Battery question

[curtasian]

I have two 18650 batteries in series to power the lights on my bike. When I get home and put them on the charger usually only one battery will show that it's charging. I'm confused as to why this is. I could understand if they were in parallel and one had slightly more capacity but if they are in series the current has to flow through both of them. Can anyone explain this?

For reference sake one is a new battery (this is the one that shows charging) and the other is an older battery. Also if I discharge them enough the charger will indicate that they are both charging so I know the second will discharge.

[Franklin97355]

Have you checked the voltage when they are put on the charger? Using unmatched LiPos together is a bad idea for this reason. If the batteries are not matched and balanced during charging they do not provide power is a predictable way.

[adafruit_support_bill]

Connected in parallel, the current through both of them will be the same. But that doesn't mean that both will be contributing equally to the total power. With lithium cells, it is important to keep the cells 'balanced'. If one cell has significantly different charge/discharge characteristics, this may be impossible. Charge/discharge characteristics do change over time with use. I suspect that is the case here.

[curtasian]

I know that charging unmatched LiPos in circuit is not a good idea but these get charged individually. I take them out of the battery case and put them in a charger. I have measured them before but I can't remember what the values were. When I get home tonight I'll measure them before and after I charge them.

[curtasian]

I was thinking that in a series circuit if I draw 1 Amp then 1 Amp flows through every component in the circuit.
Wouldn't that mean that both batteries have put out 1 Amp.

Where I'm confused is, if both batteries show full charge in the charger, why after using at least 1/4 of their rated capacity does one battery not appear to be discharged? (It is always the newer battery that appears discharged to make it even more confusing) I would expect them both to charge but for different lengths of time - not one charge and the other not.

[adafruit_support_bill]

I was thinking that in a series circuit if I draw 1 Amp then 1 Amp flows through every component in the circuit.
Wouldn't that mean that both batteries have put out 1 Amp.

No. In extreme cases a battery in poor shape can become more of a load than a power source.

if both batteries show full charge in the charger, why after using at least 1/4 of their rated capacity does one battery not appear to be discharged? (It is always the newer battery that appears discharged to make it even more confusing)

The newer battery is the one doing all the work. It most likely has a lower internal resistance and is generally in better health than your old cell.

[curtasian]

That makes sense, thank you.