# 62 cell bms @ $1.30/cell?



## dcb (Dec 5, 2009)

ug, well thinking of distributing the bleed resistors (since the wire voltage drop isn't going to be terribly predictable but substantial @ 8 feet 1 amp, and I want 22awg). So that will bump the per-cell price up a bit (keep the pcb tiny and add an extra wire). But adds some measure of user control of bleed resistor size


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## rwaudio (May 22, 2008)

In every ADC I've seen the inputs are ground referenced, exactly how are you going to deal with the fact cell 62 is at a couple hundred volts?


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## dcb (Dec 5, 2009)

rwaudio said:


> In every ADC I've seen the inputs are ground referenced, exactly how are you going to deal with the fact cell 62 is at a couple hundred volts?


simple voltage divider, liX assumed, so if it wants 10k impedence for the adc and you have 62 cells where 4.3v is the most you expect out of them,

62*4.3 =266.6

so the adc for cell 62 has a 10k resistor to ground and a 61k to cell62+ and reads pack voltage as 20 bits (4.3v signal). 

adc for cell 61 has a 10k resistor to ground and a 60k to cell61+

etc.

it may already have pulldowns built in and it has diode protection on each pin

Though the thought of hooking up 125 wires is a little disturbing, vs fully distributed and one wire between each cell for data, but the price/cell goes up. less so if you use spi-like daisy chain, low speed bus with cheap optos, but more wires between cells. I suppose it could be a circular loop too, and be one wire between cells. The nodes at the top would spend a lot of time relaying data from the lower cells.

edit: they also talk about differential readings using internal op amps on this chip, but I havent got my head around it yet. It is a fascinating piece of hardware: http://www.cypress.com/?docID=47570

I'm going to re-evaluate the cost per cell w/one internode wire circular, distributed, it might be even cheaper now that I realized how to do it. Plus it would be arduino friendly


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## rwaudio (May 22, 2008)

Since you mention 4.3v I'll assume a Li-ion style 3.6v or 3.7v nominal.
In that case fresh off the charger cell 62- which has a 10k resistor to ground will be at 256.2v, so the current through your 10k resistor is 25.6ma or 6 1/2 watts?!?!?!

Hopefully I'm simply mis-interpreting your plan, but I'm guessing you've overlooked a few things.


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## dcb (Dec 5, 2009)

rwaudio said:


> Since you mention 4.3v I'll assume a Li-ion style 3.6v or 3.7v nominal.
> In that case fresh off the charger cell 62- which has a 10k resistor to ground will be at 256.2v, so the current through your 10k resistor is 25.6ma or 6 1/2 watts?!?!?!
> 
> Hopefully I'm simply mis-interpreting your plan, but I'm guessing you've overlooked a few things.


Yah, plus resistor tolerances, etc. I'm gonna scrap it, especially since I was planning on distributing the resistors. EDIT, it is 266.6v over 70k ohms, so only 1 watt, but still.

an attiny13 is $0.50 in bulk, has internal voltage reference, can control a bleed resistor, run right off of battery voltage, much simpler to hook up (1 wire between cells) just needs one opto to isolate the nodes in the chain, and I can bitbang the protocol if necessary. It is looking way more cost effective, and simpler, and 10 bit adc directly from the cell is good enough. should be trivial to expand and only need boards for the exact number of cells you have. Plus you can order it to go to sleep (and unground the adc divider).


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## Tesseract (Sep 27, 2008)

dcb said:


> simple voltage divider, liX assumed, so if it wants 10k impedence for the adc and you have 62 cells where 4.3v is the most you expect out of them,
> 
> 62*4.3 =266.6
> 
> so the adc for cell 62 has a 10k resistor to ground and a 61k to cell62+ and reads pack voltage as 20 bits (4.3v signal).


Yeah, sounds simple. It isn't. The late Jim Williams (of Linear Technology, Corp.) wrote an excellent application note called "Developments in battery stack voltage measurement" that addresses why, but briefly, the 3 gotchas here are: 1) the precision required for each resistor divider becomes absurd as you attempt to retain the ability to resolve millivolt changes in cell voltage on a cell that is hundreds of volts displaced from ground; 2) the common mode voltage wreaks even more havoc with your measurements; 3) each divider must be different in value so each cell will be loaded differently.

Anyone who thinks they are going to develop a BMS for cheap really ought to read that application note first.


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## dcb (Dec 5, 2009)

Yah, I'm not doing the multi ADC on one chip thing. I'm over it  I do think the daisy-chained attiny - opto route has merit, but it is going to be slow (like 4800 baud) and lots of handoffs, but cheap and easy to hook up, otherwise it is "full featured". the controller issues commands at the 1st node and gets responses from the last node, and the node hardware is identical.


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## dcb (Dec 5, 2009)

with the daisy chain the nodes are addressable, obviously, so you can get specific voltage readings and turn on specific bleed resistors. Addresses can be determined at initialization so you don't have to hardcode the address. And adding cells is as easy as reissuing the init command (controller passes 0 for the address part of the command, node 1 stores it and adds 1 and twiddles the opto and passes it to node 2 (which stores it and adds 1, etc). Controller gets the response from nodeN and then knows how many nodes there are. thinking 7 address bits is probably enough.


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## sholland (Jan 16, 2012)

Just stack a bunch of ICs that are designed for the task... like the bq76PL536A, available from Digi-key.


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## dcb (Dec 5, 2009)

sholland said:


> Just stack a bunch of ICs that are designed for the task... like the bq76PL536A, available from Digit-key.


That is a cool chip! semi distributed (6 taps per node, 6 bit node addressing = 384 nodes). But $10 per chip is painful, plus lots of unintuitive wiring. I'm thinking ~$2 per cell is doable with inexpensive node level processors, and the single signal wire is in parallel to the battery connectors. He is looking at ~$12/cell if you have a multiple of 6 cells with the shield.


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## sholland (Jan 16, 2012)

Good luck!


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