[UBCECC]

Hi, I am looking into using a micro controller to read all the cell voltages of a lithium pack 96v. What is a simple yet elegant way to read the voltage of all the cells?

[ahambone]

What is a simple yet elegant way to read the voltage of all the cells?

This isn't an easy question. I've been working on this problem myself. The commercial solution is a PakTrakr. It's fairly expensive. The DIY solution is to use a microcontroller, scale the voltage of the cell down to the 0 to 5V range that most micros can read on their analog sensors, record that, and the graph the data using some kind of output device.

I'm not sure how far along you are, but here are some topics to look up:

- Voltage Divider or Voltage Dividing Resistors or Resistor Ladder
- ADC or Analog to Digital Converter
- Optical Isolation or Galvanic Isolation

I've got a little 4 cell monitor working on a lab bench. It was more of a challenge to connect up than I was expecting. What kind of micro controller are you using? I've been tinkering with Arduinos (ATMega 168 chips). They have 6 ADC channels so I could, in theory, use 1 micro to read 6 cells.

Here's an example of the data I was able to read off my my lab bench. This is 4 LiFePO4 cells connected to make a 12V battery. I ran a hi-beam headlight off of it for ~45 minutes until one of the cells in the series string hit low-voltage for the chemistry and I stopped collecting data to re-charge the cells.

test2-cellVolts.png

Cheers,
--Adam

[TheConverted]

Could you not use a multiplexor/s to the microprocessor to time slice each of the cells?

[ahambone]

You could use a MUX to time slice the cells but the problem becomes voltage potential across the MUX. Most analog MUX chips are rated at 40 volts of potential across them, at a maximum. This would probably be fine for lithium cells but for PbA I think the 4th cell would exceed the rating for the mux chip.

I suppose if you put the MUX chip inside the voltage divider resistors then this could work. I'd have to see that sketched out and think about it some more to decide if it would work. I think you'll have problems with the analog reference voltage in the micro being tied to the most negative side of the pack being monitored - especially if the micro pulls power from the pack.

You can't isolate the + and - from a cell if the micro isn't self powered or if the micro is pulling power from the pack. This is specifically why I've kept my little experiment to 4s segments. I plan on using and opto-isolated i2c bus to connect multiple 4s segments together for examining larger packs.

Cheers,
--Adam

[zwmaster]

Originally Posted by UBCECC

Hi, I am looking into using a micro controller to read all the cell voltages of a lithium pack 96v. What is a simple yet elegant way to read the voltage of all the cells?

Hi.

Maybe you check LTC6802-1 battery stack monitor IC from Linear.
I don't know if they give some free samples.
It could be a good start.
Hopefully I will also use this chip in my all-in-one controller.

Keep us posted.

[ahambone]

That LTC6802 looks interesting...

Each LTC6802-1 can measure up to 12 series connected battery cells with an input common mode voltage up to 60V.

That makes it good for lithium. Not a bad start. You'd still need voltage dividers for PbA users. I'm kinda suspicious of this:

Many LTC6802-1 devices can be stacked to monitor the voltage of each cell in a long battery string. In addi- tion, the unique level-shifting serial interface allows the serial ports of these devices to be daisy-chained without optocouplers or isolators.

I'll have to read the data sheet to figure out how that works. At $14 each this is certainly an elegant solution. It even includes mostfet switches for high charge shunt balancing. Neat.

Cheers,
--Adam

[UBCECC]

wow I like what LTC6802 can offer! I will order them and give them a shot.

[Amberwolf]

FOr the LTC6802, check this Endless Sphere thread...
http://www.endless-sphere.com/forums...hp?f=14&t=6602
especially last page.

Methods on there is also looking for anyone wanting in a group buy to get the 100-piece price break.

This older one has some info but not much
http://www.endless-sphere.com/forums...hp?f=14&t=9493
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