Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Reexamination Certificate
2002-04-02
2003-04-29
Tso, Edward H. (Department: 2838)
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
C320S132000
Reexamination Certificate
active
06556020
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention includes methods and means for indicating the remaining charge on a battery, and particularly, the state-of-charge of a lead-acid battery used to power electric lift trucks and the like.
Electric lift trucks employ large lead-acid batteries to power their traction and lift drives. Trucks are typically operated in continuously and when the battery becomes discharged, it is replaced and the truck is immediately put back into service. In a warehouse serviced by many such trucks, batteries continuously cycle through stages including: recharging (typically 7 to 8 hours); cool down period (typically 7 to 8 hours); and use (typically 7 to 8 hours). Because recharging the battery in a truck can take the truck out of service for a period of time, an objective in this industry is to operate the truck as long as possible on a battery charge. To do this, however, one must accurately know the state-of-charge of the battery.
Because a wide variety of different batteries may be used in an industrial lift truck, methods which rely on knowledge of the particular battery to measure its state-of-charge are not practical. Some of the battery variables common to batteries used in a fleet of lift trucks includes voltage (e.g. 24, 36 or 48 volts), their capacity (e.g. from 21.5 kWh to 43 kWh), their temperature (−10° C. to +45° C.) and their condition (e.g. new/old, sulfated, leaky). Each battery is made of a number of cells connected in series to achieve the desired voltage. For example, a 24 v battery has twelve 2 volt cells in series, a 36V battery has 18 cells and a 48V battery has 24 cells. The capacity of a battery is determined by the number and size of plates per 2V cell. If more plates are used the capacity is increased and the internal resistance is decreased. Battery temperature affects the rate of the chemical reaction in a battery cell. This affects the ability of a battery to deliver current and it manifests itself as a change in battery resistance and a change in its effective capacity.
When a battery ages or is not maintained properly, the lead plates become “sulfated”. Deposits of lead sulfate form on the plates and do not disappear when the battery is charged. These deposits effectively reduce the active area of each plate and consequently reduce the battery capacity and increase internal resistance. State-of-charge methods which rely on battery capacity as a known value will be very inaccurate unless the condition of the battery is carefully maintained.
Two practices are employed with lift trucks that further complicate measuring the state-of-charge of their batteries. First, it is common practice to partially recharge the truck battery at moments during the work day when it is not in use. Such moments occur, for example, when the operator takes a short break for lunch or the like. And second, such partial recharging also occurs when the lift truck traction drive employs regenerative braking. This is a form of dynamic braking where the traction motor is momentarily turned into a generator. The current which is produced flows into the battery to partially recharge it, and the same current produces a negative torque which opposes travel. While each braking event may only yield 1 watt-hour of energy, the event may occur 600 times during a work shift. State-of-charge indicators which do not account for such intermittent recharging are very inaccurate and result in taking a truck out of service 30 minutes or more before it is necessary.
There are many different techniques for measuring state-of-charge of a battery. One of the simplest methods is to measure battery voltage. Indicating state-of-charge by monitoring battery voltage can work in applications where the load on the battery is constant, but in an application such as a lift truck where the load (and consequently battery voltage), varies widely and unpredictably, the result is inaccurate. The accuracy can be improved by averaging voltage readings over a period of time, but the 30 to 60 minutes required to average out sudden changes in load is too long in most applications.
A number of more complex state-of-charge measurement methods are known. These methods are applicable when considerable information is known about the battery and its use. Such methods may require information such as the capacity of the battery, its temperature, or the specific gravity of its electrolyte. These methods are appropriate in applications in which the same or an identical battery is always used with the state-of-charge indicator circuit.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for measuring and indicating state-of-charge of a battery while the battery is in use powering a vehicle or other variable load. More particularly, the battery voltage and battery current delivered to the device are sampled during operation and internal battery resistance is calculated from selected voltage and current samples. A dynamic battery voltage is calculated from the measured voltage and current and the calculated internal battery resistance; and this calculated dynamic battery voltage value is used to determine a state-of-charge value for the battery. A stored look-up table is used to convert the dynamic battery voltage to a state-of-charge value, and this table can be used with a wide variety of lead-acid batteries.
A general object of the invention is to accurately determine the state-of-charge of a battery while it is in use with a variable load. The battery voltage and current are sampled periodically during use, and selected samples are used to make the calculations. Factors such as battery capacity, temperature or condition do not affect the results, and do not, therefore, need to be known inputs to produce accurate results.
Another object of the invention is to produce a state-of-charge value which accurately reflects intermittent battery recharging during its sue. The method does not require that the battery only be discharged during use and it does pick up the charge added by events such as regenerative braking.
A more specific object of the invention is to provide a state-of-charge indicator for an industrial truck which may employ a wide range of lead-acid batteries. The present invention provides an accurate state-of-charge indication regardless of the manufacturer of the lead acid battery, its voltage, its condition, its capacity, or its temperature.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.
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Baldini Augustus R.
Driscall Daniel Robert
Harvey Dean
McCabe Paul Patrick
Quarles & Brady LLP
The Raymond Corporation
Tso Edward H.
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