Measuring state-of-charge by voltage is the simplest method,but it can be inaccurate.Cell types have dissimilar chemical compositions that deliver varied voltage profiles.Temperature also plays a role.Higher temperature raises the open-circuit voltage,a lower temperature lowers it,and this phenomenon applies to all chemistries in varying degrees.The most blatant error of voltage-based SoC occurs when disturbing the battery with a charge or discharge.This agitation distorts the voltage and no longer represents the true state-of-charge.
To get accurate measurements,the battery needs to rest for at least four hours to attain equilibrium;battery manufacturers recommend 24 hours.Adding the element of time to neutralize voltage polarization does not sit well with batteries in active duty.One can see that this method is ill suited for fuel gauging.Each battery chemistry delivers a unique discharge signature that requires a tailored model.While voltage-based SoC works reasonably well for a lead acid battery that has rested,the flat discharge curve of nickel- and lithium battery or lithium-based batteries renders the voltage method impracticable.
And yet,voltage is commonly used on consumer products.A “rested” Li-cobalt of 3.80V/cell in open circuit indicates a SoC of roughly 50 percent.The discharge voltage curves of Li-manganese,Li-phosphate and NMC are very flat,and 80 percent of the stored energy remains in this flat voltage profile.This characteristic assists applications requiring a steady voltage but presents a challenge in fuel gauging.The voltage method only indicates full charge and low charge and cannot estimate the large middle section accurately.
When measuring SoC by open circuit voltage,the battery voltage must be truly “floating” with no load present.Installed in a car,the parasitic load present makes this a closed circuit voltage (CCV) condition that will falsify the readings.Adjustments must be made when measuring SoC in the CCV state by including the load current in the calculation.In spite of the notorious inaccuracies,most SoC measurements rely on the voltage method because it’s simple.Voltage-based state-of-charge is popular for wheelchairs,scooters and golf cars.
Lead acid has diverse plate compositions that must be considered when measuring SoC by voltage. Calcium,an additive that makes the battery maintenance-free,raises the voltage by 5–8 percent.Temperature also affects the open-circuit voltage;heat raises it while cold causes it to decrease.Surface charge further fools SoC estimations by showing an elevated voltage immediately after charge;a brief discharge before measurement counteracts the error.Finally,AGM batteries produce a slightly higher voltage than the flooded equivalent.