“Build A Smart Battery Charger Using A Single-Transistor Circuit,” an Idea for Design, has generated lots of traffic since it was first published on November 25, 2002. But it has a serious flaw that makes it unsafe and less reliable, either in a product or in a hobby circuit to be used at home.
The main problem with the circuit is that the battery-charging current is totally uncontrolled, so it is not suitable to charge any battery that is expected to have a reasonable operating life. This circuit also can be dangerous and cause a fire hazard, depending on the type of battery and the transformer being used.
If a sealed lead-acid battery is charged with excessive current, its gasket may blow. If a flooded lead-acid battery is charged with excessive current, water will evaporate, increasing acid concentration and reducing battery capacity. If a nickel-cadmium (NiCD) or nickel-metal-hydride (NiMH) battery gets excessive charging current, internal pressure will rise, possibly bursting the cell. If a lithium-ion (Li-ion) or lithium-polymer (Li-poly) battery gets excessive charging current, its temperature will rise and it may catch fire and burn. This can happen even before the battery voltage rises to a preset charging-stop level set by trimmers. Hence, RL1 won’t prevent battery damage.
In addition, this Idea for Design does not specify the load current rating for transformer T1 or the capacity and type of battery BT1, both of which are important for design. Consider two scenarios.
In the first scenario, transformer T1 is a heavy-duty device that supplies high load current. As a result, it has low winding resistance. Battery BT1 is a low-capacity device, i.e., it has a larger internal resistance. For example, T1 and BT1 are a 12-V/4-A transformer and 9-V/1-AH battery, respectively.
With ac power on and battery voltage lower than the threshold set by a trim-pot, relay RL1 is off and the output of diode D2 is directly connected to the battery. The 12-V transformer and bridge-rectifier and filter (C1) configuration generate an open circuit voltage of:
12*(sqrt(2)) = 16.8 V at pin1 of D2 (or 16.3 V at pin2 of D2)
This voltage is directly applied across the battery terminals, so the charge current is only limited by the winding resistance of the transformer and internal resistance of the battery. The charge current essentially is uncontrolled and will destroy the battery due to excessive heating or buildup of excessive pressure inside the battery even before the battery voltage rises high enough for RL1 to disconnect the charging circuit.
I wouldn’t want to be near BT1 if it were a Li-ion battery when it was getting charged because it would be sure to explode, catch fire, and burn.
In the second scenario, the transformer isn’t a heavy-duty device, but the battery has a high capacity. For example, there is a 12-V/1-A transformer and 9-V/10-AH battery. Since such a battery will have relatively low internal resistance, it will essentially present a short to 9 V at pin2 (cathode) of D2.
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