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How to Make a Constant Current Lithium Ion Charger

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!

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.

ww.zeva.com.au/index.php?product=122

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

http://www.ebay.com.au/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xlithuim+BMS.TRS0&_nkw=lithuim+BMS&_sacat=0

merlinj (author)  liquidhandwash1 month ago

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.