Good Toyota NiMh battery repair video along with simple theory explanations

Don't know if this has been shared here before but quite a good Gen3 Prius HV battery repair video along with basic theory of operations and repair explanations

Not to mention the nicely prototyped auto charge/discharge cycling jig haha

 

haha wasn't my work. Just found the video of this guy's work. Certainly a newer battery would have been preferably for longevity. I think old modules and mixing modules naturally results in poor longevity.

Speaking of batteries, my other car is a Tesla Model S with lithium ion batteries. Lithium ion chemistry is more volatile than Prius's NiMH and mixing battery modules almost never work very long (even within just a few charge sessions).

Whenever Tesla cars have replaced a single "module", it has failed soon after even with module voltages balanced manually along with similar capacity and charge/discharge rate. Reason is the tight margins of the more volatile lithium ion chemistry. BMS is watching for like < 100mV deviation amongst 96x 3-4V (depends on SOC) bricks (74 18650 cell in parallel = 1 brick, 6 brick in series = 1 module, module is a single physical unit like Prius modules, 16 modules in serials = 1 pack). What happens with "mixing" modules that weren't born and lived together is it will go above this difference at some point in operation. Discharge, charge at slight different rate, different behavior over temperature range etc. And potential differences from chemistry tweaks throughout manufacturing lifecycle. So far, the only repair solution for modules that failed due to lithium ion cells is to just reduce the number of modules in serials from 16-14 and yield a lower voltage, lower capacity car (Tesla made a cut down version so software works)

So yeah, whatever kind of battery chemistry you use, its best to have lower age and use battery and don't mix

 

Yeah the hidden secret in EVs is battery modules basically born and only live with each other. And with batteries at say 40% of EVs cost, EVs are basically disposable cars once the battery fail. Probably the reason Toyota gambled awhile back on bypassing lithium chemistry and mostly invested in solid state batteries at volume and cost manufacturing with Idemitsu last 20 years.

Tesla Model S do have battery failures but majority are actually not the cells. Its the voltage safety monitoring logic of a large pack that lives externally to the car and breathes in wet air when equalizing pressure of a large battery volume. These logic gets moisture corroded and safety logic fails and car shuts down. Tesla probably did the worst moisture handling of all EV manufacturers on nearly every part of the car. You should see where the windshield water run off dumps on haha.

 

There is a Prius battery rebuild company near me that claims they warehouse a bunch of modules. Deep cycle(s) them, characterizes them and combine matching set for longevity. Don't know the real longivity stats.

After studying Tesla Lithium Ion battery for quite awhile (and repaired a couple) There are some key fundamental factor to consider on longevity of used rechargeable batteries (of any chemistry) These fundamentals suggest there is no (or not much) "magic"

Rechargeable battery chemistry involve ionic chemical reactions which creates electron movement potential in forward and reverse directions. Discharge is one direction and charge is the other. In either direction, it is a chemical reaction of some forward and reverse chemistry equation.

Not all reactions are exactly what was intended. Just trillions of molecules doing their thing with statistical results. In basic chemistry, there is always certain % of parasitic (unintended) reactions where undesired products are made. It seems the natural result is constant reduction of original capacity of useful rechargeable ion species.

Creating a frequent forward and reverse attractive force to cause atomic scale movement results in molecules building a bridge to close the gap between anode cathode. This is how all rechargeable batteries of any chemistry shorts out. From early lead-acid chemistry to the latest lithium ion chemistry (crawling bridging fingers in lithium ion are call dendrites) For super high energy densities batteries, anode and cathode surfaces are naturally closer together.

Also the attraction potential is always present regardless of use so the above activities happen even when the battery is not charged or discharged. It still a bag of chemistry sitting in close proximity to each other. Therefore, as history has always shown, rechargeable battery just sitting idle will degrade. I doubt Toyota makes Prius batteries far in advance of sale and just leave on shelf for say 10 years. Its a lot of capital and the asset will degrade.

I'm personally somewhat skeptical of used battery longevity based on above reasons regardless of reconditioning. Ion species loss and shorting/bridging structures can't be reversed.

But Prius deep cycling and balancing probably does make some sense because Prius batteries have no active balancing logic during use that I'm aware of. NiMH chemistry is probably safe enough to just keep bleeding heat on the early charged up modules to "balance". Lithium Ion often do the lowest cost top active balancing : BMS opens a bleed path (tiny) on modules first to charge up nearly full waiting for the rest to catch up. This only happens when nearly fully charged. Lithium Ion chemistry is more volatile and therefore dangerous near 100% SOC.