Instructables

Step 3: Theory and operation

Theory and Operation

Li-Ion Batteries , when charged under no load have a Voltage of about 4.2 Volts , that quickly drops to about 3.9 Volts under load and slowly drops as the charge is used . a Li-Ion Battery cell is considered fully discharged then the under load Voltage get below 3.00 Volts.
So what i do it have the battery attached to one of the Adruino's Analog Input's , and read the No-load value. at the same time waiting for the start Button to be pressed. If the voltage input is above 3.00 volts when the Button is pressed the test will start. 
When the test starts  the Solid State relay is energized with switch the 4 ohm resistor to be a load directly across the battery. the Voltage is then read every half second and using ohm's Law current ,  I=V/R,  R= 4 ohms and V is read from the Analog input. Since this reading is taken every half second , we know there are 7200 1/2 seconds in an an hour , we just take the current  multiply it by 1/7200 of an hour and accumulate all those reading and until the Voltage  is below 3.00 volts , at that point the Relay is shut off and the grand total of how many maH of energy was released is displayed

 
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