Low-Cost Lead-Acid Charger Operates at Line Frequency

Jun 1, 2001 12:00 PM
By Paul Chinski, Charles Industries, Rolling Meadows, Ill.

The lead-acid battery charger market is established, mature, and cost-competitive. The target charger application is for deep-cycle batteries found in the “bass boat” marketplace, where fisherman want to fully charge their trolling batteries, overnight. In this industry, thermal and mechanical considerations are critical to handle heat extraction and waterproof the enclosure.

Lead-acid batteries are very rugged, yet they have a characteristic charging profile that benefits and increases their longevity ^[1]. The proper charging of sealed lead-acid batteries is critical. Undercharging reduces capacity whereas overcharging damages the battery, “boiling” the electrolyte and causing outgassing. ^{[1, 4]}

One common technique to implement a bass boat battery charger is to use a high-frequency design. The advantages for this approach are the same as for switchmode power supplies — lightweight, smaller size, and improved efficiency. An additional advantage is the control of the charging algorithm — either discretely or with an integrated chip. This results in a desirable charging profile of the bass boat lead-acid batteries, often referred to as a three-state or four-state charging algorithm ^[2].

Fig. 1, on page 50, illustrates a timed out three-state charger that maintains the current at some predetermined high rate of charge for a specified time and then reduces it. Fig. 2, on page 50, illustrates a normal volt/current vs. time graph in which current decays with time. Some charging algorithms provide a fast charge characteristic and then transfer to a slow rate of charge. Reference 4 describes the battery charging characteristics for slow vs. fast charging for sealed lead acid batteries. The trade-off is in the additional cost, the same scenario as in the linear vs. switching supply. The high-frequency charger generally costs more than other approaches.

Another common technique is to use phase-control technology for the battery charger. This approach has been in existence for quite some time, and offers cost advantages in high-volume production. The parts count is low. The major power processing portions are the line frequency transformer and the SCRs. The trade-off here is a loss of the optimum-charging algorithm and the increased weight.

An older, mature design for battery charging is the ferroresonant type. It utilizes the transformer inductance with a specialized capacitor to obtain resonance, thus it's very sensitive to line frequency variations. For example, a 60 Hz design will not operate at 50 Hz. The control of battery charging is very poor, with the possibility of outgassing and boiling batteries. Fig. 3, on page 50, illustrates the volts/amps vs. time charging profile for a ferroresonant charger.

Line Frequency Design

Some low-cost, line frequency designs use phase-controlled SCRs to control output voltage. Controlling the SCR turn-on in the rectification bridge maintains the output voltage. Logic gates, counters and analog-to-digital converters, microcontrollers, or ICs can control the SCR ^[3].

Another means of controlling the output voltage is through the use of one switch instead of controlling all four SCR switches. Using a low R_DS(on) MOSFET as the main switch can take advantage of its lower costs in high volume production. This allows the use of low-voltage, high-current Schottky rectifiers as the bridge rectifying elements. They have a lower voltage drop and lower conduction losses compared with a silicon rectifier or SCR. The control of one switch compared to four SCR switches is another advantage. The MOSFET has low losses at line frequency, with on-state losses predominating and is easier to turn on and off than an SCR.

You can use a current sensing element for implementing current mode control with voltage mode control. Using voltage mode and current mode control in the high frequency switching designs is effective. Using them in this line frequency application, the time constants are much longer in this realm, but you can still apply the concepts. You can use a resistor or current transformer as the current sensing element and implement the voltage and current control loops discretely if board space isn't at a premium. This allows the use of available commodity items such as op amps and comparators to manipulate small signals associated with the loops. This design can be interesting with ways to implement the control algorithms.

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