Lithium battery BMS questions

[TurnNBurn]

I'm sourcing some Nissan Leaf batteries which are lithium based batteries. The setup I'm looking at getting is an 18 cell pack. Where do I start looking for a BMS system that works with these batteries?

Checking Electric Motorsport I see things like:

BL20 Cell Balancer for GBS 20Ah - BATTERIES - EV Parts Store

and

Elithion Lithiumate Lite 20-Cell BMS for Prismatic Cells - BATTERIES - EV Parts Store

The second link is for 20 cells. So I was looking for something a little more custom so I'm not paying more for those extra cells I'm not needing. Are there any BMS setups you'd recommend over others?

[mechman600]

Most cheap DIY-style BMS systems (such as Mini-BMS) are meant for LiFePO4, so a different voltage than what you need. That first link is meant for GBS (nominal 3.20V, shunting at 3.55V). I don't know for sure, but I would guess that your batteries are nominal 3.80, and max charge 4.2V, which is typical for, say, the lithiums in a Fisker Karma or something like that. The second link looks like it may be fully adjustable, but you would have to contact them to be sure.

I would go the fully DIY route. Learn Arduino if you do not already know it, and come up with something yourself. That way YOU know exactly what you are getting. I would bottom balance and then rely on an Arduino based system to provide voltage warning (at an individual cell level) and to control charging. Low cost would be a nice side effect - probably under $100.

[TurnNBurn]

Originally Posted by mechman600

Most cheap DIY-style BMS systems (such as Mini-BMS) are meant for LiFePO4, so a different voltage than what you need. That first link is meant for GBS (nominal 3.20V, shunting at 3.55V). I don't know for sure, but I would guess that your batteries are nominal 3.80, and max charge 4.2V, which is typical for, say, the lithiums in a Fisker Karma or something like that. The second link looks like it may be fully adjustable, but you would have to contact them to be sure.

I would go the fully DIY route. Learn Arduino if you do not already know it, and come up with something yourself. That way YOU know exactly what you are getting. I would bottom balance and then rely on an Arduino based system to provide voltage warning (at an individual cell level) and to control charging. Low cost would be a nice side effect - probably under $100.

Those two links were just examples. I know the GBS wouldnt work because the cells I'm looking at are 3.8V if I remember correctly.

I'm going to research the arduino DIY BMS.

[P-hack]

A couple issues with the bms node:
BL20 Cell Balancer for GBS 20Ah - BATTERIES - EV Parts Store

It uses a shunt, which is ok for bottom balancing occasionally, but they are using it to top balance, which can kill one of your cells since some reach the bottom before the others, but the pack voltage is still fairly high.

They make heat instead of moving charge from cell to cell, possibly not an issue on a bike.

The bolt-through-pcb is an issue, different rates of expansion for pcb and steel.

The bolt pattern probably doesn't fit leaf cells.

But perhaps most importantly is they are designed for a different chemistry than is in the leaf:
- Nominal Cell Voltage: 3.20V
- Load Turn On Voltage: 3.55V
recall the leaf "cells" are 2.5v min, 3.75 nominal, 4.2v max (32ah) and each leaf module is 7.5v 65ah (2s2p).


I wouldn't worry about bms for starters, you can test 18 cells with a vom pretty easily and a simple circuit will tell you if it is obviously out of wack and needs a bottom balance.

9 "leaf" modules will get you to 67.5v nominal fyi.

[TurnNBurn]

are there any pre-assembled all in one type BMS setups you'd recommend? I'm seeing this Orion one, but it's like $700.

[mechman600]

Originally Posted by P-hack

I wouldn't worry about bms for starters, you can test 18 cells with a vom pretty easily and a simple circuit will tell you if it is obviously out of wack and needs a bottom balance.

If you know what you're doing you can bottom balance with a 1 ohm, 2 watt resistor. Way cheaper.

In my designing a battery monitoring system for my Electric Booger, I designed a lithium one as well. Pretty simple, actually. Keep the cells between 2.7V (or 2.5V) and 3.6V. Balance them at the bottom. Stop the charge when the first one hits 3.6. Pretty easy. Edit voltage for different battery chemistry.

Since an Arduino Nano has 8 analog inputs, that's 3 boards for you. On eBay that's $20. 8 inputs * 4.5V (ultimate max per cell) = 36V per board / 1024 (10 bit) = a resolution of 0.035V. Plenty good enough.

