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.