If I’m packing an empty box with smaller cubes, I’ll start on one side and methodically put the smaller cubes in. Halfway through, one side of the box is fully packed, the other side is empty.
If I’m pumping air into a vacuum, I put the nozzle on the valve and let loose. Gradually, and mostly evenly, the air pressure rises through the entire container.
If I’m growing a bacterial sample, I’ll seed the petri dish. The bacteria spread from the point of contact, eventually covering the entire dish.
Which of these, if any, is the best metaphor for charging a battery? That is, if I stopped charging the battery halfway through and sampled various points in the battery’s volume, what would the charge’s distribution be like?
This is probably a very stupid question. Batteries continue to confuse me.
It’s going to charge evenly across throughout. I think that’s partially just because the electricity is traveling through it, but also because the engineers design it to do that with lots of plates or a mesh that makes for a lot of surface area for the electrolyte to be in contact with.
Edit: trying to find some cites to back up what I said, and I realize that I think you’ll need to clarify your question a bit. Batteries are (usually) made up of multiple cells. I believe an individual cell will charge up, more or less evenly (excluding things like surface charge or chemical stratification across the cell), however, charging the individual cells, within a battery evenly, is a whole other thing.
But I still think the short answer to your question is that they’ll charge and discharge evenly.
Electrochemical reactions are complex, and require a good understanding of chemistry to fully understand them. Suffice to say, an electrochemical battery stores energy in electric fields. So fundamentally, they’re no different from capacitors. Only the “mechanics” are different between the two.
The battery has plates separated by the chemical compound. Electrons flow all over the plates. They interact with the compound where there is the opportunity. So the charging is happening all over the plate areas at the same time and approximately the same rate. Over time there are fewer opportunities for the electrons to interact with the compound. Charging will stop. Variations in the compound and plate thickness and purity will cause slight variation in areas.
Multiple batteries charged or discharged in series or parallel act in a similar way.
A bad section of a battery can cause more current to pass across a certain area. Wasting power, increasing charging time. Often causing excess heat. Failure will happen sooner.
I think that the batteries in EVs are a lot more sophisticated than just hooking a bunch of cells together in series, parallel, or a combination of those. The control electronics certainly could selectively draw down or recharge some subset of the cells without affecting the other (yet). Whether they actually do that, I don’t know.
The air-into-a-vacuum is probably the closest of your options.
In a lithium battery, electrons are pushed into the cell from the cathode when charging. The electrons attach to lithium molecules, which are free to move around the cell’s lattice structure. Since the lithium ions are now negatively charged, they’re pushed away from the cathode and towards the anode, seeking equilibrium.
The “filling a box” analogy does not work. The cells are conneted essentially in parallel. (Or at least, banks of them are). There’s essentially no difference in resistance between the first and last cells in the row, difference in resistance due to the length of wires is essentially nothing.
Battery management software is important. One of Tesla’s higher tech adjustments is to monitor each bank of batteries and charge them according to how well they take a charge, so as batteries begin to deteriorate (slightly) you are not overloading one bank by feeding it the same current as a better bank. (and apparently the same with drawing load)
There’s some inhomogeneity due to current density gradients. The graphite undergoes a phase transition, but not everywhere. Then there’s plating and dendrite formation. But I’d still say it’s more homogeneous than box-stacking.
It’s a question that has been the basis of important research. If it is stupid, kindly send us more stupid questions.
But maybe take some of the responses from non-chemists with a grain of salt.
I don’t think many chemists would describe chemical potential as “energy stored in electric fields”. I certainly do not. But now I’m wondering what field I’m supposed to be embiggening when I make a cell with a larger anode.
The OP asks how the lithium is distributed. Capacitors don’t answer that.
Sure, batteries and capacitors are both, ultimately, all electromagnetic. But then, everything in human experience except for gravity is ultimately all electromagnetic. I’m not sure how enlightening that is.