# Battery Management



## narithota (Jun 29, 2011)

Hi

I am working on battery management system using 3.6V 20AH LI-ION cells in series to get 72V.
Many companies are providing the battery monitoring ICs & BMS ICs. Can anybody guide me , how I can proceed.

Thanks & Best Regards


nari


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## Semper Vivus (Apr 13, 2011)

Hello,
maybe you have to specify your project a little bit further. So do you want to monitor every single cell or only pack voltage...what about pack current? Do you want a digital bus to read data etc.?

Best Regards,
Tom


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## narithota (Jun 29, 2011)

Hi

Thanks for the reply

Iam Planning to to moniter every single cell by using the PCB placed in between the +ve & -ve terminals of the cell.Communictaion between the cells to the Master control will be SPI or I2C. 

Battery pack voltage will be 72V ,Normal power 8Kv,Peak power 15kv.Single cell capacity is 3.6V 40AH.

Thanks & Best Regards

Nari


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## ElectriCar (Jun 15, 2008)

There are a few threads on here which if you read you will know more than you ever thought possible. Many including me chose not to monitor individual cells but do monitor the pack in other ways. In my case, look in my "upgrade to calb" thread for ideas. I could have bought a BMS cheaper than the route I chose but I feel better about my choice than to go with a BMS. Of course you must do what will make you sleep at night.

Among other devices, I use a split pack monitor which if all cells are balanced manually, the voltage will read 0.00V. However to manually balance that close is very time consuming and unnecessary IMO. If you lose one cell completely, goes shorted, the meter displays half the voltage of one cell or about 1.65V for a Calb. I detailed operating scenarios in the thread for interpreting the voltage displayed. 

The methods I use only require me to have 1 wire connected to the pack and that is to the middle of the pack. 

Whatever method you use, good luck!


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## Semper Vivus (Apr 13, 2011)

All the designs that I've seen here with PCBs on every cell use a microcontroller I think. With a microcontroller you are more flexible with the communication. And using both, uC and a BMS IC on every single board wouldn't be profitable I think.
If you are looking for BMS ICs I would suggest looking on the Texas Instruments page. They are one of the market leaders in such ICs I think.
For one single board, maybe the Linear LTC6801/6802 would be a solution. That is used in the Mitsubishi i-MiEV...but very expensive.
For a design, using only microcontrollers, you will find many (diy-) examples on the web.

Best Regards,
Tom


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## narithota (Jun 29, 2011)

*State of charge*

HI 

Thanks for the reply

I have seen the LTC6801/6802 from the linear, this IC will do only the cell Balancing up to 12 cells in the series & many BMC ICs will do the cell balancing.

How to find out the State of charge(SOC) of the Li-ION battery Pack(72V,40AH) which used in the Electrical Vehicle. To measure the SOC how to find out charging & discharging Current , Cell temperature , Cell voltage.

Thanks & Best Regards

Nari


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## Semper Vivus (Apr 13, 2011)

Hello,
the Linear LTC680x ICs are not specially for balancing, they are ICs for monitoring cell voltages. They also have inputs for temperature sensors. Just read the datasheet. Additionally the LTC6802 has outputs to drive MOSFETs to do balancing, that's right yes.
To meassure SOC you just integrate the actual current over time (with a shunt). I doubt that you will find a ready "black-box-IC" that gives you the SOC of your pack. But there are many current sensing ICs that will help you meassuring the voltage over a shunt. The final SOC estimation then is the task of your microcontroller.

Best Regards,
Tom


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## DIYguy (Sep 18, 2008)

OK, here is a great opportunity for someone who wants to get into this. Go 
to http://www.zanthic.com/bms.html 

I have no connection to this company other than a bunch of email with Steve. He is looking for someone to take his design and be the seller/system support. 

The design you describe already exists.It can be used a monitor or you can add the balancing for a full BMS. This is based on the LTC6803. The guys name is Steve Letkeman. What he is offering is the complete design and also the firmware. All he wants to do is sell the processors to you. He has done all the work by the look of it. Component pricing examples are there. Board layout is there. He is offering;



Pre-programmed Freescale 9S12C microcontroller with built in Cheetah bootloader for firmware updates and BMS firmware that can be custom configured for a number of different applications
Schematic examples of demo hardware for different variations of master and slave configurations
Bill of materials for all boards and approximate price listings for budgeting purposes
Configuration software for Windows PC
Pre-built demo boards for bench top evaluation
CAN-4-USB/FX interface for configuration and demonstration
I was going to buy enough demo boards to do a monitoring only system...but this way is too expensive. If on the otherhand, you were to take his design and go out for build, especially with a larger quantity this could be a decent business addition.

here are pics of the master and slave demo boards.


