Electrical – LiPo battery protection circuits

This is quite the multipart and deep question. You seem to understand the basics but just in case, I’ll recommend this website as a good overview, albeit dated in terms of current ICs and BMS competitors. http://liionbms.com/php/index.php

Chips This is dependent on application. For small packs like the one you’ve drawn, there are a wide variety of chips available. For larger packs, Linear (LTC6803) and Maxim (MAX11081) are the two main competing suppliers of BMS solutions. They provide the most integrated solutions for multi-cell management. The main differences from smaller packs are the higher series voltages, daisy chain communication and noise immune signaling.

Techniques In either case, voltage monitoring, temperature monitoring and active balancing tend to be the primary needs of a BMS. Other features such as redistribution tend to be less critical and often do not warrant the added cost and hardware.

Solutions Unfortunately, even though nearly all BMS designs have the same goals, rarely is there an all in one solution. Solutions are based on number of cells, voltages (chemistry), tolerance for loss, communication method etc. These variables are not all standardized between one project and another. Furthermore, technologies keep changing. Linear is considering replacing CAN-BUS with shielded Ethernet. ADC accuracy is increasing. Sample rates are increasing. The tried and true method is just simple active balancing. That is extra voltage on a cell is bled off resistively. Other than that, the rest of the system is a simple matter of reading all the cell voltages efficiently.

The Future Eventually, we will see a more robust landscape with better standardization. Companies like Elithion or Nuvation are designing BMS solutions that are nearly drop in solutions. Chip designers are getting a better understanding of customers needs and have already sought to distinguish between the different types of li-ion needs based on different industries. This will mean that we’ll start only paying for features we need. Even battery cell manufacturers are standardizing cell types. For now though, any battery design remains quite customized.

Because charge efficiency of the battery and control circuitry is not 100%, you're just going to end up wasting energy and adding cost for minimal returns. As long as your batteries have the same specifications and are properly balanced prior to use, then your risk of damaging one battery before another during discharge is pretty remote. You're much more likely to damage an unbalanced cell during recharge.

Generally it's bad practice to fully discharge lithium ion and polymer batteries as it results in shorter battery lifespan. So assuming you're only running your batteries down to 50-80% depth of discharge (DoD), the risk of damaging an unbalanced battery is negligible.

To ensure quality of your packs, I would recommend putting them through an 80% discharge test as soon as they're ready. Measure the voltage at that DoD to establish a baseline. Anything below the normal average you'll know has a problem.