The automotive lamp wired in series is still a good feature with any type of battery charger or power supply, when wired and sized appropriately it can act as an indicator, and a safety feature (current regulator) it acts like a resistor in the circuit limiting current, with a visual indication of its voltage drop. In fact it works better than a normal resistor will in this case because the lamps filament varies with heat, the brighter the lamp the filament has a higher resistance.
Actually using a lamp acts as series load. Since it will limit current to non-destructive level, light bulb which will vary in its resistance by 1:10 from cold to hot resistance of the filiment. (as opposed to a fixed resistor), the lamps resistance can vary with its current through it. being in series it has the same current as your electrolytic cell draws, The light bulb, provides a nice visual indication of the current drawn by the circuit and of your electrolytic cell, it can be a visual indication of the cells resistance and the current it is drawing from the power supply, the lamp acts as an indicator of what is happening inside the cell.
How this works is an incandescent light bulb filament has a low resistance when cold, acting as a low ohm resistance (where almost all of the voltage drop will be across your electrolytic cell). When a higher current begins to flow and filament heats up, and resistance rises (the lamp becomes a larger resistor), if the electrolytic cell was to short the lamp will rapidly to act as a sort of current safety valve, so the lamp somewhat acts as a variable resistor, as the cells resistance changes so does the current through the circuit (the lamp wires and your electrolytic cell), and thus the lamps brightness, if the cell becomes a dead short the lamp will glow full bright, and limiting the current going through your cell (which can be determined by the wattage of the lamp you put into your circuit) so essentially you can choose a lamp to limit a maximum amperage or current you wish your cell to draw from your power supply (a good way to keep a small cell from overheating itself , limiting the cells watts (heat) of the cell.
In a series circuit, the amperage is the same, the voltage divides.
This means the lamp in series with your electrolytic cell, the current or amperage flowing through your lamp and cell will be the same, but with the lamp in series the voltage is divided between the lamp and your cell, with low current through the circuit the cold lamps filament has a lower resistance, this means it will not drop much of the voltage across it of the total circuit, and most of the voltage will be dropped across your electrolytic cell (determined also by the cells resistance which varies during operation or conditions), your electrolytic cell is also acting like a varying resistance as its conditions change.
As your the electrolytic cells resistance becomes lower, trying to pull more current the power supply the current flow through the series lamp also increases, making it brighter, and the filament becomes hotter, thus the lamp becomes more of a resistance with a greater voltage drop across it, which limits the voltage drop across the electrodes of the electrolytic cell (and its current flow), if the electrolytic cell becomes shorted out, or a dead short, all of the current is drawn across the lamps filament (heating the filament), the lamps hot filament resistance now will limit the current draw from the power supply for the whole circuit (to the lamps wattage, and the circuits maximum amperage) with the lamp taking all of the voltage of the circuit (the lamp dropping all of the 12 volts in this case), and the electrolytic cell not having any voltage drop across it.
As the cells resistance changes so does the lamps brightness, if you do not have much current flowing through your cell (less metal in the electrolytic solution) the lamp is dim, as the cell pulls more current the lamp brightness increases, and if the electrolytic cell, or the electrodes or circuit wiring shorts out, or draws excessive current the energy is absorbed by the lamp and doesn't cause further damage to the circuit or your cell or blow expensive fuses.
All the while giving you a visual indication of what is happening, it is kind of like using an ammeter in the circuit where you can see how much current is flowing through the circuit.
In this case it is a direct indication of the current flow through your circuit (or your electrolytic cells resistance).
Higher wattage lamp have lower resistance than lower wattage lamps!
To determine what wattage bulb to use you can use Ohms law.
Watts/Volts = Amps
This was a trick used in trouble shooting electronic circuits, I have used this trick in repairing TV's and many other electric and electronic circuits, it can be used for circuits like in troubleshooting a short in a vehicles wiring (by wiring in a small lamp in the place of the vehicles fuse in its fuse box) where if the circuit had a short or a problem (the lamp glows bright as long as the circuit is shorted, removing the short the lamp goes out, or if the circuit is normally powering its load the lamp is dim because of the normal resistance of the normal load of that circuit), with the lamp in series you could power up the circuit to find the problem without burning up the circuit, because the lamp can act as a current limiter of the circuit , or can help to keep from putting an excessive load on the power source, the lamp was also a visual indication of a problem, or what is happening in the circuit, and also as protection of the circuit.
This can also save money on fuses when trying to troubleshoot problems with a vehicles circuit, or some other circuit, where you are not blowing handfuls of fuses to find a simple short in the vehicles wiring, also as a visual indicator I can be wiggling wires a under the hood, or disconnecting connections, looking for the lamp to go out when I remove the short from the circuit...
P.S. I have used a buzzer for similar type of troubleshooting also where I can hear if I found a problem using the buzzer as an aid to hear if I found a problem...
There are many other tricks like this I use when troubleshooting.
Sometimes a lamp will work where a voltmeter will not to find a problem.
A lamp also work as a load, your voltmeter will not load a circuit, so you can have a bad switch in a circuit and your voltmeter will read voltage, but when you load the circuit with a lamp you will see the bad switch would not pass current through the bad switch's resistance...
