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How to Make a Constant Current Lithium Ion Charger Answered

Hi Guys!

I have a bunch of 18650 lithium ion batteries that can handle a charge rate of 5C. Each one is 3,000 milli-amp hours, and I would like to build a charger for them as my project. I understand the mechanics of charging lithium ion batteries, and that they need to be charged at a constant current up until that current reaches 4.2 volts, than switch over to constant voltage. The constant voltage charging makes up about 20% of the battery capacity, and I am willing to forgo that amount. What I am trying to build is a constant current charger for it that would  charge at a current of 1C, and shut off when it reaches 4.2 volts, and I would like some help as to how I would do this.

I was thinking that I would have some sort of current source that supplied 3 amps and then have a relay that would shut off the current as soon as a voltage detector reached 4.2. I do not know much about current sources, so I am asking here for some help.

Thank You for Your Help!

2 Replies

liquidhandwash (author)2017-03-31

You need a BMS or battery management system, which will ensure all cells are charged correctly, and no cell can be over or under charged.

Im just in the process of building a battery pack for an electric go kart and have used one of these.


There are cheaper versions around, But Im new to this and im not sure how good or bad some of these products are.


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merlinj (author)liquidhandwash2017-04-03

Thank You Liquidhandwash for your reply, but I believe I may have misexplained my project (sorry).

My Project is to build a battery management system for a single cell (1s1p). Because of this I do not have to worry about cell balancing. Interms of over charging and under charging, I will use a lipo alarm to shut read the low voltage (around 3.4v in this case), and once it hits that, a relay hits and the charging shuts off. As for the 4.2 volt over charge I will also have a detector that will through a relay as soon as it reaches that voltage. Because of that, it will not be going through its constant voltage phase, and thus will not charge past 80%, however this will increase the longevity, and allow me to also use only constant current for charging it. I need a way to build that constant current charger.

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