“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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The transformer will be overloaded, and load current is limited only by the winding resistance of the transformer. As a result, the transformer will run very hot during most of the charging duration. The transformer will burn out due to excessive heat generation soon enough, due to insulation breakdown or the insulating material melting on the windings.
If BT1 happens to have relatively high internal resistance, the charging relay will chatter because when charge voltage is applied to such a battery, its terminal voltage will suddenly rise and the charging relay will change state. As soon as the charging stops, the battery terminal voltage will drop and the relay will change state again and connect the charging circuit. This will repeat forever. The hysteresis provided by the VR1/VR2 network will not prevent this chattering because the voltage swing across the battery terminal will be large when charging compared to not charging.
In addition, different battery chemistries such as NiCD, NiMH, Li-ion, and lead-acid will require different charging algorithms (often sophisticated ones) as well as detection of negative-delta, battery temperature rise, and charge current detection and limiting to get good performance out of the battery and long life.
Many batteries will require float charge to replace the charge lost due to self-discharge characteristics after regular charging is complete, while some other batteries should not be given float charge.
In summary, connecting the filtered output of a bridge rectifier directly to a battery until the battery voltage rises to a preset value is not a good way to charge a battery. If the charge current is not controlled, it can have an adverse effect on battery life in the best case and disastrous results in the worst case.
Getting good life out of a rechargeable battery requires a carefully designed charging circuit that often monitors not only battery voltage but also monitors and controls charging current, monitors battery temperature, and can detect a sudden rise in temperature or sudden change in battery voltage.