# BMS For Bottom Balancing



## glyndwr1998 (Apr 27, 2013)

Hi sunking,

You already probably know this, what about cell loggers?

I use them to monitor my 76 cell pack in a prius plug in hybrid conversion kit.

I have the cell loggers programmed to cut off the charger at 3.5v, and to audible alarm ( but could easily use it to cut off the contactors) Al low end to 2.65v.

The output signal from the cell loggers goes to a small breadboard that has 11 opto isolators that when the cell loggers trip, they open the opto circuit and cut the 12v feed to the charger relay and the contactor relay if you would wish it too, very cheap to t up and wire, and so far very reliable.
As for amp hour counting I ise a jld 404 with a 75 ma shunt, this too works very well. 

Thanks.


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## Sunking (Aug 10, 2009)

glyndwr1998 said:


> Hi sunking,
> 
> You already probably know this, what about cell loggers?


Yes that is what I am thinking. Just commercial versions. Just have no need to Vampire Bleeder Boards or Top Balance. I do not mind using a control unit made for Bleeders if that will work. Just new to BMS and and do not know what manufactures are out there and looking for products to choose from.

Basically a unit that is programmable, at least 16 cell monitoring, with I/O features like LVD signal, HVD to turn a charger off based on cell voltage value, and some SOC or Coulomb Counting if possible. 

I could use Arduino but I do not care to design or build it for this application.


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## glyndwr1998 (Apr 27, 2013)

Go,

There is a guy on a uk site called batteryvehiclesociety.org who is called Greg forsythe he has designed a board using a ltc chip that monitors 12 cells per pcb that I have talked to him about on a few occasions, his project is open source so anyone can copy the pcb and code for their own use, he uses it very successfully, no balancing purely voltages and current measurement with cut off both ends. It's expandable for as many cells as have.

If I came away from the cell loggers I would probable do the same as his project as it wasn't expensive per cell and is basics but pretty programmable and expandable.

Have a look at it, it's in the bms section of the forum he has a topic on it all on his own, some 8 pages showing his progress from start to finish.

Thanks, anthony.


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## glyndwr1998 (Apr 27, 2013)

Also a guy called Peter perkins has something similar but His bms balances, his design has been replicated many times by others and has been working on his pack for over 5 years reliably, and in that time his 50ah cells from new were measuring 35 ah after 5 years with no cell failures, I think that's pretty good. Tolerance I think was 3.65v upper limit and 2.6v lower.


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## dcb (Dec 5, 2009)

I looked up Gregs Design, fyi:
https://github.com/Greg-Fordyce/evBMS

I'm leaning more towards the multiple cells per node approach these days, simply because of cost/cell.


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## glyndwr1998 (Apr 27, 2013)

Hi dcb,

I'm not 100% sure but I think greys latest slave board monitors 12 cells per slave.



dcb said:


> I looked up Gregs Design, fyi:
> https://github.com/Greg-Fordyce/evBMS
> 
> I'm leaning more towards the multiple cells per node approach these days, simply because of cost/cell.


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## dcb (Dec 5, 2009)

glyndwr1998 said:


> Hi dcb,
> 
> I'm not 100% sure but I think greys latest slave board monitors 12 cells per slave.


I think so, been looking at the ltc68xx chips, pretty sweet, 12 bit 12 cell, some can be daisychained.


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## glyndwr1998 (Apr 27, 2013)

I think Greg daisy chained his using standard usb cables, pretty neat and cheap too. 



dcb said:


> I think so, been looking at the ltc68xx chips, pretty sweet, 12 bit 12 cell, some can be daisychained.


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## Sunking (Aug 10, 2009)

Hey guys thanks for the comments but can we keep this thread on track with commercially available monitors.


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## PStechPaul (May 1, 2012)

If you just need a way to put a power resistor across each cell to discharge it, I made a charger/BMS that uses inexpensive components with total cost less than $1/cell. Use an optoisolator driven by the monitor, and the output can be connected from one side of the cell to the base of an NPN or PNP BJT that has a power resistor to perform the shunting or discharge. I have several thousand MJE170s (PNP) that can work up to about 2 amps. That's about 6 watts per cell. Higher currents would need a different BJT and higher power resistor, which may affect board layout, size, and cost. A 5 cent LED across the resistor would indicate discharging or shunting.

My most recent concept for a BMS would use an 8 channel analog MUX to read up to eight cells in a string with just one ADC, and the eight shunt circuits can be turned on or off with eight I/O lines. This could function as a bottom balancer as well as a shunting charge limiter and a continuous SOC monitor. When I get it finished I will probably have PCBs made and it will be essentially open source. I will probably spend $150 to get 10 boards made and the parts will be about $5/board plus $1/cell for the shunt circuits, which can be removed and mounted to each cell.

If something like this would be of enough value to the EV community and people would be willing to spend, say $50 per unit (for 8 cells), it might motivate me to devote more time to it and "git 'er done".


