Electrical – Monitoring Lithium Polymer Battery Voltage from Microcontroller

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.

The voltage divider needs to join the MCU in deep-sleep mode then... This can be achieved with a P channel FET (for instance).... When the MCU wakes up, it will want to measure the battery voltage so what it can do is turn on a circuit formed around a P channel FET that connects the battery +V to the voltage divider: -

The ADC input is shown to the right and there will be no voltage reaching it unless the MCU has activated the BC547 via the 10k resistor. Without activation, the P channel FET is turned off and virtually open circuit. If you can program the MCU to have a pull-down on its control pin when asleep that should be it, else add another (say) 10k resistor from that point to ground - this ensures the P channel FET is completely off.

A small word of warning, choose a P channel fet with low leakage current when off else there will be a slight drain on battery life but most fets are going to be under 100nA and many in the region of 1nA.

One final thing - how does the voltage regulator perform on it's standby current when the micro is off - do you need to take care of it as well?