BMS design guidelines

Thanks for starting this thread. The book on Amazon looks very good, and I see that the author is Davide Andrea of Elithion LLC. Having a comprehensive book such as this can be valuable, although in the electronics field the material can quickly become outdated. How much has changed since 2010, when it was published? And it was probably mostly written in 2008-2009.

Another good source of information is companies who make batteries and those who offer ICs for various aspects of battery management. Some such sources are:
http://www.linear.com/products/battery_management
http://www.maximintegrated.com/products/power/battery_management/
http://www.ti.com/lit/sg/slyt420/slyt420.pdf
http://liionbms.com/php/index.php (Elithion)
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2087
http://www.freescale.com/webapp/sps/site/taxonomy.jsp?code=BATTMNGT
http://minibms.mybigcommerce.com/

I have several ideas which may be realized as separate designs or combined into a complete system

1. A simple single cell monitor which presents a minimal current drain (10-100 uA) and flashes one or more LEDs showing SOC based on cell voltage from 2V to 5V. Target cost < $5/cell.
2. Single cell monitor and charge/discharge controller. Flashing LEDs for indication of SOC. Bypass charge current upon maximum voltage. Isolated signal to charger to taper off charge and shut down when all cells fully charged and balanced. Open cell detection and protection: For charging by shunting charge current and sending shutoff signal to charger. For discharging avoid cell reversal by detecting low voltage and activating bypass circuit, and/or using power diode, and sending shutoff signal to controller and main interlock. Target cost < $10/cell.
3. Single cell analyzer, which can perform a pre-programmed sequence of charging and discharging at various rates and store data or send to PC via USB connection for further analysis. Test profile can also be changed and controlled by USB. Target cost $50-$200 depending on cell A-h rating and complexity of software.
4. Integrated modular battery pack for small cylindrical cells. Package 12 cells in series for 38.4V Li-Ion or 44.4V LiFePO4. Internal relay disconnects module under internal fault conditions, and requires external 12V signal to activate. Eight modules in series supplies 307V or 355V for 230 VAC VFD, or 15 in series for 576V or 666V for 460 VAC VFD. Modules may be connected in series or parallel for various voltage and current capacity.

Those are my ideas at this time. I want to do some testing on the inexpensive Li-Ion 18650 cells I found, as well as the NiMH which may be a viable alternative. Another version of these devices could be made for 12V SLAs and Flooded Lead batteries. My particular immediate interest is for use in small vehicles such as lawn tractors, wheelchairs, ATVs, and bikes, but may be scaled up for larger lithium cells of 20-100+ Ah capacity for road vehicles such as cars and trucks. I am focusing on standard industrial 3 phase motors so the battery pack voltage will be 300-600 VDC nominal, and 2-5 HP motors drawing 3 to 10 amps where one or two strings of 3600 Ah cells would be sufficient.

Here is a very simple and cheap single cell BMS which uses only about $3 in components and can be adjusted to limit charge to any voltage from about 3V to 4.5V. Here it is set to 3.62 volts.



The functionality of the circuit could be replicated with much higher accuracy using a small PIC, with little difference in cost, and it can have a better SOC indicator and shutdown signal. The ASC file for this design is available at: http://enginuitysystems.com/pix/ShuntChargeLimiter_NMOS_PNP.asc

For the simple circuit I show above, during charging, if a cell opens up, the voltage is held at about 4 volts and the shunt resistor and MOSFET draw up to 20A. This voltage would be maintained as long as the charger were properly current limited, and of course this would be a fault condition which would be detected by the BMS (assuming it uses a PIC or other processor), and it would signal the charger to disconnect within 100 mSec or so.

In the case of a 150V pack with a 1 ohm (150A) load, the MOSFET reverse diode would conduct, and the 0.2 ohm shunt resistor would limit the current to 130 amps and there would be -27V across the cell BMS. The resistor would dissipate 3000 watts and the MOSFET would see 143 watts. But it would be easy to add an optoisolator so that any reverse voltage would turn it on and send a signal to the main interlock contactor to shut down. This would happen as soon as the voltage reached about -3V, at which point the shunt resistor saw about 12 amps which would be 28 watts, and the MOSFET would see 9 watts. If the cell opened gradually (as in 100 mSec or so), the voltage would rise slowly enough for the fault protection circuitry to shut down the pack before severe damage (or any danage) would occur.

Another safeguard would be a TVS diode across each cell that would limit the voltage to 6 volts or so. Here is a 3000W TVS that can withstand a pulse current of 326 amps and costs less than $1.
http://www.mouser.com/ProductDetail...=sGAEpiMZZMsYiK5PgaDog9xriego/BKRX2LxccMvWjs=

Or a 5000W version that can handle over 500A for about $2 in quantity:
http://www.mouser.com/ProductDetail...EpiMZZMsYiK5PgaDog38p0a8Da%2bhYvYJr%2bz8m5XI=

A 3.5 volt version may be better:
http://www.mouser.com/ProductDetail...GAEpiMZZMsYiK5PgaDogyJ4ny/JxXeZY8cFkHLu%2blg=

What happens when a cell opens without any BMS or other form of protection? Or with a commercially available BMS?