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## CKidder (Dec 12, 2009)

Sunking said:


> Perhaps BMS is the wrong term to use because most BMS systems I have seen for sale are for using Balance Boards or Top Balancing.
> 
> I am looking for a system tailored to Bottom Balance. One without Bleeder Boards, but rather monitor each cell with programmable low and high voltage set points that can operate both a LVD and HVD Contractor Relays. Additionally if available one with a Display with basic Coulomb Counting, SOC, Alarms.
> 
> ...



The EVTV people (disclaimer: that includes me) have two systems something like what you want. There is the JLD505 which will do pack voltage monitoring and current monitoring. This can do coulomb counting and has outputs. It is meant as a beefed up version of the JLD404 except that it includes no screen. You're supposed to use an Android tablet or something to monitor things. It'll have support for OBDII compliant output so that you can use your favorite app. It has bluetooth for external connectivity.

The other option is the EVTV BMS which monitors pack amperage and quadrants of your pack in voltage and temperature. This allows for cutting off charging or discharging if the quadrants get too far out of balance. This option has no actual outputs but interfaces with something that does, such as the GEVCU board. 

Both of them are essentially BMS's meant specifically for bottom balancing. Neither of them technically exist for sale yet. But, both of them should be available very soon.


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## arber333 (Dec 13, 2010)

I use DIY BMS designed by Neville Harlick here: 
*http://forums.aeva.asn.au/forums/low-cost-bms_topic2753_page1.html

*It is very cheap and very simple - low parts count. It uses single voltage reference for voltage reading and temperature reading. It uses PIC12F1822 chip. Code is written in C and it works even in my EMI noisy car.
https://mazdamx3ev.wordpress.com/tag/bms/

At first it worked to bottom balance LiFe cells to 2,8V and then disconnect when first cell reached preset value. System worked i just didnt agree with bottom balancing.

On my request Neville checked it and preprogram it to work top ballance with minimal hardware changes. Since then i adapted code for LiPo cells and i am trying to make system autonomus.
Note also that i changed bipolar smd transistor for trench mosfet. Problem was pic12F wasnt able to provide more than 125mA current to drive bipolar above 500mA shunting and so transistor was suffering since i used 3R3 5W resistors. When i chahged to mosfet i could reliably balance at 2A using 2x 3R3 resistors in paralel! Now i use 1x 3R3 and balance at 1A.

Here is another iteration of software for the same hardware. This one uses advanced error reporting and pwm balancing. It uses ASM though...
http://sourceforge.net/projects/low-cost-bms/

A


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## PStechPaul (May 1, 2012)

That's rather similar to the BMS I have been working on, and it even uses the same PIC12F1822. I plan to use an optoisolator driven by the PIC at about 20 mA, which can supply a similar current or greater depending on the CTR, which can be 50-600%. I am using an MJE170 PNP BJT, which has an hfe of about 40 at 1 amp and 20 at 2 amps, which would need base current of 25 and 100 mA respectively, but I can simply add a 5 cent 2N3904 to get full saturation up to its maximum of 3 amps. But a MOSFET is just as good, if you can find one that switches on reliably at 3 volts or so.

It is not good design practice to draw more than the absolute maximum rating of about 25 mA on any pin of a PIC, but you can use several pins in parallel for higher current. You can use a 14 or 20 pin device at about the same cost with extra I/O, which might be able to drive a BJT directly. Or you could use a Darlington.


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## Sunking (Aug 10, 2009)

arber333 said:


> I use DIY BMS designed by Neville Harlick here:
> *http://forums.aeva.asn.au/forums/low-cost-bms_topic2753_page1.html
> 
> *It is very cheap and very simple - low parts count. It uses single voltage reference for voltage reading and temperature reading.


Took a look at, but am I correct it is just for 4 cells?


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## arber333 (Dec 13, 2010)

Sunking said:


> Took a look at, but am I correct it is just for 4 cells?


Nope, those 4 SMD resistors are just in paralel to dissipate cca 500mA off one cell. It is a distributed BMS with one master. Modules do not engage untill comms are not active. That way there cant be power drain due to some EMI... 

@PStechPaul: Check the thread at the end, there is one guy who made the code with pwm and error reporting. 
Also i have some 40x empty modules at home, since i ordered 100x PCBs but if i would to source a new set i would draw another 10k resistor from mosfet gate to gnd plane, to be sure it would not be left floating in any case.
It is not a problem though, cmos pin on pic12F goes below gnd when shutting down, hence mosfet always shuts down. 

There are also ideas to implement current measuring to master, since code is like half full... Also if you would not need LCD pin A0 you could use this as input pin from LEM module hall sensor.

I was trying to built some analog voltage regulator/shunt when i found this, and there really is no excuse NOT copying design that is allready proven to work. Rather i would expand on it...

A


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## dcb (Dec 5, 2009)

arber333 said:


> I was trying to built some analog voltage regulator/shunt when i found this, and there really is no excuse NOT copying design that is allready proven to work. Rather i would expand on it...
> 
> A


It is good to see another example, and I'm always glad when people share their designs, but there is room for improvement on that design. I can spot a few things but the big one is the battery terminal bolt clamping directly on the pcb. It has a different rate of expansion, and usually you see a metal tab with a hole in it (or a lead with an eyelet) to connect to the battery terminals on more current designs. I don't know how accurate or fast it is, but there have been a few new components available since 2011.


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