula is a "rule of thumb" and if you know all the specific conditions it may apply pretty accurately. I suspect at a bare minimum this "rule of thumb" applies at a specific temperature, and there is a temperature coefficient that would need to be applied.
Otherwise, it appears while the voltages calculated are certainly close, probably not close enough to "calibrate" a voltmeter or hydrometer.
so some searching and I found this paper on battery chemistry which expands on the above equation.
lead acid battery chemistrybut then, it's written at a college level chemistry class so see what you remember!
Anyway, I'll trust my Freis (sp?) hydrometer and Tek TX3 true RMS DMM which is calibrated with NIST traceability rather than the formula.
