Scope Creep - -

Kai had added a comment asking the question:

From what I understand, you also want to include cell-balancing for LiPos into this BMS. Don't you think this makes the whole project overly complex? There are so many dedicated devices for this task and only very few people are using LiPos on a boat or in an off-grid location anyways...BTW, I'm observing your projects for quite a while now. You are really doing a fantastic job, thank you! I'm in an off-grid situation with solar, batteries, an inverter and a 24v generator. A combination of your alternator regulator, bms and possibly also the mppt solar controller would be perfect for me!
All the best from Germany,
Kai

Good question, and as I started to type the reply it become obvious I could not keep my thoughts to a few lines.  So - the topic of Scope Creep. 

All projects have it.  There is an old, but perhaps not PC, saying in the Tech industry that at some point in time Marketing needs to come into the lab - gas all the Engineers and take what is setting on the bench; else nothing will every get to market.  My original DC generator started out as a simple regulator, expanded to include engine start/stop, then throttle control.  And hey, how about a remote panel as well!  So, what about this Cell balancing I poke at..

I think one will find we are rather close to a cross-over between traditional FLA batteries and LiFeP04.  Much depends on import duties and taxes, as well as if the LiFeP04 cells live up to their projected life expectancy, but given their wider usable range - as well as better charge efficiency, once might find they are very close to a Return On Investment cross over point with FLA..  This is a long way to say;  I think we will see more LiFeP04 deployments over the coming years.

But I also think there will continue to be lots of traditional lead-based storage out there as well.  Hence my general goal for all these projects:  Multi-chemestry, and 12..48v deployments.

But what about the Cell Balancing:  There seems to be a bit of dynamics in the LiFeP04 world to simply monitor, or have active balancing.  Will take time for that to settle.  My idea is to have as an option the ability to either monitor for imbalance, and/or provide for some type of active balancing capability.  This could range from simple monitoring using an existing modules such as the: http://www.cleanpowerauto.com/ modules. ( I have shared a few Emails with him.)  to doing an full monitor/balance.  

My work plan is to settle on a concept for the cell monitoring /balancing, as that will drive what hardware interface is needed in the main module.  Then develop the main module  (Battery MONITORING module) to work in conjunction with the MPPT controller and a future DC Generator  / alternator regulator.  Once all that is completed, can go back and add what every approach for the balancing / monitoring.  But in any case, it will be an add on option, and it is a 2nd tier work effort.  Just need to get a vision of direction today!

 (And again Thank you Kia for your questions:  Is nice to know some folks are out there)
: 

Sometimes waiting help - a lot...

Look what TI released just last month:  http://www.ti.com/product/bq34z100-g1

A kind of 'super INA226', it is built upon a small uC and included not only the Vbat and Amps measurement capability, but a column counter as well as a whole host of firmware to monitor and calculate a batteries SOC.  Still interfaced via I2C, and under $5

Am thinking to use this as the foundation for the monitoring portion of the BMS - add an Arduino based uC to provide CAN bus communications and controlling of the cell-management logic (See prior post:  "Central or Distributed") and external alarming - - - -  With some smart power management to reduce overhead..

Central or Distributed?

Now that I have made some progress on the MPPT controller, I have been thinking ahead towards the BMS.  At 1st I had in mind to simple create a Battery MONITORING System, as opposed to a Battery Management System - with perhaps the biggest delta being if one is able to recognize, and actively rebalance batteries who's cells have drifted apart.  (Perhaps most commonly seen in LiFeP04 deployments).

This extra capability requires some attachment to each cell in a battery, and there are two ways to do it:

1. Have a master-controller, and then on each cell place a module to measure and report back status.
2. Still have a master-controller, but instead of communicating with individual cell monitoring/management devices, have a centralized one co-exist with the master-controller. 

As always there are ++ and -- to each approach.  The Distributed approach keeps the cost of the master-controller down when there is no need to actively monitor / manage at a cell level.  Perhaps only some extra logic is needed to provide an 'expansion' port.  But the distributed system might be a bit more costly, as it would require a small uC at each cell - and there are those who comment that placing something on each cell will also, over time, cause the cells to go out of balance due to slight variations in the cell level boards...

The centrally located one is perhaps a bit cleaner, as only one (or two) wires need to be routed to the junction between each cell - as opposed to a small board and some communication wires for the distributed option.  And one could do an expansion board for the centrally located master as well, or just integrate the logic into the master and perhaps make it a depopulation option....


