When Is a Battery Fully Charged?
The most important part in fast charging is when to stop. Overcharging a battery can damage it and is dangerous. In
the graph on the next page we deliberately overcharged a NiMH battery to show you what happens. Look at the
battery temperature line. It rises slowly at first then quickly rises when the battery is almost 100% charged. The fast
charging stops when the temperature reaches the maximum temperature of 45ºC as set in the BAR. If the fast
charging didn't stop then the temperature would continue to rise and the battery would eventually vent (release
electrolyte) causing permanent battery degradation. Overcharging a lithium-ion or lithium-polymer battery can result in
the battery bursting into flames.
Now look at the voltage graph. This is the most common method to terminate a NiCd/NiMH quick charge as it requires
no temperature sensing. The "X" axis is the fraction of charge being put into the battery. At "1.0", in theory, the battery
would be fully charged. In actuality the battery isn't fully charged until a bit past this value. The line at "A" is the point
where the battery voltage has dropped enough (about 2mV/cell) so that a peak charge would have stopped there.
Notice at "A" the battery receives about a 5% overcharge (based on a theoretical 100% efficiency charge) which is
acceptable.
Now look again at the temperature graph. We've drawn two temperature curves - one for the battery temperature and
the other for the rate of battery temperature change. The temperature peaks at line "B" which is when the charger was
turned off. A temperature cut-off works best as a backup method of ending the battery charge cycle. Temperature
rate of change (dT/dt) is a better method of determining when the battery is fully charged. In this example, if you would
have specified a maximum temperature rise of 1ºC/minute then the charging would have terminated exactly at the
100% efficiency fully charged mark. Since rate of temperature change is dependent on the battery pack construction
(the less cooling the faster the temperature will rise) it's difficult to come up an optimum value to use.
What's best? We recommend that you start with a maximum battery temperature of 35 to 40ºC and a maximum dT/dt
of 0.5ºC/minute to 1.0ºC/minute. If the charging is stopping too early then increase these values. You can tell what
caused the charging to stop from the text at the bottom of the results viewer.
You can use the UBA to create your own chart. Run a battery test with a temperature probe. Then import the results
file into a spreadsheet program and use a simple formula to get the dT/dt (in degrees Celsius per minute): ((Temp2-
Temp1)/(Time2-Time1) * 60). For this graph we matched up the end of fast charge and beginning of slow charge
which compensates for the voltage drop of the cable (during peak charge the battery voltage is sampled with the
charge current on, afterwards the battery voltage is sampled with the charge current off).
The UBA Let's Get Started Guide
Page 31
