RU2321105C1 - Method for equalizing capacity of nickel-hydrogen storage battery incorporated in space vehicle - Google Patents

Abstract

FIELD: electrical engineering; power supplies for off-line power consumers such as spacecraft.

SUBSTANCE: proposed method for equalizing battery capacity makes use of tight dependence between rate of self-discharge of nickel-hydrogen cells and temperature and is implemented by disconnecting battery and load, its heating and storage at temperature not over 50 °C for time sufficient for complete discharge of all cells, that is, for voltage drop below 0.5 V. Proposed method is characterized in that it wants no special measures for its implementing as it uses heaters incorporated in storage battery.

EFFECT: ability of equalizing capacity of all battery cells dispensing with extra charges for implementing this method.

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Description

The present invention relates to electrical engineering and can be used in consumer power systems installed on autonomous objects, in particular on spacecraft.

There is a known problem with the battery run-up by capacity during long-term cycling in a serial battery circuit, as a result of which when the battery is discharged to the load, individual batteries are “reversed” (reverse polarity) and limit battery discharge. The mechanism of the “run-up” of batteries by capacity using nickel-cadmium batteries as an example is described in the work of A. Zolotov. "The operating mode of nickel-cadmium batteries and their service life." [Electrical industry. Series Chemical current sources. 1980, issue 5, p. 22, 23]. A feature of the operation of batteries in a battery is that all of them are charged and discharged with the same current and the same capacity as the entire battery, while batteries may have some differences in performance. The sources of battery breakdown by capacity can be:

- differences in the initial capacity in the batteries when they are selected in the battery;

- differences in the value of the internal resistance of the batteries due to technological variations;

- differences in the degree of degradation of batteries at the service life;

- differences in battery temperature and degree of self-discharge.

Therefore, developers of power systems take various measures to equalize the battery capacity. The simplest method is the discharge of batteries widely used in terrestrial conditions to individual discharge resistances that are connected to the battery for discharge. However, due to the complexity of its implementation for the automatic mode of operation, when the intervention of maintenance personnel is excluded, he did not find application in autonomous objects.

In space conditions, two methods of leveling the capacitance are widely used:

- discharge of the battery for total resistance (this method is widely used in the USA, for example, on the Intelsat satellite [Levy E., Ir.Osugi Fred s Design and Performance of Intelsat - IV Power Subsystem. "7 ^th Intersoc. Energy Convers Eng Conf ., San Diego, Calif., 1972 "] - analogue;

- recharging the battery with a small current after a full charge (the so-called Trickle charge), see, for example, [A Power Subsystem for telecommunication Satellite / L. Croci, P. Galantini, C. Marana Proceeding of the European Space Power Conference Held in Graz, Austria, 23-27 August 1993 (ESA WPP-054, Ang. 1993)] is a prototype.

The method of equalizing capacity by discharging the battery to the total discharge resistance (analogue) is not without drawbacks. Firstly, it requires additional mass costs (resistors, switching switches with deep redundancy to ensure high reliability for any type of failure), complicates the battery monitoring and control circuit. A typical discharge algorithm for aligning batteries is to discharge the battery until the voltage on any of the batteries drops to 0.5 V, then decrease the discharge current (increase the resistance of the resistor), re-discharge to 0.5 V on any of the batteries, etc. The alignment procedure ends with the last discharge up to 0.5 V on the “weakest” battery. At the same time, despite the complicated discharge procedure, the complete discharge of all batteries to 0 V with this method is not guaranteed.

The essence of the second method (prototype) boils down to the fact that all batteries, regardless of their individual state, are fully charged after a full charge.

The magnitude of the recharge current is selected from the analysis of many factors:

- self-discharge values of batteries;

- temperature;

- cooling method;

- battery cycling mode, etc.

