1. Technical Field
This invention relates generally to rapid charging of lithium-based batteries, and more specifically to a circuit and method for modulating pulsed current in a rapid charging system.
2. Background Art
Nothing is more frustrating that a dead battery. If you are hungry, and want to order a pizza, the hunger pangs pang much louder when you pick up your cellular phone to call in the order and find that the battery is dead. Similarly, if you attempt to check your schedule in your personal digital assistant (PDA) and find that the battery is dead, you run the risk of missing your job interview, your reminder to pick up a loaf of bread at the store, or the worst case scenario: your anniversary. Tell your spouse, xe2x80x9cmy battery was dead,xe2x80x9d and you""ll end up sleeping on the sofa.
When people do find that the battery in their phone, radio, pager, laptop computer, PDA, or MP3 player is dead, they want to charge it as quickly as possible. No one likes waiting around two hours for a phone to charge. You could have made a pizza from scratch in that time. Consequently, battery charger designers are constantly working on ways to charge batteries faster.
By way of background, several prior art methods are known for charging batteries. For example, a constant-current, constant-voltage (CCCV) method is widely used. This method allows for the charging a battery with a constant charge current till a voltage threshold is reached (e.g. 4.2 V in single cell applications), after which the charge is continued by holding the lithium-based batteries at the voltage threshold of 4.2 V and continuing to provide current, wherein the current tapers sufficiently to maintain the 4.2 V threshold. When the current has fallen to a minimum threshold, a full charge indication is made and/or the charging is terminated. This method is effective at fully charging a battery, but it demands a charger that is capable of implementing the above regime. For a variety of reasons: cost, complexity, legacy system compatibility; a CCCV charger is neither preferred nor acceptable in some host power systems. Moreover, often for cost and design simplicity, the battery will have no control or influence over the charging parameters inherent in the charger, such as voltage or current. Finally, a certain battery may be expected to work with a variety of chargers where each charger has a different or unknown charging parameters.
In addition to the problems listed above, CCCV chargers may also become problematic when trying to reduce charge time. The way to rapidly charge a battery with a CCCV charger is to set the current limit very high so as to simply dump large amounts of current into the battery. Putting large amounts of current in, however, can be like trying to get a square peg into a round hole due to a phenomenon known as xe2x80x9cionic relaxation.xe2x80x9d Ionic relaxation is known in the art. It was recited, for example, in U.S. Pat. No. 6,127,804, issued Oct. 3, 2000, entitled xe2x80x9cLithium Ion Charging Means and Method Using Ionic Relaxation Controlxe2x80x9d, which is incorporated herein by reference for all purposes. For convenience, the fundamentals of ionic relaxation will be recited in the following paragraph.
Without being bound by theory, lithium-based batteries have active particles called ions that convert chemical energy to electrical energy. When a rechargeable lithium battery has been at rest for some time, i.e. with no charger or load attached, the ions become evenly dispersed throughout the cell. Evenly dispersed refers to a state where the electric field is evenly distributed, resulting in an electric field gradient of zero across the cell. This state may be referred to as xe2x80x9cionic restxe2x80x9d as there is no migration of ions within the cell.
When an external voltage, current, or load is applied to a cell, the electric field gradient is disturbed and the ions migrate to accommodate the new external terminal voltage requirements. This state may be referred to as xe2x80x9cionic agitationxe2x80x9d. Ionic agitation is analogous to poking a stick into a hornet""s nest, with the stick representing the external stimulus and the hornets representing ions.
When the external voltage, current or load is removed from the cell, the cell begins to return to the state of ionic rest. This return process from agitation to rest is called xe2x80x9cionic relaxation.xe2x80x9d The rate at which the cell xe2x80x9crelaxesxe2x80x9d may be approximated mathematically as an exponential decay in the form T=Cexe2x88x92kt, where C is a constant proportional to degree of agitation, t is time, and e is the exponential constant. The term k is a constant and may be referred to as the ionic relaxation time constant. It is related to the rate of ionic mobility of the ions in the electrolyte of the cell. Typically, the time required for relaxation under a normal stimulus is somewhere between 30 and 300 seconds.
Ionic relaxation impacts a cell when charging. When a cell is being charged at a high rate, the voltage across the cell increases as the cell absorbs energy. If the charge current is suddenly interrupted, the cell voltage drops a certain amount almost instantly due to an equivalent impedance within the cell. Following the initial drop, the cell voltage will continue to drop exponentially until a lower steady state voltage is reached. This exponential decay is a result of ionic relaxation.
In a similar fashion, when charge current is applied to the cell, the voltage instantaneously increases due to the equivalent series resistance. This initial jump is followed by an exponential increase in voltage due to ionic agitation. These xe2x80x9csmall signalxe2x80x9d, exponential, voltage swings are superimposed upon a slower, more linear xe2x80x9clarge signalxe2x80x9d voltage increase that is increased and decreased by energy storage and discharge within the cell.
In addition to energy storage, ionic agitation and relaxation cause resistance and inefficiency when charging cells. When ions are aligning themselves due to energy storage, they xe2x80x9cbump into each otherxe2x80x9d along the way. This interference generates heat and unwanted gas. In order to rapidly charge a battery, one tries to align ions as rapidly as possibly while reducing the incidents of inefficient collisions.
Charging a battery is similar to, and thus may be visualized as, filling a mug with creamy, frothy root beer. Imagine that the mug is the battery, root beer is energy, and the foamy head is an undesirable increase in cell voltage and impedance caused by inefficient agitation. The goal is to fill the glass with root beer as quickly as possible, i.e. fast xe2x80x9cchargingxe2x80x9d, without any of the foamy head overflowing the mug. Pouring in one continuous stream is the same as charging a battery with a constant current; it generates a substantial amount of head. If, however, one puts in a little root beer and waits for the head to disappear (i.e. allow ionic relaxation to take place), then puts in another burst and so on, the glass can be filled (or battery can be charged) much more quickly. This is the motivation behind pulse charging in order to rapidly charge batteries.
Prior art ionic relaxation solutions, like U.S. Pat. No. 5,808,447, teach an ionic relaxation method that operates thusly: Charge a cell to a maximum voltage and turn the charger off (e.g. charge a single cell to 4.2 volts and stop). Wait until the battery voltage has drooped below a threshold (e.g. wait until the battery drops below 3.9 volts). Turn the charger back on. Repeat.
While this solution is better that simply dumping a large amount of current into a cell, it would be advantageous to have even faster charging methods due to the frustrations listed in the first paragraph. After all, no one likes sleeping on the sofa. There is thus a need for an improved pulse charging method for rechargeable batteries.