I think this comes down to two approaches for this project.  A Master controller with CAN, overall battery voltage and  current monitoring and then either:

• A common Cell module which can monitor voltage as well as temperature - communicating back likely over a LIN serial network.  --OR--
• An expansion board which attached to the top of the master controller and support 6 batteries.  Communicating perhaps via the SPI bus, and only able to monitor voltages of each cell (no temperatures).  Should have the ability to add up to 4 expansion boards, for 24 cells (48v battery support being a common goal across all these projects)

Either would also include the option of enabling resistive "bleed-down" cell balancing if desired / needed.

Perhaps I will mock up a couple ideas and price them out - see if that gives any additional insight.

Still Here Post

Just a quick post to let any know this project is still alive, but taken a back set to the solar MPPT controller as well as work to enhance the core regulator engine in the External Alternator Regulator as well as the DC Generator projects.

My timeline is to complete the above tasks, bring the MPPT controller to a stable state, and then begin working on a CAN based communications protocol based on the open  RV-C spec.  Then work will resume on this BMS system, with perhaps two variants:  Monitoring only, and monitoring combined with cell balancing capability.


links:

http://smartMPPT.blogspot.com/

http://arduinoAlternatorRegulator.blogspot.com/

http://smartDCgenerator.blogspot.com/

http://rv-c.com/

Some existing works.

Very few ideas are truly new or unique.  Often they are re-application of work done elsewhere, or more often - ideas and concepts independently developed (it is, after all, a very big world out there).  There are two kind of key concepts for this line of BMS / CAN charging systems:

• Ability to communicate battery status and needs out to many charging sources.
• System enforced coordination of those multiple charging sources.

The benefits of these two include:  Elimination of individual sensing wires from each charging source to the battery (We have  6x sets of voltage sensing wires attached to our house battery, 5x temperature senders..).  And the other benefit is to allow charging resources to work together in a unified way; eliminating the not uncommon teeter-tottering back and force fight between two or more independent sources as they play 'King of the Mountain'.  It also opens up the potential for more intelligent deployment of resources, ala - let the Solar panels do the final finishing charges, as opposed to keeping the generator running under a light load...


Here are a couple of examples of prior work that touches on these concepts:

The 1st illustrates the idea of using the CAN bus to inform remote devices of the battery's voltage/current/temperature status: "Distributed Power Supply Control Using CAN-Bus":
    www.ixs04.aps.anl.gov/News/Conferences/1997/icalepcs/paper97/p155.pdf


And here is a high end Solar MPPT controller who advertises a value of "Don’t waste solar power: all chargers will always be in the same state.." and more:
 

http://www.victronenergy.com/blog/2013/11/15/synchronizing-multiple-mppt-15070-charge-controllers-2/


Both of these are existing examples of the key goals of this 'systems' project, and highlight why selecting a CAN protocol is a bit difficult.  Note that the Victron MPPT controller uses its internal VE.connect protocol extensions to enable the coordinator, not NMEA-2000 - that is just used to report out the aggregated results.

Refinement of draft Schematics - BMS + a draft CAN Alternator Regulator

Along with looking at CAN standards, I have been refining the draft schematics for the BMS, and crafted up an initial cut at a simplified CAN Alternator Regulator.   Here are a couple of snap-shots; higher resolution .pdf files can be found under the Schematics resource tab above.

Click for larger, or see .PDF file in Schematics resource tab above

Major changes to the BMS include:

• Addition of small Switching PS for better energy usage.
• Revised USB chip for more widely supported (driver wise) component.  Is also simpler to solder, and lower cost!
• Improved CAN electronics, considering 'system wide' deployment.  There likely will be some additional enhancements / changes here - open to input from anyone with experience in this area.

I considered changing out the CPU for an integrated CPU/CAN device (ala, the 90CAN32), but am staying with the current solution as it appears to be able to provide lower power consumption.   In addition to more consideration on the CAN interface, I want to consider the optional BMS Cell loop - currently it is a Yes/No design in support of the Clean Power Auto cell monitors (http://www.cleanpowerauto.com/).  I want to consider how a simple protocol can be transferred over this same line to gain perhaps individual cell voltage status.  Maybe even enabling the OneWire standard - Or reusing the Auto industries LIN protocol (a kind of CAN lite) ??





I also drafted up a potential Alternator Regulator for use with the CAN system.  It is based on the Arduino Alternator Regulator (http://ArduinoAlternatorRegulator.blogspot.com/) with several key differences:

• Battery measurements are utilize the BMS
• There is no on-board remote battery voltage connectors..
• Same of battery Amps, and Temperature
• Eliminated Charge Pump (May add back), will cap max field drive to a duty cycle of 99.6%

Click for larger, or see .PDF file in Schematics resource tab above

 Some new features include:

• Isolated CAN Bus interface
• USB built in, just as the BMS is.