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## narithota (Jun 29, 2011)

Hai
Thanks for the reply

Aim looking for single cell monitering.If my battery pack is having 30 cells means i required 30 number of PCBs. Single PCB arranged in between the Cell +ve & -ve terminals. This method is somewhat complex but In critical applications we can know the individual cell information & Monitoring very accuratly. 

Could you suggest me the single cell solution Or ICs for BMS.

Regards

Nari


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## DIYguy (Sep 18, 2008)

narithota said:


> Hai
> Thanks for the reply
> 
> Aim looking for single cell monitering.If my battery pack is having 30 cells means i required 30 number of PCBs. Single PCB arranged in between the Cell +ve & -ve terminals. This method is somewhat complex but In critical applications we can know the individual cell information & Monitoring very accuratly.
> ...


Hi Nari,

With the system I showed in my above post, it is more advanced than what you describe. Each slave board connects to 12 batteries. Each cell is monitored for voltage and temperature and hi/lo alarms can be set for each parameter on each cell as well as complete pack. If balancing is used, you can even force individual cells to balance at different voltages if you want. Below are a few screen shots of the display used. Note individual cell voltages. Also shown is set up screen showing 3 slave boards and a total of 28 cells. 
Each slave wires to max 12 cells. Each slave is daisy chained via ethernet to one master. Display is connected to master. No spaghetti wiring. For 30 cells you would need 3 slave boards and one master board. look at this document... http://www.zanthic.com/project31.htm


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## narithota (Jun 29, 2011)

Hai
Thanks for the reply

In the project i have to monitor the individual cell , to measure

Cell Voltage/Cell Balancing
Cell Charging / Discharging Current
Cell Temperature
State of Charge 
Columb Count
State of Health

Please can anybody suggest Single cell solution for above parameters.

My load is 6KV Motor, Single cell should withstand for this power rating.

Regards
nari


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## DIYguy (Sep 18, 2008)

narithota said:


> Hai
> Thanks for the reply
> 
> In the project i have to monitor the individual cell , to measure
> ...


Did your read the article? Maybe you should do some research.

Cell voltage and balancing is there with limits set-able high and low.
There is a current sensor for measuring charge and discharge. The current in a series string is the same no matter where you sample it. 
Cell temperature is there with set-able alarms high and low.
SOC is there and calculated by the pack's Ah rating and measured current.
Columb count is there expressed as Amp-hours.
State of health is a subjective term... but I think with the parameters mentioned and alarms shown you can draw your own conclusion. Ability to show single highest/lowest voltage cell tells u a lot.

As for your motor, I suspect you meant 6kw although this seems pretty low. 6kv is 6,000 volts, and this is not a measure of "load" nor is it practical.

There are other systems out there also to give you this stuff. Go have a look.


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## narithota (Jun 29, 2011)

Hai
Thanks for the reaply

Our plan is , we are arrange the batteries in the series, ex:72V 80AH means 20 number of 3.6V Li-Ion cell in series. if any cell goes bad while in the travelling , we want *by pass* the that cell for contentious run(vehicle) without replacing the cell. For this application we required the single cell BMS solution.

For measuring the *SOC , coulomb count, SOH* we required the Current measurement. 

How to measure the current in the cell level. Suggest open loop current sensor for EV/HEV applications.


Regards
nari


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## Semper Vivus (Apr 13, 2011)

Hello again,
as DIYguy said, when you have all cells in series there is only one current to meassure. There is no need to meassure the current at cell level.
For bypassing every cell on the BMS board with a MOSFET, you have to route traces for over 200A (with your mentioned 15kW peak). In reality that value will be higher because the pack voltage will sag and so more current has to flow. That will result in very very big PCB traces... Of course you can also build a big relay crave for bypassing.

I think you have to do some research about basic electronics and BMS stuff first. And then come back with more precise questions. Then the guys here can help you much more efficiently.

Best Regards,
Tom


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## narithota (Jun 29, 2011)

Hai every body

Just i have the some confusion regarding the my concept ,once that is clear i will start my research.

