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I am currently in search for a battery charging ic that has a variable output voltage (so that I can set it to 3.6V to 3.9V) or a fixed range, similar to what I would like to have.

But why would I want that? Its because most of the charger ics are charging the batteries up to 4.2 which is considered 100% of charge.

This will use every bit, the battery has to offer, but will also damage it in the long run, so I would like to maintain my level of charge between 20 and 80% which is 3.6-3.9V According to this image: From here

enter image description here

I have a hard time finding a suited chip, any advice is greatly appreciated!

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closed as off-topic by PeterJ, Daniel Grillo, Nick Alexeev Feb 22 '15 at 0:15

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  • \$\begingroup\$ The intention has merit BUT that graph has essentially nothing to do with what you said or think it does. That is related to discharge voltage and charge voltage has its own set of rules and reason. Read my answer here CAREFULLY, look at Table 4 (at the end) and the 1st graph. That will tell you what you want to do. Then we can discuss how. Ask again when ready. If you have not read the above and somewhat understood it then there will be no point in proceeding. \$\endgroup\$ – Russell McMahon Feb 20 '15 at 11:49
  • \$\begingroup\$ NB: This is a good question - it just needs some assumptions sorted out. \$\endgroup\$ – Russell McMahon Feb 20 '15 at 11:50
  • \$\begingroup\$ I suggest that you update your question to correct the incorrect assumption re the graph. I note that now 2 mo... er mo... er imb ... er whatever nice folk have voted to close - anonymously of course, as such mo ... er nice folk almost always do. \$\endgroup\$ – Russell McMahon Feb 20 '15 at 15:25
  • \$\begingroup\$ CN3058e (from memory) is a LiFePO4 charger IC. You can almost certainly fool it with any voltage over about 3.6 or 3.7V by dividing Vbat down. Will need to look. BUT as you will want to use CC mode and not use CV tail then not much more than a comparator will be enough. \$\endgroup\$ – Russell McMahon Feb 20 '15 at 15:27
  • \$\begingroup\$ I would like to correct my assumption but i don't really know whats wrong with it. Isn't the Voltage the cell voltage and should be related to the state of charge in the same way for charging and discharging? \$\endgroup\$ – Jonas Eschmann Feb 20 '15 at 16:33
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This is a placeholder for a more detailed answer once the question is sorted out. I'm posting it like this to ensure I'll be able to expand it in due course.

The intention has merit BUT that graph has essentially nothing to do with what you said or think it does. That is related to discharge voltage and charge voltage has its own set of rules and reason. Read my answer here CAREFULLY, look at Table 4 (at the end) and the 1st graph. That will tell you what you want to do. Then we can discuss how. Ask again when ready. If you have not read the above and somewhat understood it then there will be no point in proceeding.

These excellent graphs and tables are from this Battery University page. Read it too. The graph and table 4 are of most use to you.

enter image description here

enter image description here

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  • \$\begingroup\$ At first, hanks for the answer! The page was a great read and I think what I am aiming for is: "Chargers for cellular phones, laptops, tablets and digital cameras bring the Li-ion battery to 4.20V/cell. This allows maximum capacity, because the consumer wants nothing less than optimal runtime. Industry, on the other hand, is more concerned about longevity and may choose lower voltage thresholds." \$\endgroup\$ – Jonas Eschmann Feb 20 '15 at 14:45
  • \$\begingroup\$ Also "In terms of optimal longevity, a voltage limit of 3.92V/cell works best but the capacity would only be about half compared to a 4.20V/cell charge (3.92V/cell is said to eliminate all voltage-related stresses)." Sounds good. My first guess of 3.9 was more or less random, but seems to be scientif ically reasonable :). I am also perfectly okay with only having about half of the capacity available. \$\endgroup\$ – Jonas Eschmann Feb 20 '15 at 14:49
  • \$\begingroup\$ Another question incoming: Why do you only address the upper bounds of the cell voltage, when, according to table 2, level of discharge is similarly important for battery longevity? \$\endgroup\$ – Jonas Eschmann Feb 20 '15 at 15:11

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