Possible modification to HV battery

You're on track with "low memory problem" aka: low capacity. Toyota design battery packs to function normally down to 50% capacity. Once you you get that low you can experience that sudden drop on the meter. Key to cycling the pack through three increasingly deeper discharges and recharge is being able to monitor the voltage draw down of each of the 28 modules so you can find a bad one before it goes bad.

 

Basically, you want to get as much data as possible from each of the 28 modules so you can rebuild a pack that will last... Here's what my spreadsheet data looks like on one of my packs:

 

For starters, trickle charging doesn't do anything to address battery memory / reconditioning lost capacity needs. You need deep discharging and recharging. Here's the exact science that explains what that does:
BU-807: How to Restore Nickel-based Batteries – Battery University

Second, what are the specs of the charger you're using? To trickle charge a NiMH high voltage battery pack you need high voltage above or equal to what the pack produces at full capacity and if voltage is stuck on weakest voltage module number then your charger is not strong enough or not charged long enough.

The Prolong charger uses the NiMH charging algorithm under 1 amp loads (0.03 amps per module) that's modified slightly based on sensor feedback (still need to confirm) and this charge once it has the pack up into the 240v - 245v range / 8.6v per module and higher the balancing stage begins and fully charged modules will dissipate excess charge through heat while the modules not fully charged catch up so the whole packs has 28 modules that are all the same voltage / balanced once you're done.

 

Hybrid Automotive used to recommend going all the way down to 0.6 per module on 3rd and final discharge but that's alot of time and weaker modules were failing that way so now they only go down to 3 volts on deepest discharge:

 

Raphael,

Toyota uses a band of ~43% to ~65%. This is to provide a long useful life.

Think of it like this.

An NiMH battery has a finite number of uses or full cycles available before it's just worn out. (we're assuming no battery maintenance here) If you were to take a brand new Prius HV battery and discharge it completely to 0%. Then charge it to 100%. That is one full cycle. A typical NiMH battery can do that ~1000 times. and last about 3-5 years. After that, it's useful life is over. This is similar to a cordless drill.

Now, lets do the same experiment, but limit ourselves to discharging to 42% and charging to 65%. A very low stress band of energy cycle for the battery. Now I may get 75,000 cycles out of it, with the same exact car performance characteristics. Now the battery lasts potentially 10-15 years and Toyo can actually warranty them for 10-15 years.

this is very similar to the reason why many Gen 2 Prius vehicles have an 8+ year old 12v battery. The 12v battery is only used to boot up the electronics. It has a very easy life.

 

Well... Sometimes...

Prolong whole pack charging is ideal when the pack is in the car and put together and individual module work is best when the pack is out of the car and partially disassembled. But yes, there are times where I charge and balance with Prolong in the car, then pull the pack and get self-discharge voltage reading from each module a day or many days after the pack has sat so as to find bad modules with growing self-discharge failings.

 

There's 6 cells in a 8.2volt module and there's 28 modules in a pack... You can only replace a module, you can't replace a cell... But because the pack is wired up in series and not parallel, if one of those 168 cells fail, the whole pack has technically "failed" and you're bleeding out in shark infested waters in terms of how many dishonest people, especially stealerships will be willing to take more than the car's value in repair costs... But if you're eager to learn DIY you just replace one module for $30 and all is well for a year or longer depending on how thorough your effort is.

 

Can we now sort of say, hyper milling, can get us to maybe, say, 75k cycles?