I just found this resource that seems rather comprehensive and worthwhile looking at. I found it when searching for "open cell protection lithium battery pack EV".

http://www.nfpa.org/assets/files/PDF/Research/RFLithiumIonBatteriesHazard.pdf

Some other links with useful info:

http://www.o2micro.com/prods/o2m_intbattery_catalog.pdf
http://www.ipd.anl.gov/anlpubs/2011/10/71302.pdf
http://www.ibt-power.com/Battery_packs/Li_Ion/Lithium_ion_tech.html
http://www.eosenergystorage.com/documents/2012_JES_Christensen_Kojic_Critical_Review_Li-air.pdf (Lithium-Air cells)
http://www.batteryspace.com/prod-specs/6059.pdf (testing)
http://industrial.panasonic.com/www-data/pdf/ACI4000/ACI4000PE5.pdf
http://www.transportation.anl.gov/b...oundtable/battery_mgmt_system_tsinghua_lu.pdf
http://www.mpoweruk.com/protection.htm (also many other topics)
http://www.transportation.anl.gov/pdfs/TA/149.pdf (costs)
http://www-scf.usc.edu/~rzhao/LFP_study.pdf
http://www.evolveelectrics.com/Lithiumate_Lite.html (Elithion BMS)
http://www.te.com/content/dam/te/gl.../knowledge-center/documents/mhp-techpaper.pdf
http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/apesaran.pdf (thermal issues)
http://www.batteryfaq.net/battery-news/how-much-do-you-know-batterys-protection-circuits-1056.html
http://ecec.mne.psu.edu/Pubs/2006-Smith-JPS-1.pdf (power and thermal)

and the list goes on. There is a huge amount of information out there, but really not very much specifically addressing the problem of open cell failure.

I think the 1/2 amp bypass specification is only for charge equalization, where it removes, say, 5% of total charge current of 10 amps until other cells with greater capacity reach maximum SOC. But if a cell opens, with a constant current source, there will be a voltage imposed on the open cell which will be the full charger compliance voltage less the voltage of the intact cells. So a battery charger which uses a 170 VDC peak voltage (from a 120 VAC rectified line voltage), on a 120V nominal pack with 38 cells at 3.2V each, will have 170-(37*3.2) = 51.6V applied to it. If the rest of the pack cells have internal resistance of 0.05 ohms (probably much less), it only takes 20*1.9 = 38 VDC to maintain a 10 amp charge.

The situation is much more dire in the case of discharge during maximum current draw, where the entire remaining pack voltage can be applied to the open cell, and the inductance of the motor could generate a much higher voltage. So it is important to have a fast passive means to maintain continuity of the circuit during fault conditions, long enough for a secondary means of shutdown can be activated.

As I explained before, having multiple strings of cells in parallel eliminates most of the problem by providing auxiliary paths for the current, and a simple disconnect of a defective string, or bypass of an open cell, can permit continued use of the EV until the defective cell can be replaced.

I have made an initial design of a single cell BMS system:



It has a shunt MOSFET and a 0.47 ohm power resistor which is designed to limit charging voltage to a voltage determined by the PIC, and it can shunt up to 7 amps before the voltage will climb above 3.3 volts. The top balance voltage can be programmed to anything from 3V to 4.5V, and at that point it will start shunting the charge current and also will activate the optoisolator which can shut down the charger. It is possible to connect the optoisolator outputs in series so that shutdown will occur only when all cells have been balanced, or if connected in parallel the first cell to reach maximum voltage will shut down the charge. There is also a visual indication from the red LED.

If a reverse voltage is applied, as may happen if a cell is depleted during discharge, the MOSFET reverse diode will conduct and the current will be shunted by the 0.47 ohm resistor. It will also activate the optoisolator which should be connected so as to shut down the pack. Of course, in normal operation, the PIC would sense a drop in cell voltage before reversal occurred, and it would flag the error and shut down. But in case of a sudden open circuit of a cell, it would also protect the system by limiting the voltage.

The green LED is used as a status indicator. It can be set up to flash at a variable rate to indicate battery voltage. Thus during charging it can show the progress as the voltage increases, or it can also use the time elapsed as a measure of SOC. This design does not have a current sensor, so it cannot comput Ah, but that is more a function of the charger.

In the absense of charging current, the PIC is powered from the cell, and can provide a flashing green LED signal to indicate open circuit voltage. The PIC itself draws only about 25 uA when operating and as little as 20 nA standby. The LED is visible at current as low as 2 mA and it can have a duty cycle of 100 mSec/10 sec which is the equivalent of 20 uA. So an average draw of 50 uA on a 3 Ah battery would drain it to 90% capacity in 6000 hours. It could also be programmed to enter standby mode if the open cell voltage was less than, say, 2.8 volts, so it would not deplete the battery even after years of storage.

The total cost of this design is mostly the PIC ($0.50), MOSFET ($0.75), Optoisolator ($0.25), and power resistor ($0.50). The other components are mostly penny and nickle parts or maybe $0.25 total, and the PCB would be about $1 in 100pc quantity. So about $3.25 total. Of course this is designed for a small pack of 3-10 Ah capacity, but the design could be scaled up to handle 40-200 Ah cells.

This is still in its "infancy" so it may change as I dig into details. Also I have chosen the smallest and cheapest PIC I could find, just to see if it could work, and I have a couple of development boards using this part. A more capable part with more I/O pins would still be less than $1. I also need to deal with the problem of reading the cell voltage which will be the same as or greater than the supply voltage. A voltage divider would be the obvious answer, but the ADC requires a maximum source impedance of 10kohms, which would be 320 uA on a 3.2 volt cell. Probably still OK. But I could use an external MOSFET to connect the ground side of the divider only during measurement. Such things are part of the challenge and allure of electronic design. So please tell me any pros and cons of this circuit, and any features that might be desired.

Thanks.