Battery Performance
Contents
Overview
Research pertaining to Battery performance.
Research
Note on efficiency measures
The charge/discharge (also called round trip and overall) efficiency is the performance parameter that we care about, of course. Hypothetically it should be equal to the charge efficiency multiplied by the discharge efficiency. In this case the efficiency is energy efficiency.
The charge efficiency is often quoted in absence of the discharge or overall efficiency, which is very annoying as it makes it harder to compare different battery designs that are mentioned in the literature. In marketing materials it is probably used in the hope the customer will erroneously assume it is equivalent to overall efficiency, since it is always higher than overall.
The discharge efficiency is rarely referred to in isolation for some reason, probably because it usually more or less commensurate with charge efficiency and can be computed easily from the overall and charge efficiency. Commensurate does not mean equal though.
Not energy efficiency:
Faradaic and coloumbic efficiency: see wikipedia article. Unfortunately it is apparent in patents and other documents it is apparently that this is in fact often, either due to ignorance or more likely sloppiness, used to mean charge efficiency or overall efficiency so watch out for this.
Polarization of the electrodes, see wikipedia.
Overpotential, see wikipedia.
Non-chemical factors affecting performance
At low temperatures with some iron electrode designs sulfur can accumulate on the iron electrode and increase internal resistance (designs which incorporate elemental sulfur, patents indicate this can be rectified by using a sulfide salt of low solubility like iron sulfide FeS).
The conductivity of the electrolyte goes up as the temp goes up. This is desirable but other undesirable reactions increase in rate at higher temps so there is a compromise there. Also this is one of the main mechanisms that can lead to thermal runaway during constant voltage charging as it can form a positive feedback loop. The solution is to just not use constant voltage charging, which is easy.
The factors like the energy to weight ratio and power to weight will of course tend to be affected by any non-reactant materials used to e.g. reduce cost etc, but fortunately those are of relatively little importance in the context of OSE for off grid electricity and farm equipment, although they may be important for the electric car. Basically we want to produce something that can replace lead acid which is cheaper, easier to work with and make and maintain, more durable with abuse and longer lasting, and not as bad for the environment. Ideally both for starting lighting ignition (SLI) batteries and also storage. Batteries described in documents can handle 6C and more without seriously hard to manufacture materials and additives so SLI is definitely an option. See related pages section. None of these require high energy density or power to weight ratio although they are always nice.
The fraction of active material that actually is utilized if low will reduce power and energy to weight and volume ratio. Can be affected by particle size, additives and also any particles of active material that are not in reasonable electrical contact with the current collector for whatever reason will remain unused.
The level of the electrolyte can if not maintained in a non-sealed battery, drop below the level of the plates, and the uncovered portion of the plate may remain unused, decreasing capacity until the electrolyte is replenished.
There is always some distance that the current needs to travel to get from the reaction area to the current collector, and to do this is has to pass along the active material. Therefore the conductivity of the active material is an issue which significantly affects internal ohmic resistance (the term internal resistance is often used to refer in a catch all way to the current draw vs. voltage output relationship even though this is due to many factors besides ohmic resistance). See wikipedia for polarization for more information that relates to the voltaic efficiency.
For a vehicle: Power to weight ratio is affected strongly by the surface are to weight ratio of the electrodes.
The pressure the casing needs to stand increases as the internal pressure dose of course, see sealed battery section, if sealed it will need to stand significant pressure and therefore be relatively heavy.
