Can you connect in series 2 unequally charged batteries?

Question

So I have 4 batteries. 2 of them are charged (about 70%) and 2 about 10%. I have a flashlight (which uses 2 batteries in series) which I would like to power at small current and voltage because the light produced is too powerful. Would combining a charged battery and discharged battery be a good idea? If not, what can I do? I thought of placing a piece of iron where the discharged battery would be so it conducts the current from the first one. Would that work?
Note that they are single use batteries.

Thanks in advance!

Answer 1

This sounds like a bad idea. Besides the fact that it says not to do this on the battery package, here is the reason:

^{simulate this circuit – Schematic created using CircuitLab}

As you can see, the batteries that are used have a lower cell voltage, but also a higher internal resistance. They aren't contributing much to the power delivery, and they will dissipate quite a lot in heat! This will eventually rupture the battery.

Your bypass might work if the circuit is capable of using a lower voltage, and the batteries you use are capable of delivering a higher current to compensate for the reduced voltage.

Answer 2

Would be putting a 70% cell in series with a 10% cell be a good idea?

No. Once they are down to 60% and 0%, the battery at the end of its life will have its terminal vpltage collapse, and get reverse biassed. Not that this matters much to a single use battery, you were going to throw it away anyway, but it won't light the torch.

Two discharged batteries in series, or one well charged cell in series with a conductor would work better.

Have you considered using a bulb with a suitable design voltage for your batteries?