In a proposed method of controlling the electric current supplied for charging a nickel/hydrogen battery, the rate of evolution of heat would be taken into account along with the electrical quantities (voltage, current, and/or charge as functions of time) that are traditionally taken into account. This method might also prove useful for controlling charging currents in batteries based on different chemistries.
The parameters related to limitations on the lifetime of an Ni/H battery are depth of discharge, temperature, and overcharge. The major proximate cause of failure is gradual swelling of plates, associated with overcharge and with the generation of oxygen in the overcharge reaction sustained over thousands of charge/discharge cycles. In principle, the lifetime could be prolonged by minimizing overheating and precisely limiting the amount of overcharge.
The rate of heating cannot be measured directly but can be estimated from other quantities that can be measured. Because an Ni/H cell is a pressure vessel, its internal pressure can be used as an indication of the amount of hydrogen present and thus of the state of charge. Assuming that (1) the void volume in the battery remains constant; (2) the pressure, temperature, and number of moles of hydrogen are related by the ideal gas law; and (3) the charging efficiency is 100 percent, then the rate of increase of pressure during charging should be proportional to the rate of charging; that is, to the charging current.
Any deviation from this proportionality would be attributed to a decrease in efficiency associated with the overcharge reactions. The portion of applied current diverted to the overcharge reaction would not be available for the main charging reaction that generates hydrogen; as a consequence, for a given charging current and temperature, the rate of increase of pressure would decrease as the battery went into overcharge. The oxygen generated in part of the overcharge reaction is also quickly recombined in another part of the overcharge reaction; the net effect of the portion of the charging current diverted to the overcharge reaction is to generate heat. Thus, by use of basic equations of thermodynamics, it should be possible to determine the charging efficiency and the rate of generation of heat from measured values of pressure, temperature, and voltage as functions of time.
The proposed method would be implemented in control software. In this method, the rate of generation of heat computed as described above would be used as feedback in a control algorithm that would taper the charging current to maintain the lowest possible battery temperature and minimize the generation of oxygen. The maximum allowable rate of generation of heat would have to depend on the temperature of the battery because charging efficiency decreases with increasing temperature.
This work was done by Paul Timmerman of Caltech for NASA's Jet Propulsion Laboratory. NPO-20470
This Brief includes a Technical Support Package (TSP).
Improved control of charging current for Ni/H battery
(reference NPO20470) is currently available for download from the TSP library.
Don't have an account?
Overview
The document titled "Improved Control of Charging Current for Ni/H Battery" discusses advancements in the charging methodology for nickel/hydrogen (Ni-H2) batteries, particularly in the context of space applications. Authored by Paul Timmerman for NASA's Jet Propulsion Laboratory, the report emphasizes the importance of optimizing charge control to enhance battery performance and longevity.
Ni-H2 batteries are critical for spacecraft, but their efficiency can be compromised by factors such as temperature, depth of discharge, and overcharge. The document outlines a proposed methodology that leverages telemetry data to monitor internal heat generation within the battery during charging. By calculating the charge current based on this heat generation, the system can maintain optimal battery temperatures, thereby minimizing oxygen generation and extending the battery's lifespan.
The report explains that the internal pressure of the Ni-H2 cell can serve as an indicator of the state of charge, following the ideal gas law. As the battery charges, the pressure should increase proportionally to the charging current, assuming 100% efficiency. However, deviations from this proportionality indicate reduced efficiency due to overcharge reactions, which generate heat and can lead to battery swelling and failure over time.
To address these challenges, the proposed control algorithm would taper the charging current as the battery approaches overcharge conditions, thereby limiting heat generation. This real-time control requires a dedicated test stand and sophisticated software capable of processing data and adjusting charging parameters dynamically.
The document also highlights that while traditional methods, such as voltage-temperature (VT) curves, have been used for charge control, they have limitations. The new approach aims to provide a more precise and responsive charging strategy that could become the industry standard for Ni-H2 batteries and potentially be applicable to other battery chemistries.
In conclusion, the proposed charging control methodology represents a significant advancement in battery management for space applications, focusing on real-time adjustments to enhance efficiency and prolong battery life. The implementation of this technology could lead to improved performance in future spacecraft, making it a valuable contribution to the field of aerospace engineering.
