Knowledge Domain: Power Supply
Unit: Batteries
Skill: Building a charger
Tools and Parts Required:
1) Digital multimeter (DMM)
2) Power supply
3) Low value, high power resistor
4) Rechargeable battery
Introduction
Rechargeable batteries are common in medical equipment. Batteries convert chemical
energy into electrical energy. This process can be reversed in rechargeable batteries,
converting electrical energy into chemical energy.
Once a battery is recharged, remove it from the charger. The battery can be damaged
by overcharging. The best way to charge batteries is to add charge slowly.
Example
The two most common types of rechargeable batteries are nickel-cadmium (NiCd or
NiCad) and lead-acid. NiCd batteries are typically found in smaller medical devices.
Lead-acid batteries are most common in automobiles.
Identification and Diagnosis
Verify that the battery is rechargeable. Measure the voltage of the battery. If the voltage
is below the expected value, the battery may need to be recharged.
Procedure
The following procedure can be used to configure a rechargeable battery for charging.
Measure the voltage of the battery before beginning. Verify that the voltage is not below
the allowable voltage for the battery. If the voltage drops too low, the battery may be
permanently damaged and unusable. A 1.2 V NiCd battery is destroyed when its
voltage drops significantly below 1.0 V. A 6 V lead-acid battery is destroyed when its
voltage drops well below 5.2 V. A 12 V lead-acid battery is destroyed when its voltage
drops well below 11 V. If the charge on the battery is allowed to drop too low before
recharging, it cannot be recharged.
1. Look on the battery for the capacity. This value is measured in milliamp hours
(mAh). Look on the battery for the cell voltage, measured in volts.
Capacity = __________mAh
Cell Voltage VC = __________V
2. Measure the output voltage of the power supply. An adjustable power supply will
have a range of possible output voltages.
Vs = __________V
Cell Voltage Capacity 3. Choose a recharge rate between 1/100th and 1/10th of the battery capacity.
Choose the largest rate possible that will not destroy the battery. You may need
to recalculate this based on other values. Select a starting value now. You
should not be afraid of changing this number as needed.
Rate = __________
4. Calculate the maximum charge current to the battery, IC. This is Capacity * Rate.
For example, if your capacity is 700 mAh, and your rate is 1/10, IC = 700 * (1/10)
= 70 mA. If there is a standard IC labeled on the battery, use this value.
Ic = __________mA
Standard Charge Current 5. Use a resistor, R, to limit the charge current to IC (Step 3). Calculate this value
based on your power supply voltage, VS, your cell voltage, VC, and IC. Use the
formula shown below.
R=
‫܋܄ି ܁܄‬
۷۱
= __________Ω
6. Calculate the maximum power from the power supply to the resistor, PR. Verify
that this value is below the power rating of the resistor. If PR is above the power
rating of the resistor, begin again at Step 2. Choose a lower recharge rate and
calculate Steps 2 - 5. If the recharge rate is less than 1/100th of the battery
capacity, use a higher power resistor.
PR =
ሺ‫܄ି ܁܄‬۱ ሻ૛
‫܀‬
= __________W
7. Calculate the maximum power delivered by the power supply, PS. Verify that this
value is below the power rating of the power supply. If PS is above the power
rating of the power supply, choose a lower recharge rate and begin again at Step
2. If the recharge rate is less than 1/100th of the battery capacity, use a higher
rated power supply.
PS = VS * IC = __________W
8. Connect the charging circuit. Wrap the battery connection wires in a spiral shape
for a better connection.
Charging Circuit Diagram Power Supply (VS) Negative Lead Positive Lead Wrap the battery connection wires in a spiral shape for better connection.
Negative Lead Charge current (IC) Positive Lead Resistor (R) 9. After verifying that the correct charge current is being delivered, you may
reconnect the circuit in a more permanent setup. Check the battery and resistor
after 15 minutes to insure that no overheating occurs.
Negative Lead Positive Lead Full Charging Setup Check battery and resistor for overheating after 15 minutes. Exercise
Use a power supply and resistor to charge a battery. Measure the amount of current
flowing in the circuit using a DMM. Your instructor must verify your work before you
continue.
Preventative Maintenance and Calibration
Do not let batteries discharge below their lower voltage limit. Medical equipment with
rechargeable batteries should be checked every six months to insure that the user is not
overcharging the batteries. Medical equipment with rechargeable batteries should be
checked every six months to insure that the batteries still hold a charge. Always
calibrate every medical device before returning it to use.