My doubts is:

Battery pack State of Health(SOH) & State of charge(SOC) is depend on the individual Cells. If i have to find the SOH & SOC, i have to measure the cell current or Battery current. But in the series only one current to measure. If one cell gone bad in the series how I can track defect cell in the battery pack. 

My intention is i have to monitor the health of the battery ,it is depends on the individual Cell.

Finally Please tell me How find the health of the battery(Age) & State of charge.

Thanks & Best Regards
nari


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## Semper Vivus (Apr 13, 2011)

narithota said:


> Hai every body
> 
> Just i have the some confusion regarding the my concept ,once that is clear i will start my research.
> 
> ...


Hello,
of course the battery SOC and SOH depends on the individual SOC and SOH of every cell. But you want to know the SOC and SOH of your whole battery, not of one individual cell. So it's adequate to meassure only current and battery/pack voltage.
Measuring of the individual cell voltages is only for fault detection. For example a fault would be if a cell reaches the lower or upper limit of the specified voltage operating range. Or the voltage difference between the highest and lowest cell reaches a threshold.
For balancing you will also need the measuring of the individual voltages of course. But now we are only talking about monitoring.


SOC estimation in THEORY:
Let's use your 20S1P battery with 72V 80Ah for this example.
Let's also assume it's fully charged. Now you track the battery current with your microcontroller. Let's say the controller samples every one second a new value. Because your battery is still fully charged, your SOC is 100%.
Now you start the drive. Let's say the controller meassures 80A and it does meassure that value also at the next hundret measuring cycles (100 seconds). The battery has lost 80A*100s/3600=2,22Ah of its capacity. At this point your SOC is (80Ah-2,22Ah)/80Ah=97%.
If you continue your drive with this current for further 3500 seconds your battery will be empty... So just calculate the lost capacity at every sample cycle (current*cycle time) and add it to the previous value. With that value you have also your SOC based on the full charged capacity.
Of course you have also consider the negative values while charging or regen.

That easy example will work at the cell's begin of life. But if your cell is at end of life (gives only 80% of its rated capacity) you will get in trouble if you let the discharging process continue to for example 20% SOC based on the rated begin of life value. At this point the Battery would be at 0% in reality.
So it would be ideal to use the real capacity for SOC calculations. 
You get the real capacity by measuring the resting voltage. When measuring load voltage, the internal resistance will distort your measurement. So you have to wait until there is no current flow at the battery. And then you have to wait further until the cell is fully rested.
Doing that process once in a while gives you the possibility to use the real (actual) capacity for SOC estimation. Maybe it would also be a advantage for this process to measure the voltage of every single cell.


SOH estimation in THEORY:
The battery's end of life (max. rated cycles by the manufacturer) is normaly calculated for the point where the battery has lost 20% of its rated begin of life capacity. 
So the simplest way for SOH estimation would be to say the SOH is 100% when the battery has the rated capacity and 0% when it is 0.8*rated capacity.


Conclusion:
In real life measuring the resting voltage is a hard task (depending on the application)...maybe you could do that at night where the EV is not in use. The accuracy also hardly depends on the cell chemistry. So when you use LiFePo4, resting voltage will only be usefull at begin and end of the discharge curve, because between the curve is virtually flat.
So the easy way is to just calculate the SOC by measuring current, don't take care of resting voltage and SOH and use the single cell voltages for fault detection.
My example also doesn't take account of the battery's temperature.
If you have mathematical background and much time you could also do Kalman filtering...that will surely solve all these problems ;-)


Best Regards,
Tom


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## DIYguy (Sep 18, 2008)

narithota said:


> Hai
> Thanks for the reaply
> 
> Our plan is , we are arrange the batteries in the series, ex:72V 80AH means 20 number of 3.6V Li-Ion cell in series. if any cell goes bad while in the travelling , we want *by pass* the that cell for contentious run(vehicle) without replacing the cell. For this application we required the single cell BMS solution.
> ...


I'm sorry, but I think your plan is completely not worth the money and effort. 
The need to switch out a cell somewhat automatically and keep going is silly when you consider the cost/complexity to likelihood of it happening. Use a half pack bridge and carry an extra jumper or two if you want. 
What cells are you using that have a 3.6 volt nominal?
If you select quality batteries and install correctly.. you should not have any issues.
Also, the system I mentioned *does measure current*.


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