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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?

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  • \$\begingroup\$ The spec is always in series. And yes, the parallel connection makes it think you have fewer, higher-capacity cells. (if it even cares about capacity) \$\endgroup\$ – AaronD Jun 29 '17 at 6:41
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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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The specifications of the chargers are for battery cells in series.

When you connect n batteries in series, then the total voltage between the "-" of the first battery and the "+" of the last battery is equal to n times the voltage of one battery.

On the other hand, when you connect n batteries in parallel the voltage at their pins is equal to that of one battery. They will be like a big battery with a capacity equal to n times the capacity of one cell.

So, it makes sense that the chargers are specified for a maximum number of cells in series, since in that case the charger will have to be able to provide this total voltage that is created by all these batteries in series. Otherwise it will not be able to charge them.

If you connect the charger to batteries connected in parallel, the charger will not know anything. Only thing that will happen, is that it will need a long time to charge all of them, longer than it would need for only one cell. That's because the charging current will have to be distributed to more than one cells.

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