Battery charging voltage additive?

Question

I've got a 18.5 volt battery made up of 5 3.7 volt cells (Li-Ion). For charging the datasheet for the single cells says the battery requires .5 volts more, to counteract the internal resistance, I guess.

My question: Since the nominal voltage is additive per cell, is the additional voltage as well? As in: do I have to charge at 21 (18.5 + 0.5*5) or at 19 (18.5+0.5) Volts?

I believe it is the the former, since in a serial circuit resistance is additive as well, but since I don't want to make a mistake, I decided to simply ask.

https://voltaplex.com/media/whitepapers/specification-sheet/Sony_V3_Specification_Sheet.pdf

Answer 1

Consider that the cells don't know they're in a pack. Your responsibility is to each cell. Each cell requires 4.2V to charge effectively, so that's what you have to provide.

The simplest way to do that is to charge at up to 21V, and hope that the voltage is evenly distributed between the cells. Generally it will be, unless one of the cells is significantly weaker than the others, or damaged. If so, it may exhibit an internal resistance or cell voltage significantly different to the other cells, and therefore see a different voltage to the other cells.

Often these non-ideal effects are compensated for by individual cell electronics, but if you don't have that luxury then you can still provide some protection by simply setting an overall current limit. Without the current limit, you risk destroying cells or even starting a fire.

A sensible current limit for Lithium Ion cells is generally "0.5C - 1.0C", which means half the one-hour capacity to one times the one-hour capacity. Your cells are 2.15Ah, so 1A is a safe current limit, and you can go up to 2.15A if you need to charge quicker.

Answer 2

Since the nominal voltage is additive per cell, is the additional voltage as well? As in: do I have to charge at 21 (18.5 + 0.5*5) or at 19 (18.5+0.5) Volts?

Yes, the 'additional voltage' is additive, but there really isn't additional voltage. The nominal voltage of a lithium ion battery is 4.2V, and that voltage is multiplied by the amount of cells you have in series. 5 cells would be 21V. Lithium ion cells can actually be charged by a lab power supply, you put the supply on constant current, then on constant voltage (which would be the max voltage for the cells in series). (Which is a really fun educational activity when done properly and safely). The curve looks like this:

Source:https://www.electronics-lab.com/using-bench-power-supply-charge-lithium-ion-batteries/

Answer 3

The 100mA cut-off current in the datasheet you provided is not a recommendation, it's an obligation. Not respecting this could lead the cells to overheat and/or explode. The proper way to charging Lithium elements is to use a charger compatible with that particular technology, and to follow the cell's datasheet for the charging current and end of charge condition.

As Heath Raftery said, you are responsible for balancing the charge in between the cells. Circuits exist, they are called "cell balancers". in case any one cell charges faster than the others, this circuit will ensure the cell never goes beyond the 4.2V limit. Even if each cell had its own BMS (and I don't think yours does), you shouldn't rely on the BMS to properly charge the cells.

Charge management recommendations by TI: http://www.ti.com/lit/an/slyt334/slyt334.pdf