 It still supports 12v .. 48v batteries and fields, independent of each other.  It also includes local voltage sampling (at the Alternator) for the purpose of early detection of load-dumps, as well as fall back modes in the case of a failure in the BMS CAN bus communications.

Note that it has an isolated CAN bus, I will be key for any high current device (ala, charging sources) - as the voltage drop over even large cables can be significant.  And when paralleling the small signal gauge wires using in CAN wiring, one could end up trying to carry several amps of ground loop current.  Isolation prevents this situation.

Going forward I may look again at the Power Supply - perhaps also changing out of a switching mode PS. - one of the challenges is there is a need to pass though voltages under 12v to allow the FET driver to work.  So, and power supply design has to work in two modes:  Buck as well as pass-though....

Reviewing potential CAN protocols

CAN is largely the hardware used to connected different nodes.  On top of that there are oh so many 'protocols' which can be carried, Wikipedia has a few nicely listed here:  http://en.wikipedia.org/wiki/CAN_bus#Higher_layer_implementations


From a higher level protocol I am looking for the ability for the BMS to broadly do two things:

1. Effectively communicate the current status and health of the battery it is managing.
2. Provide information useful to external charging sources for better coordination.

#1 is the classic:  Volts, Amps, temp, Ah, SOC, etc.  But for some battery types, notably LiFePO4, would include High Voltage and Low Voltage warnings as well as alarms and cut-off / faults.  #2 contains some of the same items as #1, but extends to include desired charge states, target charge voltages and currents, etc.   There needs to be some way to address multiple charging sources, ones of different capabilities and a way to prioritize them (ala, let Solar do the final top-off charging as opposed to running the generator).   And these need to be usable for batteries deployed in a house-bank usage, contrasting directly to those deployed in Electric Vehicles.

Of the ones I have looked at (not totally in depth), here is what I have found out so far:

SAE J1939-xx

•  Set of industry specific standard developed for many segments:  Transportation, industrial, etc.
• Special interest J1939/75: Generator and Industrial
• Portions of this have been adopted by NMEA-2000.
• Includes some concepts of battery instance and Charger Instance
• Support 10mV / 100mA resolution
• Strong / well developed standard
• Closed, and costly.

OBD-II Mode#22 (aka: SAE J2190)

• Variation of J1939 / OBD-II designed for EVs.  
• Widely adopted by EV community for their BMS / Charger communications
• Includes concept of BMS directed charging sources! 
• Vbat resolution is 100mV, way too coarse for house battery usage.
• Closed, and costly.

NMEA-2000

• Built upon J1939, DeviceNet, and others - with marine specific extensions.
• Implemented by some marine equipment suppliers (notable Victron)
• Very fred DC/Battery PIDs defined - Most venders augment heavly via priority extensions..
• There is a LiFePO4 working group that has been established.
• Closed, costly.

RV-C

• Open standard, targeting RV industry
• Includes house battery and charger concepts.
• Also included Generator, Autogen functions.. 
• 50mV battery voltage resolution. . . .
• Has many characteristics inline with J1939, but not a related standard. 
• May be open to accepting extensions as needed
• Open and Free!

 

CiA  / CANopen

• Wide support and deployment, mainly in industrial applications.
• Includes modules targeting Batteries (CiA-418) and Chargers (CiA-419)
• Support for 1mV resolution!
• Can not locate any Generator functions...
• Semi-open/closed:  Free to use spec, membership needed for changes.

Each spec brings somethings, from BMS coordinated charging, cabling specs, generator integration, etc.  But some common problems include insufficient battery voltage resolution, closed / expensive standards to support.  Of them, my current thinking is along these lines:

• J1939:  May have all that is needed, but clearly not open-sourced. 
• using it would likely pick up to a large part NMEA-2000 compatibility.
• J2190: Has key concepts for BMS directed charging sources, but some problems:
• Grossly insufficient voltage resolution for House battery voltages (designed around 70-300v banks)
• Assumes simple charge then discharge usage.  Does not support simultaneous use/charge modes.
• Does not support different sized chargers.
• NMEA-2000:  really just a repackaged J1939 spec with DeviceNet connectors.
• CiA:  Promising.  May need some 'hacked in' extensions to fully support needs - lacks Generator support and not sure how would do different charging source coordination.
• RV-C:   IF changes are accepted, can have what is needed.  Plus free licensing / usage.

At this point, I am going to continue to dig into information around the J1939-75 spec, I have started working with the RV-C team on some ideas for proposed changes, and I will keep in mind CiA as well. 

Does anyone out there have any thoughts / insights into this topic???