Accumulating Trend Analysis over a Period of Years
Trendingofperiodic Mhosmeasurementscan yield valuableinsightsfor estimating theremaining lifeof thebattery. Suppose that
a4.0Ahbatteryhasbeeninserviceforthree years andthatthe LifeTester measurementsafterthefirstyearwas90Mhos,85 Mhos
after thesecond year, and 80Mhos afterthe third year. Since the battery is still at 80Mhos and is dropping an average of 5 Mhos
peryear, itis highlylikelythat thisbattery will notneed replacingbefore the endof the fourthyear. Ontheotherhand, supposethat
thereadings were90 Mhos after thefirst year, 82Mhos afterthe second year, and 70 Mhos afterthe third year. The rateof decay
has increased from 8 Mhos to 12 Mhos per year. This indicates that although the battery is still serviceable, it is degrading more
andmore rapidly. Itis unlikelythat thisbattery willbe serviceable for another year. Underthesecircumstances, either the battery
shouldbe replaced early or the service interval should be shortened tocatch this battery before a system failure.
Why Mhos instead of Amp Hour
The LifeTester measures the Mhos (conductance) of a battery using anAC impedance measurement algorithm. Every battery
manufacturing process produces a slightly different conductance value for a given battery size. Elk Products elected to display
consistent, accurate Mhos (conductance) values, rather than just estimated Amp Hour values. The only accurate method for
measuringa battery’sAmp-hour capacityiswith a long discharge test, which actually decreases the life of the battery. Estimation
ofAmp-hour capacitywithoutadischarge test isa complexerror-proneprocess involvingtheconductancevalue,the batterystate-
of-charge, the voltage, the temperature, and the many varying design and production variables for each battery type. Ordinary
testerswhich measurestaticcriteriaorAmp-hourare inherentlyinaccurate acrossvarying batterydesigns. In contrast,extensive
testing has proven that when the conductance of a charged battery is tested and has declined to approximately 70% of its full-
capacity reference, the battery is unlikely to deliver its rated capacity and should be replaced.
Is Mhos relational to Cold Cranking Amp
TheLifeTester isdesignedto measurethe conductanceof abattery atfrequencies indicative of the capacityof a battery inAmp-
hour. ConductancemeasurementsforestimatingtheCold CrankingAmpsfor startertype batteriesareat a muchhigherfrequency.
Although the measurement techniques are similar, there is little correlation between the Cold CrankingAmps measurement of a
batteryandthe Mhosreadingof theLifeTester,orvice-versa.
Using Mhos to Estimate Battery Life
The Mhos reading of a new battery out of the box, will generally not be the same as that of a fully charged battery. This is due to
theplates notbeing totallyformed duringmanufacturing. Once thebattery hasbeen oncharge forsome timethe plateswill finish
forming and the peak Mhos reading will normalize. For standby batteries, peak capacity is normally reached after about three
months on a float charging system. For cyclic use batteries, full capacity may not be reached until after ten to thirty cycles,
dependingupon the depthof dischargeand the chargingmethod.
Ifa newbattery has been in storage for anextended period,say over six months without being charged,plate oxidationfrom self-
discharge will occur, causing a decrease in the Mhos reading. Plate oxidation also occurs in standby batteries during a power
failure, particularly if a battery remains in a highly discharged state for an extended time period. Plate oxidation is unhealthy and
candestroy a battery’scapacity. Once a batteryis weakenedby plate oxidation,it isdifficulttorecover fullcapacitywithoutspecial
chargingor conditioning methods. Insome casesa battery willrecover andpassatest afterbeingrecharged. However,asecond
test should be performed a few days later after recovery to accurately assess the overall life of the battery.
Theaverage lifecycle of a sealed lead-acid battery instandbyuse on a float charge is 3to 5years. Abattery with a Mhosreading
of less than 70% of its original capacity is no longer considered serviceable and should be replaced.
Instructions ELK - BLT
