It's significantly more accurate than watt-hour counting.
It is not very accurate except for certain short time operations where the error accumulation is relatively small.
The greatest inaccuracies, in my experience, are dealing with absorption and adjusting for self-discharge. When batteries are in absorption, a growing amount of energy is converted to heat. With self-discharge, it just disappears.
With watt-hour counting, charging watt-hours have to be adjusted by voltage elevation from high rates of charge, and discharging watt-hours have to be adjusted by voltage depression. Using amps instead of watts adjusts for both of those scenarios pretty well.
Adjustments are needed for amp-hour counting as well. You seemed to imply that amp-hour counting is accurate without keeping up with what goes on in the charge-discharge cycle.
tallgirl said:
You'd have to know the entire history of the charge and discharge cycle to know the state of charge if you use watt-hour counting.
It is simply a matter of fact that amp-hour counting alone is only useful for certain short time operations where the error accumulation is relatively small. To be accurate, amp-hour counting must be supplemented with other measurement and book-keeping tasks. These can include stabilized voltage measurements, EMF measurement/determination, table comparisons, or other techniques to re-synch with the battery's real status. Additional adjustments must frequently be made for changes in capacity, self-discharge, charge/discharge efficiencies, memory effects, etc. as well as other battery model parameters that change during the usage and life cycles.
In order to make an accurate book-keeping system, amp-hour counting must be part of an adaptive system.
Battery capacity is the battery?s electrical energy content expressed in ampere-hours.
In battery mangement, key items including charge control, capacity monitoring, run-time status, and charge-cycle counting but the two most important tasks are optimizing the use of the energy in the battery and preventing battery damage.
Monitoring is a sub-set of functions that supports the over-all battery management. State-of-charge indication is just one function used to manage the energy in the battery. Outside of determining delivered/recieved power, amp-hour counting alone is a useless function and one should recognize the underlying purpose is to monitor the battery capacity and prevent battery damage.
The other monitoring functions include state-of-health and remaining run-time. The remaining run-time is an estimate based on loading and is a function of remaining battery capacity (the energy we can expect to get out of the battery).
In addition to current and time, other measurements are used to determine the battery's remaining capacity including voltage, impedance, temperature, etc.