Electronic – What does it mean when a charger supports n cells

battery-chargingchargercharginglithium ion

I want to preface this by saying that I'm a beginner and this might be a very silly question but I've spent a while looking at datasheets and online forums and I still don't get this.

I'm trying to select a charger for a project where I'll be charging a Li-ion battery pack with a dynamo hub installed on a bike. Most of the charger ICs I've come across say that they support n cells in series where 1 <= n and at most I've seen 8 or 6 cells being supported by the charger IC.

The typical circuits in the datasheets for these charger ICs show n cells connected in series. My question is, if a charger supports 2 cells does that mean it can support both 2 cells in series and alternatively 2 cells in parallel? I'm confused because if you connect two cells in parallel the charger would see about 3.6V across them and it sees 7.2V if you have 2 in series. In the parallel case, does the charger know that its actually two cells or does it think its one cell with twice the capacity of a single cell?

Best Answer

Check out how many terminals your charger has. Assuming connecting cells in series, a 2-cell charger should have three leads, usually labelled (-), (+3.6) and (+7.2) or something similar. The (+3.6) lead goes in middle point between the cells and is used for balancing. Li-ion cells cannot self-balance in series, so it's pretty much mandatory.

If you're connecting cells in parallel, a single-cell charged will do just fine, since parallel cells do self-balance by passing current from more charged cells to less charged ones. There are two things to remember when connecting cells in parallel:

  • Cells must be charged to the same voltage before you connect them together. A voltage difference of 0.1V or less guarantees that the balancing current will stay below 0.5C which is safe for most cells.
  • Connecting N cells in parallel will result in N times the capacity rating (mA*h), but the C rating will be lower. Unless you use cells with closely matched internal resistance, drawing high current may result in only a few cells with the lowest resistance to be loaded, which may result in a failure.
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