The current value is selected on the basis of a compromise between ensuring a full degree of battery charge and preventing individual batteries from overheating. To avoid imbalance, the recharge current should be sufficient to "pull up" the most "weak" battery, i.e. a battery having a maximum self-discharge and a minimum charge efficiency. Therefore, in all cases, due to the individual characteristics of the batteries at the selected recharge current, the battery has batteries that are subjected to the greatest recharge. It is known that overcharging a sealed battery leads to its overheating and the appearance of oxygen in the gas phase, which adversely affects the battery performance. Therefore, in real practice, you have to choose the lesser of evils:

- or allow overheating of individual batteries and take the risk of deteriorating battery life, but at the same time ensure that all batteries are fully charged in the current time period;

- or allow a limited imbalance of batteries in capacity with a decrease in the discharge capacity of the battery.

Equalization of capacity due to recharging is associated with additional management difficulties, since Due to the resource degradation of the batteries, the required recharge current should be adjusted during the operation of the spacecraft.

In the proposed method of aligning the batteries by capacity, the property of a nickel-hydrogen battery is used - the so-called property of "selective catalysis", which is expressed in the fact that the main reaction:

NiOOH + 1 / 2H ₂ ⇒ Ni (OH) ₂ .

When the external circuit of the battery is closed, the reaction is current-generating and goes much faster than without closing the circuit. In the second case, it occurs due to a chemical (rather than electrochemical) process and determines the self-discharge of the battery.

The rate of the chemical process of hydrogen oxidation is highly temperature dependent. In the same work, experimental data are presented, from which it is seen that a decrease in battery capacity to a level of 50% of the initial value during storage occurs at a temperature of 15 ° C for ~ 20 days, at 30 ° C for 5 days. A similar relationship is confirmed by many other sources.

The essence of the proposed method consists in heating the batteries and storing the battery in a state disconnected from the charge and load until the voltage on all batteries is lower than 0.5 V.

According to numerous sources, the battery capacity is almost completely exhausted when the voltage drops below 1.0 V even when discharged to an external load by a small current. When stored without external load, this condition is even more fulfilled.

The duration of storage until guaranteed complete self-discharge of each battery is determined by the temperature and degree of charge before the operation. According to the technical conditions for nickel-hydrogen batteries, the maximum allowable temperature of the batteries during storage is allowed no more than 50 ° C.

That is, during operation, to equalize the batteries in terms of capacity, the battery is periodically disconnected from the external electric circuit, heated, stored at an elevated temperature for a time sufficient for the complete self-discharge of all batteries, after which the heating is turned off and the battery is connected to the external electric circuit, charged and continue to operate. The proposed alignment method allows for independent self-discharge of each battery and does not require additional mass and hardware-software resources for the developer of the power supply system. This can be explained by the example of a geostationary connected spacecraft:

- to ensure the reliability of the spacecraft, at least two rechargeable batteries are used as part of the power supply system, which in total are designed to provide the required capacity when passing shadow portions of the orbit. The period with shadow portions of the orbit is 45 days for six months. The rest of the time, about 5 months, the batteries are in standby mode and can be used to alternately equalize the capacitance without violating the operating mode of the power supply system and there is no threat to the survivability of the spacecraft;

- on most spacecraft, methods of cooling batteries with heat radiation into outer space using special radiation panels are used. At the same time, special heaters are used to protect the batteries from hypothermia (in those cases when they are not subjected to deep cycling). They are connected either automatically when the battery temperature drops to the set value, or on command from the Earth. The same heaters can be used to heat the battery when leveling the capacitance;

- when implementing the method on a specific spacecraft, various combinations of storage duration and battery heating temperature can be used. For safe operation of the batteries during further operation, the maximum temperature must be limited to plus 50 ° C.

Thus, a distinctive feature of the proposed method is the ability to fully equalize the capacity of the batteries in the battery with extreme simplicity and the absence of additional costs for its implementation. The proposed method is proposed to be used in the development of the spacecraft.

Claims (1)

A method of leveling the capacity of a nickel-hydrogen storage battery, which consists in disconnecting it from an external circuit, characterized in that the battery is heated and stored off at a temperature not exceeding 50 ° until all batteries are fully self-discharged below 0.5 V, after which it is cooled to operating temperature and connected to an external circuit.