Maintain VEX Rechargeable Power Packs Performance Objective: Given a VEX robotics system, maintain VEX rechargeable power packs so the system functions as required. Enabling Objectives: define the following terms: ohms, volts, milliamps, amp hours, electricity, alternating current, direct current identify 7.2V and 9.6V power packs test power pack voltage using a multimeter explain how to charge the 7.2V power pack versus the 9.6V power pack set-up the battery charger identify the charging sequence when charging two power packs describe the relationship between the status lights on the charger and the condition of the power pack(s) being charged
Battery Packs • 7.2 volt robot battery • Capacity: 1000-2000 mAh • Charging time: 1.4 – 2.8 hrs • 9.6 volt transmitter battery • Capacity: 700 – 1000 mAh • Charging time: 1.4 – 2.0 hrs NOTE: the batteries used to power VEX systems are also known as power packs
Electricity Electricity is the movement of electrons; particles with a negative charge. Electrical systems utilize either alternating current (AC) or direct current (DC). VEX robotic systems utilize batteries that supply DC.
Electrical units of measurement • There are three units of measure that are important when looking at an electrical circuit: • Amps • Volts • Ohms
Amps (A) Amps is the measure of electrical current. Current is the amount of electrons flowing through a conductor in a given period of time. Amps can be measured using a multimeter.
Volts (V) Volts is the measure of force or pressure moving electrons. Volts can also be measured using a multimeter.
Ohms (Ω) Ohms is the measure of resistance to the flow of electrons. Multimeter can be used to measure ohms.
Digital Multimeter A multimeter can be used to measure both AC and DC voltage, amperage, ohms, and continuity.
Digital Multimeter AC Volts DC Volts DC millivolts • Continuity Ohms DC Amps AC Amps
Measuring DC Voltage Step 1: Set multimeter on DC voltage Step 2: Plug red probe in volts, ohms, and continuity socket Step 3: Be sure black probe is in the com socket
Measuring DC Voltage Step 4: Place probes in socket of battery pack, red to red (+), black to black (-) Step 5: Note voltage value from the multimeter If reading is negative, you can either ignore the minus sign or reverse the probes
Normal battery readings A freshly charged 7.2 V (nominal value) battery will read approximately 8.2 volts A freshly charged 9.6 V(nominal value) battery will read approximately 10.4 volts When a battery reaches its nominal value, it will need to be recharged
CAUTION using multimeter DO NOT test the batteries using any other set-up on the multimeter or you may risk damaging the multimeter.
Milliamp-hours (mAh) An amp-hour is the amount of amps a battery can deliver for one hour The prefix milli is used to indicate one thousandth (1/1000) of a unit in the SI measurement system For example: a millimeter (mm) is one thousandth (1/1000 or 0.001) of a meter; a milliamp-hour (mAh) is 0.001 of an amp-hour.
Milliamp-Hours (mAh) • 1000 mAh • The battery can produce a steady charge of 1 amp for one hour 2000 mAh The battery can produce a steady charge of 2 amps for one hour
Battery Charger • Transformer and chord Battery charger and battery packs
Setting up the battery charger • Plug transformer’s 16VDC cord into the battery charger Plug transformer ‘s 120VAC cord into wall outlet
Battery charger default setting • Default overrideswitches:These switches will be engaged when the 7.2 Volt battery is properly installed Battery packs properly installed in charger
Properly inserting batteries Switch is pressed When charging 7.2 volt batteries it is very important the default override switches are pressed down. If these switches are not pressed down, the 7.2 battery pack will be charged to the higher default setting, ruining it.
Charging order of batteries Whichever battery is plugged in first is the first one to charge. The second battery will be charged when the first is completely charged.
LED Status lights Whichever LED is on indicates which battery is charging If the LED is flashing it indicates there is a problem If the LED is off it means the battery is fully charged or waiting for the other battery to charge
Flashing LED If LED flashes, try a different battery pack If the LED still flashes, try another charger
The End Have fun and be safe