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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 if you are barely above the chemistry voltage.
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 |
|
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 4.2V capacity |
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0% |
2.9% |
5.0% |
8.6% |
36% |
62% |
73% |
83% |
89% |
94% |
100% |
105% |
106% |
| 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 Technologies: 1163 S. 1680 West. Orem Utah
84058
Phone: 801-764-9060
Fax: 801-764-9061
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Battery charging for lithium ion batteries. |
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