Physics 310 Spring 2019
PROTECTING THE BATTEY FOR EXTENDED CHARGING
An important aspect of using Lithium batteries is protecting the battery from charging past it's rated voltage. It was noted by a group who had worked previously on improving Donette's bicycle that ensuring optimal charging meant risking that the battery could be charged past it's rated voltage, which can deteriorate the battery and be dangerous. We set out to fix this issue in order to ensure we can charge the battery efficiently without risking anything.
CIRCUIT OPERATION
Theory of How the Above Circuit can be Used to Protect the Battery
The key to the operation of the above schematic is the 1N5369B Zener Diode. The diode has a forward Zener Voltage rated at about 50V; per our calculations, 50V represents the maximum power at which the battery can be charged without damaging the cells. The Circuit operated by having the high power NPN transistor, Q, operate normally for voltages which are less than 50V. If a voltage higher than 50V is delivered the zener diode will begin to operate, providing a path to ground and disabling the transistor Q1.The capacitor is included as a crude representation of a battery, as it has charge/discharge characteristics. The practical operation of this circuit has been simulated and can be seen below.
Simulated Results of Above Schematic
In dissecting the above waveform, it should be noted that the blue waveform is the voltage supplied to the circuit to test it's operation, the red waveform is the voltage across the capacitor (used to represent battery voltage in this case), and the grey waveform is the current delivered to the capacitor. Therefore the above simulation shows that in the initial ramp from the input the battery is able to charge up only to the rated 50V value, and doesn't change from that value even with the input voltage continuing to vary. The current delivered to the load peaks just below the batteries rated 6A for charging, and then dips once the maximum voltage is reached; this is promising as it illustrates to things required for proper operation. The first of these things is that the NPN transistor is capable of sinking this rather high current, and the second is that when the battery is charged no current is delivered to the load.