Charging DeWalt 26650 cells in series

I'm in the process of building myself a bike light powered by either 4 DeWalt 26650 cells in series, or 4 of these in series. I don't care about or need rapid charging. Overnight charging is fine since I won't be using the bike until the next night once I start charging the batteries. I was thinking of a simple constant voltage (3.65V x 4 = 14.6V)/constant current circuit, perhaps with a current limit of around 0.2C. I also plan to have an overdischarge cutoff in place which shuts the light down when the battery pack reaches maybe 11 volts. Under this scenario would any kind of cell balancing be necessary? If so, would something simple suffice, such as a circuit which shunts part of the charge current when the cell reaches 3.65V?

yes ballancing is a must, a pcb is a must for li ion in series, for charge and discharge.
the same site you linked, sells pcb for various cells, find the one you need current\feature wise, and use it,
\do not charge discharge li ion cells in series with no pcb.

Keep in mind that these are LiFePO4 cells, not Li-ion. The only PCB I found which is applicable for my use is this one but is doesn't do balancing. Still, it provides overcharge/overdischarge/overcurrent protection, and at $6.95 it's cheap enough that it doesn't make sense to make my own protection circuit.

still all the rules apply as for li ion. the pcb does ballancing, it doesn't have equilibrium function, which is not the same, their chart is a bit missleading, they all charge batterys more or less equaly, without pcb you will not charge cells like that evenly in series, equilibrium, otoh, after all cells are charged, will discharge cells to equal their soc to cell with lowest soc, turning energy into heat, so those pcb with equilibrium, can't be sealed or placed next to cells. you can call them and ask them about how their pcbs work, i did, and that is what they told me.
also don't rely on pcb to do overcharge priotection, it is there as a last resort, charger should stop charging well before pcb protection kicks in, if not look for another charger.

alpg88 said:

still all the rules apply as for li ion. the pcb does ballancing, it doesn't have equilibrium function, which is not the same, their chart is a bit missleading, they all charge batterys more or less equaly, without pcb you will not charge cells like that evenly in series, equilibrium, otoh, after all cells are charged, will discharge cells to equal their soc to cell with lowest soc, turning energy into heat, so those pcb with equilibrium, can't be sealed or placed next to cells. you can call them and ask them about how their pcbs work, i did, and that is what they told me.also don't rely on pcb to do overcharge priotection, it is there as a last resort, charger should stop charging well before pcb protection kicks in, if not look for another charger.

Click to expand...

Yeah, I was reading the spec sheet and it seems the board protects each cell individually from overcharge or overdischarge. And no, I'm not relying solely on the board for overcharge protection. My plan is to build a simple CV/CC circuit which is limited to 14.6 V maximum. The PCB protection would only kick in as a last resort if my charge circuit failed. Naturally, I'll test everything multiple times outside the headlight to make sure it works.

jtr1962 said:

Keep in mind that these are LiFePO4 cells, not Li-ion. The only PCB I found which is applicable for my use is this one but is doesn't do balancing. Still, it provides overcharge/overdischarge/overcurrent protection, and at $6.95 it's cheap enough that it doesn't make sense to make my own protection circuit.

Click to expand...

LiFePO4 cells are indeed lithium-ion cells. They just use a different cathode than "conventional" lithium-ion cells, and therefore have a different cell voltage.

The world of lithium-ion is going to get more confusing. We typically think of lithium-ion cells cycling between 3.0 and 4.2 V. However, several new lithium-ion cells need to be charged to 4.3 V and discharged to 2.5 V to get full capacity. In the future, the voltage ranges are going to get ever more diverse as new lithium-ion chemistries are introduced to push the capacity of the cells higher.

Cheers,
BG