Analog MUX using discrete MOSFETs for flying capacitor cell measurement BMS

P E Schoen · 2017-11-10T09:04:51+00:00

My latest project is a battery management system (BMS) that can accurately measure cells in a lithium battery pack, and also perform shunt balancing and possibly charge shuttling to balance the pack. I would also like to be able to use it for a pack of four 12V SLA batteries. A DG408 could be used to select up to 8 cell voltage taps from nominal 3.2V (LiFePO4) or 3.7V (Li-Ion), with a maximum voltage of 8 * 4.2 or 33.6V, which is within the maximum voltage of 40V for the DG408. But the voltage would need a 12:1 voltage divider to read using a PIC with a 3V power supply (from the bottom cell), and this renders the precision of the 10 bit ADC to 33.6/1024=0.032V, which is really not accurate enough for lithium cell monitoring. I did a preliminary design using a DG409 which can read any one of four cells differentially, or two DG408s for 8 cells. The outputs can be fed to a "flying capacitor", which will hold a voltage sample taken from a cell, and then a pair of MOSFETs can translate that charge to ground level so the ADC can read it using the full range allowed by the reference. Another method, used by some BMS chips, utilizes a high CMRR differential amplifier to translate the samples to the ADC. It is also desired to implement shunt charge balancing, where selected cells that have a higher voltage can be discharged to match lower cells in the pack. This is wasteful, but is a commonly used method, and can be done with MOSFETs and resistors. It is also possible to use charge shuttling, where the flying capacitor can be connected to lower voltage cells so that its charge adds to that of the low cell. But this requires a fairly large capacitor and a low resistance MUX. The DG408/9 have about 50 ohms per element, so that severely limits the practicality of such charge shuttling. So, I endeavored to design a multiplexer using discrete MOSFETs. My first attempt used MOSFET opto-isolators, but they are somewhat costly and use a fair amount of current...

Nov 10, 2017 20 Replies

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Whew! I found out that my PADS Logic symbol editor was somehow corrupted. I would edit the symbol, but when I applied it to the schematic, it kept the old one. Now I think I finally have it right, and a made the arrow open rather than closed. I still like the body diode, but I see your point. Anyway, I have arranged the parts on a 2"x 6" board, and that isn't really very dense. I will be getting the prototypes made by

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, originally zoompcb.com, and they have a deal where I have gotten 5" x 5" boards (maximum is 25 in^2) for $25 each. I am considering adding more circuitry to fill up a 4" x 6" board - perhaps my design using TL222 optos and one using one or two DG408 or DG409 analog switches.

Thanks,

Paul