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Minky

Is there any effect to moving batteries around a circuit?

Imagine a really simple circuit 4 AAs then 2 3v LEDs then back to batteries. All in series.
Then imagine 2 AAs then a 3v LED then 2AAs then a 3v LED then back to original 2AAs. All in series.

Is there any difference between the two circuits?
What if one of the LEDs in the circuits were a motor?

The reason I'm thinking on this is for potentially adding LEDs to springers with limited shell space for battery packs and other potential projects.
OldNoob

The position of the battery pack is irrelevant to the function unless you move it one side of an on/off switch. Your model proposes splitting the cell count, which will change the circuit, current flows from high to low, so led's nearest the larger pack will glow brighter than ones nearer the smaller one.
If you are adding LEDs in quantities of less than 5 or so then coin cell holders are much better space wise, easily fitting in a grip or under a jam door.
SSGT

Splitting the pack in two around the circuit actually shouldn't matter. In it's most basic form you're still putting the same potential across each LED and the current flowing through the circuit won't change either (it also shouldn't make a difference whether you use a single larger resistor for all the LEDs or multiple smaller ones scattered around the circuit). If things are in series you can pretty much re-arrange them to your heart's content (this is effectively part of Thevenin's theorem, the principle you use to reduce complicated circuits down to something easier to analyse, which states that any combination of batteries and resistances with two terminals can be replaced by a single voltage source and a single series resistor - voltage sources and resistances in parallel can get complicated but voltage sources/resistances purely in series simply add together). Think of the batteries (in this case each pair of AA cells) adding voltage and the LEDs/resistor subtracting/dropping/dissipating voltage. Instead of raising the voltage in one go and then dropping it across everything else in one go you're adding a bit, then dropping a bit then adding a bit more then dropping a bit more (again, you could take that back to the pumped water analogy - rather putting the pumps next to each other to double the pressure, you raise the pressure through one, drop it through a turbine/restriction, raise it again with another pump, and drop it again through another turbine/restriction). The total in/out is the same so the circuit should behave pretty much the same. The same should apply to motors although for our use you want each motor to be in parallel with the other motors and parallel to the LED circuit (or completely separate if you're running alkalines for the LEDs). You shouldn't be running the motors in series.

I'd avoid doing it if possible though as it'll make things harder to think through and will make adding things in the future (especially adding things in parallel) a nightmare. In the above example you could instead run the LEDs in parallel, and do away with two of the AA cells completely (the run-time of the LEDs would be reduced but AAs, or even AAAs, should have plenty of capacity to run a fair few LEDs for a long time anyway). Alternatively, as Oldnoob mentioned, you could use one or more 3V button/coin cells or, if you happen to have them anyway, a single rechargeable 14500 size lithium cell. Either will take up less space than 2-4AA cells and should have plenty of capacity to run a few LEDs for a pretty long time.
Minky

Thanks both. Bearing in mind this is entirely theoretical and I'm not building anything with this set up at the mo (just speccing future builds)....

I know that mixing voltages is a bad idea as ON and boff took me through how effectively the you would end up attempting to charge the lower output cell (probably damaging it if not made for it) and I surmise that is why it is a bad idea to mix ages/used batteries as one will be further along it's discharge curve than the others and so be lower output. And I'm thinking this is why you shouldn't mix battery chemistries as they will have different discharge curves so even though they may start at similar output they would likely differ over time so, again, you end up with differing output voltages as above.

So.. With all this in mind, can you mix different sizes of batteries with the same output and chemistry? So put different sizes of 1.5v alkalines in a circuit without issues? Or will they have differing output voltages over time as well?
OldNoob

The voltage will sag quicker on the smaller cells.
Minky

Yeah,  I thought that'd be the case. Thanks ON
SSGT

OldNoob wrote:
The voltage will sag quicker on the smaller cells.


Basically this.

Different sizes (i.e. AA and C or 14500 and 18650 etc.) will have different capacities. You can put different sizes/capacities in parallel (although this isn't ideal) but you shouldn't put different sizes/capacities in series for largely the same reason you shouldn't mix chemistries or voltages (although chemistries and voltages shouldn't be mixed in parallel either, including mixing otherwise identical cells at different levels of charge).

If you put a 100mAh cell in series with a 200mAh cell to make a battery, the 100mAh cell will be completely flat when the 200mAh cell is still at half capacity. If you keep on discharging the battery until the 200mAh cell is also flat then the 100mAh cell will likely have been reverse-charged (i.e. forcibly charged the wrong way around) which, in the case of alkalines, would probably result in it leaking. Do this with lithium chemistries and best-case scenario is that you destroy the cell/pack.

If you didn't keep on discharging the battery after the first cell discharged then you'd technically be fine but at that point you might as well have used two cells of the same size/capacity anyway as the additional capacity and/or max discharge rate of the larger cell is being wasted - you'll always be limited by the smallest series cell.

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