[PStechPaul]

Perhaps a good way to measure internal resistance would be to measure the voltage difference for current of 0.5C and 1C. That would eliminate the effect of the unloaded (relaxed) condition.

 

[PStechPaul]

You could also consider silver-bearing conductive epoxy or other adhesive, but it's rather expensive:

https://www.mcmaster.com/#7661a11/=19nmwiq ($33 for 0.09 oz)

https://www.mcmaster.com/#7595a31/=19nmxr3 ($122 for 0.5 oz)

https://www.mcmaster.com/#7365a44/=19nmz0b ($49 for 0.2 oz pen)

https://www.amazon.com/Adhesive-Ele...psc=1&smid=A1665ONA14XMDL#feature-bullets-btf ($14 for 2.5 gram 0.09 oz syringe)

There are less expensive alternatives with nickel, carbon, or perhaps copper, but look at the volume resistivity. It may not require very much if the conductor and the cell are smooth and flat. You could fasten the connecting strip with a tiny drop of conductive adhesive and then reinforce it with a larger glob of plain epoxy over the assembly.

 

[PStechPaul]

You can make a datalogger using a PIC or Arduino.

http://embedded-lab.com/blog/beginners-data-logger-project-using-pic12f683-microcontroller/

http://embedded-lab.com/blog/low-cost-temperature-data-logger-using-pic-and-processing/

https://www.arduino.cc/en/Tutorial/Datalogger

https://www.adafruit.com/product/1141

Or a multimeter with datalogging function:

http://www.ebay.com/itm/US-Ship-Mul...r-Ohm-USB-PC-Upload-Data-Logger-/401090714400

Or a data acquisition module:

http://www.ebay.com/itm/Measurement...-Data-Acquisition-Multifunction-/272864779211

https://www.microdaq.com/measurement-computing-usb-minilab-daq.php

 

[PStechPaul]

I've looked into various design ideas for a BMS and cell balancer. I think this could be accomplished with a microcontroller for each set of 8 cells, and an 8 channel DG408 analog multiplexer to read the string voltages at 8 points into an 8:1 voltage divider. The DG408 can handle up to 44 volts so eight cells at 4.2 volts each would be 33.6 volts. The DG408 draws only 10 uA. A 100k voltage divider draws at most 420 uA while reading but that can be done in less than 1 mSec perhaps once every few seconds, so average current is only a few microamps.

Each set of 8 cells can perform balancing by putting loads on high cells, using transistors for level-shifting. Each 8-cell module can communicate with a central processor directly, or in daisy-chain fashion with other modules, using opto-isolators or digital isolators. The microcontroller can sleep when not sampling cell voltages or communicating. Twelve modules of 8 cells each would work for 403 volts with 4.2V Li-Ion cells.

 

[PStechPaul]

A 10 bit ADC with an 8:1 divider has a resolution of about 0.004 volts. A 12 bit ADC would make that 0.001 volt. Except for the bottom cell, readings would need to be calculated as the difference in readings for successive taps. A differential ADC might work, but requires two multiplexers.

It might be possible to design a flying capacitor circuit where MOSFETs on adjacent cell taps would take a sample of the cell voltage desired, and then another pair of MOSFETs would apply that capacitor to the ADC. This would eliminate the voltage divider and allow full 10 bit resolution. The current draw for the first sample would be significant, but for subsequent samples the only current would be that required to stabilize the reading, and would even provide some charge balancing when the capacitor charged to a high cell would impart that energy into a lower cell.

It has been considered previously:

http://americansolarchallenge.org/ASC/wp-content/uploads/2013/01/SAE_2001-01-0959.pdf

http://www.google.com/patents/US7362588

https://patents.google.com/patent/US8786248B2/en

http://www.utdallas.edu/essl/projects/charge-balancing-problem/

https://www.edn.com/design/analog/4334442/Analog-multiplexer-uses-flying-capacitors

 

[PStechPaul]

The Microchip PIC16(L)F1783 has a 12 bit differential ADC and a fixed selectable internal voltage reference, and costs well under $2 in moderate quantities. The "L" version is limited to 3.6 volts, otherwise max 5.5 volt supply.

I have also considered the possibility of measuring the voltage on several taps of a battery pack by applying the voltages through MOSFET switches to a resistor and a capacitor connected to a comparator, and use the time of charging to determine the voltage. As long as the resistor and capacitor values are known and stable with temperature, the time can be determined very accurately to as many as 16 bits. A mux like the DG408 can be used up to 44 volts, but otherwise higher voltage MOSFET switches could read even higher.

I haven't fully worked out details, but take an example of 8 cells at 3.2 volts each, with a 500k resistor and a 100 nF capacitor, and a comparator with a reference of 2.048 VDC. For the voltage at the top, 25.6V, the setpoint is reached in 4.181 mSec, while for the single cell at 3.2V, it takes 51.103 mSec. A 1 MHz counter would provide precision of 25.6/4181 = 6 mV, and 3.2/51103 = 0.06 mV.
0.

