A flyback converter implements a current-limited power supply to charge lead-acid batteries. The MAX773 current-mode controller limits the output current and the flyback transformer provides isolation and flexibility for input voltages both above and below the battery voltage. The MAX471 current-sense amplifier monitors the charging current and feeds back to a threshold detector so that below a designed threshold the flyback converter can switch to a lower charging voltage for trickle-charge mode.
The circuit shown in Figure 1 charges lead-acid batteries in the conventional way: A current-limited power supply maintains a constant voltage across the battery (2.4V/cell or so, as specified by the battery manufacturer) until the charging current decreases below a current threshold defined by the capacity of the battery. At this point, the charger is placed in a trickle-charge mode. The current threshold is typically 0.01C, where C refers to the battery capacity, which is specified in ampere-hours. When charging a battery, the term “C rate” refers to the current required, in theory, to charge a battery to its full battery capacity C in one hour. In actuality, power lost during the charge cycle ensures that all batteries charged at their C rate take more than an hour to reach full charge. Ideally, you could charge a 5A-hr battery in one hour if the charge current is 5A. Also, ideally, a C/10 charge rate (500mA) charges the same battery in 10 hours. However, the power loss mentioned previously increases these charge times beyond the two time spans stated above.
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http://www.maxim-ic.com/appnotes.cfm/an_pk/621
http://www.elecfree.com/electronic/simple-charges-lead-acid-batteries-by-max773/
