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How does capacity correlate with charge voltage for lithium ion batteries? |
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Custom design and manufacture of state-of-the-art battery chargers, DC/DC Converters, and power
supplies
How much voltage does it
take to charge a lithium-ion battery?
Motivation: Most batteries have a distinct charge voltage. Below
that voltage you cannot move the chemistry in the right direction, above that voltage you can fully charge the battery, even
though it might take a long time.
With lithium-ion (lithium-ion, lithium polymer, lithium iron phosphate, etc.) this is
not the case. There is a voltage below which there is no action, the chemistry just won't move. But a big part of the charging
process is getting ions in and out of solid compounds. These compounds have space between the crystal planes, or within the
crystal structure for small ions, such as lithium, to insert themselves. But it takes force to drive them in, and the more
force the more the loading of the crystal. This loading small ions into a crystal structure is called intercalation.
So
it makes sense that the amount of charging depends on the voltage. But how? I haven't been able to find any data on this, so we
decided to do the experiment.
Method
1. I used a 60 mAH ultra-thin lithium polymer battery. This small capacity
should be able to be charged and discharged quickly, it won't heat up, and if something bad happens it isn't able to store
pressure, and doesn't have enough energy to do any damage. In addition I can tell if something is going wrong because the
battery will start to puff up.
2. Charging was done with a lab power supply, the voltage was set and the battery
connected without regard for current limit. Typical starting currents were 60 to 100 mA at the higher voltages.
3. The
battery was discharged at 100 mA rate to 2.8 volts termination voltage.
| Summary of tests, see discharge
curves below |
| Charge voltage |
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3.3V |
3.5V |
3.6V |
3.7V |
3.8V |
3.9V |
4.0V |
4.05V |
4.1V |
4.15V |
4.2V |
4.25V* |
4.3V* |
| Capacity mAH |
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0 |
1.8 |
3.1 |
5.3 |
22 |
38 |
44.7 |
51.4 |
55 |
57.8 |
61.6 |
64.5 |
65.4 |
| Percentage of rated capacity |
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0% |
3% |
5.2% |
8.8% |
37% |
63% |
75% |
86% |
92% |
96% |
103% |
108% |
109% |
*Note: Charging above 4.20 volts is bad for the battery cycle life!
Commentary: It looks like the magic number is around 3.8 volts. Below that you don't have significant
charging, above that you do. A lithium ion battery doesn't care if it is never fully charged, so if all you have available is
3.8 volts and you don't mind the loss in capacity you could use the 3.8 volts. Unfortunately, the supply voltage is probably
3.3 volts in this modern digital age, which won't work at all. |
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PowerStream Technology: 140 South Mountainway Drive, Orem
Utah 84058
Phone: 801-764-9060, 9062, 9063, or 9064
 Fax: 801-764-9061 |
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PowerStream Technology
140 South Mountainway
Dr.
Orem, UT 84058
Phone: 801-764-9060, 9062, 9063, or 9064
Fax: 801-764-9061
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Battery charging for lithium ion batteries. |
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