Should we avoid battery rapid charging? Yes, but not for what you think...

pagheca

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As a result of an earlier discussion, I have been trying to figure out what the problem of rapid charging with lithium-ion batteries (LIBs) really is, and I think I have come with a good explanation. I write it here in case someone is interested. If no one is it doesn't matter: it was useful for me to fully understand how it works :)

Actually the problem of fast charging applies to all batteries, but not as I thought.

Let's start here: every Lithium battery has an anode (-) made of a graphite structure that traps lithium atoms between two layers. Why Lithium? Mainly because Lithium has a very low ionization energy (see, e.g., https://en.wikipedia.org/wiki/Ionization_energies_of_the_elements_(data_page). Therefore, it is easy to "snatch" an electron from it.

The battery also has a cathode (+), usually consisting of Cobalt, that is positively charged, and therefore good at attracting the electrons.

As soon as you close the circuit, the potential difference between the anode and cathode causes the electron to travel, through the circuit, from the anode to the cathode. The problem is that as soon as the electrons begin to accumulate on the cathode, a negative charge would form on it (while a positive charge forms on the anode), which the electrons do not like since they repel each other, and so the flow of electrons (current) would stop.

To avoid this, you have to neutralize that charge. To do this, you make the positive (because they have lost an electron) Lithium ions transit, in the same way, from the anode to the cathode through an electrolyte inside the battery, attracted by the increasing negative charge as the electrons reach it passing through the circuit. And this is where the problem of fast charging arises.

The first problem is that the electrolyte through which the ions pass to get from the anode to the cathode during use (or vice versa during charging), "crystallizes" over time, so the ions have more difficulty moving. In addition, crystallization involves lithium, which is then less available to release electrons, reducing the total capacity of the battery. The main cause of the phenomenon (there are other reassons) is that at the interface between the electrolyte and cathode some of the electrons react with the electrolyte and cobalt, forming a solid compound called SEI (Solid Electrolyte Interface) and thus causing a reduction in the amount of Lithium and electrolyte available in the battery...hence its lower capacity! That's it!

Now, like almost all reactions, crystallization happens more rapidly if the battery is hot, and this can happen

1) sure, because are fast charging, which heats up the battery in itself because of all those ions moving around in the electrolyte,
2) but more in general, because the battery is left in a hot environment or exposed to the sun.

So, you absolutely have to limit the time that the battery is exposed to high temperatures. Not only by avoiding rapid charging, but more generally by avoiding as much as you can heating LIB batteries during charging, discharging and storage.

In general the charging current should be so low that the dissipation keep the battery internal temperature more or less at ambient temperature, but there is no real gain to reduce the current any more. And in any case, the same happens if we make an heavy use (high power) of the battery.

For storage at low temperatures the problem is different (may require another post): in this case the battery is not really damaged (unless the temperature is so low that it modify the battery structure), but it loses charge and is no more able to be safely recharged or to be used.

I believe this is also why solid-state batteries

1) can be charged much more quickly since crystallization is not a big problem and
2) promise to last longer than Lithium Ion batteries

because a solid cannot easily crystallize as there is no liquid moving inside. In addition, the ionization energy of Lithium is not the lowest ever, so there are other elements that might be good candidates, but they are more reactive and heavier, and thus tend to have their own downside.

I hope I have explained myself and made no (big) mistakes, Should anyone find any, I will of course be glad to amend it.
 
I'm finally buying into it. This plus fabbrisd's posts on it made me open my mind a bit more.

I charged my Level.2 at 0.5 A the other day and felt a bit silly, but if one has the charger option, the time and it extends the life of the pack, why not?

I think as we get into more certified bikes and batteries, the manufacturers may go more toward proprietary plugs, thus making this more difficult. OEM chargers tend to have just one charge voltage and current output. The "better" ones are higher current and the cheaper ones are lower. Getting into using lower or higher charge currents is niche stuff. It would be nice if they had dual current charging. Fast might be 4 or 8 A and Slow might be 0.5 or 1 A. I'd pay an extra $20 for a charger like that!

As a matter of fact, I'm going to recommend that my company provides a slow charge option into our next generation battery charger now. These battery packs are EXPENSIVE! Here's our current 18 V pack: LINK Going strictly by the amount of energy stored, eBike batteries are quite inexpensive, by comparison.
 
The eBike space has been hovered around - but not committed to - battery temp sensing data to run back to the controller... that really is the next step forward - it will get there...
 
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