 

[PStechPaul]

I think it would be easier to implement than you think. A single DG408 would allow connection of any of eight voltage points from 3.2V to 25.6V. Once the comparator detects the charge to the setpoint, the MUX would be turned off, and a MOSFET across the capacitor could discharge it for the next reading. All this can be done with 3 I/O pins for address select, one I/O pin for enable, one I/O pin for discharge, and one analog input for the comparator. Six pins.

The effect of each measurement would be the energy loss of

0.5 * 100nF * 2.4V ^ 2 = 200 nanoCoulombs

The energy of a 10 A-h 3.2V cell is 32 * 3600 W-sec (Coulombs)

Even with one measurement per second it would take 3600/200 = 18E9 seconds or 5 million hours. Another way to look at it is that the measurement is equivalent to a maximum of 3.2V/500k = 6.4 uA which by itself would drain the cell in 10/6.4 = 1.56 million hours. In reality the sample has an average current of roughly 3 ua and a duty cycle of 51 mSec/sec or about 40 times less.

In an actual circuit, there will be other more significant current draw. I envision the microcontroller powered by the low cell in the stack, so it will lose more charge and eventually become unbalanced. But the same circuit could be used to perform charge balancing by discharging the other cells through the sampling resistor, and keeping the discharge MOSFET across the capacitor ON. The 500k would only be 1.6 uA on a 3.2V cell. Another problem is that all cells from the chosen tap on down would be discharged, so that might not work as desired. However, adding an opto-isolator and load across all the upper cells would probably do the job with minimal additional components. And, of course, fully selectable cell discharge could be done with eight isolators and resistors. This could easily provide 20 mA of load.

 

[PStechPaul]

Here is a preliminary design:

 

[PStechPaul]

The idea was to provide a very small amount of balancing to compensate for the additional current from BT1, which is used to power the PIC. But then I realized that it would drain all cells below the chosen tap. So that won't work.

I show an opto-isolator U3 that is not connected. If I used a 28 pin PIC there would be eight more I/O pins each of which could turn on 8 isolators that could be connected across each cell in the stack, to draw as much as 30 mA from selected cells to perform balancing. Actually, it could use a quad darlington opto that can handle up to 160 mA per channel, and costs only about $2:

https://www.mouser.com/ProductDetai...EpiMZZMteimceiIVCBy62mdAgfQXXHOTA%2bTID%2bVg=

D1 is actually a Schottky diode that isolates the PIC Vdd from the cell BT1. It might not be needed. Actually I thought it might be better to power the PIC using an inexpensive DC-DC converter from the 12V accessories battery. But I think the ideal design would be self-contained and run off one of the cells.

The Tx and Rx are the serial connections to the USART. I planned to connect those signals to a Bluetooth module for communication with a computer or a "motherboard" that would handle communications. I haven't fully thought through this part of the design. Another method would be a daisy-chain from module to module, possibly using opto-couplers or digital isolators or other method. Power consumption must be considered. Bluetooth modules draw something like 20-50 mA.

PGC and PGD are just the programming pins.

I would also like to design the dynamic charge shuttling version that requires two DG408s. It might even be able to extract voltage from each cell and transfer it to the PIC power supply. It should be easier to implement, as it would use the full range of the 10 bit ADC for each cell measurement. DG408 is inexpensive - a little over $1 in 100 piece quantity.

 

[PStechPaul]

The opto-isolators for shunt balancing are probably most effective and relatively inexpensive. Resistors should be added rather than dissipating the power in the device. Something like 10 ohms for 300 mA at 3 volts. Only 900 mW. Might be able to use a 1 watt LED instead?

The precision of the top voltage measurement is only 6 mV because of the limitation of the maximum count for the single cell measurement. Perhaps a two channel by 4 MUX like the DG409 could be used. One set of four switches would use 250k resistor and 220 nF capacitor would take 9.6 mSec for the 4th cell at 12.8V, so 9600 counts and 1.3 mV resolution. The first cell 3.2 volts takes 56.2 mSec for 0.05 mV/count. The higher 4 cells could use 500k and 470 nF, so 24.8 volts takes 19.6 mSec and 16.0 volts takes 32.2 mSec. That represents 1.2 mV/count and 0.5 mV/count.

Another "trick" would be to change the setpoint of the comparator to 1.024 volts to reduce the number of counts for the lower voltages. And also it's possible (and maybe easier) to change the clock rate for the counter depending on the voltage tap to be measured.

The isolation of the power supply from BT1 with the Schottky diode just allows the voltage to stay constant (depending on the power supply capacitor and load), if the battery voltage drops because of a heavy current load on the pack. It may even be good to use a small lithium cell to power the controller without drawing anything from the pack, except the 25.6 volts for the DG408.

Communication could use the Txd and Rxd from the USART, through digital isolators. The Si8621A has two channels, draws less than 2mA, handles up to 1 Mb/sec, and costs about $1.00.

https://www.mouser.com/ProductDetai...D0wnx/ymM3bPDptwQTOS6S9E8Ss2%2bohTUaCHX6VnQ==

This discussion has been interesting, but perhaps it should be transferred to that for BMS guidelines.