Why would 4 batteries in a device not drain equally? I noticed a motion sensor light was weak so I assumed the batteries were dying and removed them. To make sure I was right, I tested the 4 D alkaline batteries. One was completely dead. One was on the brink (of death). And the other two were about 80% charged. This was the first and only time I'd ever changed the batteries for this light. So this has happened 1 out of 1 times, and NOT happened 0 out of 0 times, to answer a commenter's question.
I don't remember the order in which I removed them, and I can't tell where the anode and cathode are - see for yourself: 
I have noticed this with another electronic (a motion light) that uses 3 [I use rechargeable] AA batteries; they slide into the tube and touch end-to-end. The first one I pull out- at the anode end- is always very dead. The next one is pretty dead, and the last one (at the cathode end) in the rod-like light is always practically fully charged. This has happened EVERY time I've replaced the batteries in this device.

Why are these devices not draining the batteries equally? Is it because they are inferior products?
The two items are motion sensor lights; they have a PIR (infrared, I believe) sensor that detects movement, then they turn on. They turn off after a certain amount of time (that I swear varies but never measured).
The AA rechargeables are eneloop NiMH 1900mAh; all go into the light fully charged. The D batteries were all brand new, unused alkaline, Amazon Basics.
 A: If the batteries are in series, they should experience the same current. However, there are two factors that will make it appear as though some batteries drain faster than others:


*

*temperature. In the case where you have batteries that drain faster at one end than the other, I suspect there may be a thermal gradient. Batteries have a self-discharge that is a function of temperature - the higher the temperature, the more the battery will lose charge

*initial charge level. Not all batteries will have identical amounts of chemicals inside, and so it is possible that some batteries will "run out" faster than others. When one battery has run out, the other batteries will continue to push current through; this current will lead to destructive electrolysis and the result is that when you measure the voltage on the dead battery, it will be significantly lower than the voltage on the others. This is also why you should replace batteries before they are completely dead: you risk getting leakage from this process.


More information at battery university