It's easy to send warning messages (high V or low V) from one board to another via a PWM signal down a single wire. I experimented with an 8 channel dip switch, sending a PWM value down a single wire (using analogRead() function), making the "reader" board decipher what switch was on or off by measuring the PWM (using pulseIn() function). With 8 bit PWM (0-255), allowing for some space between signal duty cycles, you could probably have at least 100 separate triggers down a single wire. Maybe you wouldn't be able to read the voltage of each cell, but a warning of WHICH cell is low or high would be easy, and better than the Mini-BMS which has a general warning and cannot pinpoint problem cells.

Let me know if you need help with code or wiring. I would be more than happy to help.

[P-hack]

turn, might I suggest you offer to buy mechman:
an arduino (clone for $10)
US Uno R3 ATMEGA328P ATMEGA16U2 2012 Version Board Free USB Cable for Arduino | eBay

a mux shield:
https://www.sparkfun.com/products/11723
$24.95

maybe a 20/4 serial lcd so you don't have a bunch of pack wires going to the dashboard:
Blue Serial IIC I2C TWI 2004 204 20x4 Character LCD Module Display for Arduino | eBay
$9.18 suggest 3 digit volt readings, space in between, 5 readings per line, skip the decimal point, to fit 18 reading on a 4x20.

very unbalanced example (18 readings):

000 050 470 250 270
277 343 333 320 386
312 402 412 333 262
303 389 250

plus a loud piezo if any are out of range, and flashing led backlight, and maybe some way to signal the charger or controller to stop. (or charge through the contactor and have this disconnect it maybe)

and whatever else mech needs and see if he will mess with it for you

[TurnNBurn]

Originally Posted by P-hack

turn, might I suggest you offer to buy mechman:
an arduino (clone for $10)
US Uno R3 ATMEGA328P ATMEGA16U2 2012 Version Board Free USB Cable for Arduino | eBay

a mux shield:
https://www.sparkfun.com/products/11723
$24.95

maybe a 20/4 serial lcd so you don't have a bunch of pack wires going to the dashboard:
Blue Serial IIC I2C TWI 2004 204 20x4 Character LCD Module Display for Arduino | eBay
$9.18 suggest 3 digit volt readings, space in between, 5 readings per line, skip the decimal point, to fit 18 reading on a 4x20.

very unbalanced example (18 readings):

000 050 470 250 270
277 343 333 320 386
312 402 412 333 262
303 389 250

plus a loud piezo if any are out of range, and flashing led backlight, and maybe some way to signal the charger or controller to stop. (or charge through the contactor and have this disconnect it maybe)

and whatever else mech needs and see if he will mess with it for you

That'd work for me if Mechman is willing to build the stuff. I went around and bookmarked all the parts on other sites (no ebay account here).

I like DIY stuff and doing things on my own, but I've been researching arduino BMS and DIY BMS stuff all day. Lots of soldering that I don't have enough skills in. Or the proper soldering tips.

[P-hack]

I have to mention that manually bottom balancing a 18 cell pack once a month (if it even needs it) can be pretty trivial, next to being a guniea pig. Usually you take 18 readings with a voltmeter before hooking up the charger and see if any need to be bled down with a resistor and some alligator clips.

You don't get any guarantees with the experimental approach. But if you are up for it and mechman wants to experiment w/the hardware then have at it.

I forgot current sensor FYI, and a relay if this is going to override the contactor. Guess you have room for amps in the lower right.

[mechman600]

Mux shield. Wow. That's a lot of I/Os. Does it use it's own ADCs (and if so, how many bits are they) or does it multiplex analog inputs to the ADCs in the Arduino?

The problem with using one chip on a series string is that you have to deal with upwards of 90V worth of potential voltage difference on a 72V car. Therefore, say max is 90V and you put it through a voltage divider to make 90V = 5V to the chip, that's only a 0.09V resolution with the Arduino's 8-bit ADC (1024 digital steps). I guess not too bad, but I would like better resolution than that, especially with a lithium pack. If the mux shield has a better ADC, say 10 bits, that would give you 4 times better resolution than 8.

This is why I suggest multiple Aduino chips, each taking care of only 8 inputs across a spread of only [4.5V X 8] 36V, yielding a 0.035V resolution (or even less - 4.5V X 4 = 18V with a 0.017V resolution....). Each of the two "reader" Arduinos (for an 18 cell system) can send PWM warning signals to the master board (that has 8 of its own analog inputs) via a single wire through an opto-isolator to keep voltage potential difference between two chips at different parts of the battery series string from letting the smoke out of things.