# Log of CAN messages between BMU and CMU boards ?

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Hello,

For my residential battery project, I would like to understand in detail how battery balancing works.

Does this balancing only occur at the end of the charge or also during use of the car?

Does anyone have logs of CAN communications between the BMU card and the CMU cards in different use cases?

Thanks

Best regards

and here might be a good start,

i think we determined that balancing can be done at any time if there are a couple of conditions. The balance voltage set point is sent and the balancing ON/OFF bits are set in the CAN buss messages.

Not sure how effective it can be if there are any significant current loads in or out of the pack, as the balancing current is very low. So i think another condition is very low to no Pack current.

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Indeed, I feel like balancing makes sense when the current going into the battery is low.
What I am trying to understand is the consumption curve of the charger at the end of charge (in pink on the following graph).
Why does the curve have this shape and does not instead take a step with a low consumption level throughout the balance phase ?

Indeed, I feel like balancing makes sense when the current going into the battery is low.
What I am trying to understand is the consumption curve of the charger at the end of charge (in pink on the following graph).
Why does the curve have this shape and does not instead take a step with a low consumption level throughout the balance phase ?

View attachment 212
The pink curve starts tapering off once cells are getting close to 4.1V to avoid overcharging, not due to balancing…

The pink curve is showing the CC/CV nature of the charge procedure. Constant Current is held until the cells rise up to the upper voltage level, then it holds Constant Voltage at that level while the current naturally decays and falls off as the cells reach full.

If balancing were needed then the charging current would drop off as shown and the pulsed balancing would burn off energy thru the resistors on the CMU boards. There is a video of the low frequency pulsing recorded with IR imaging, posted somewhere on this forum.

I wonder why they don't just charge to 80% to protect the life of the battery, at least turn off charging when it gets to that point (like it seems they do for the CHAdeMO port) and then if you wanna go further just plug it in again.

I wonder why they don't just charge to 80% to protect the life of the battery, at least turn off charging when it gets to that point (like it seems they do for the CHAdeMO port) and then if you wanna go further just plug it in again.
It's marketing (perceptions) vs. engineering. With a vehicle that has a range of only 62 miles (100km) it would not be good to have a reviewer say that it stopped charging a less than 50 miles of range. Don't forget, this is an early EV and manufacturers have (slowly) at least caught up to Tesla's on screen ability to vary the max charge number from 50%-100%.

I typically charge my i-MiEV up to 12-13 bars (~75%) and invariably keep my Tesla at 50% and its 12v battery on a float charger to let the car sleep (until just before I take off for a long trip), the latest 'ideal' setting for our chemistry for calendar aging being anything below 55%. For some pretty good references, refer to Post #21:
https://teslamotorsclub.com/tmc/thr...ng-percentage-band.287025/page-2#post-7296529
OOPS, sorry, apologies for going off-topic.

Hello,

For my residential battery project, I would like to understand in detail how battery balancing works.

Does this balancing only occur at the end of the charge or also during use of the car?

Does anyone have logs of CAN communications between the BMU card and the CMU cards in different use cases?

Thanks

Best regards
The voltage of the battery cell has an excessively charged range and an excessively discharged range. When the voltage of the battery cell is excessively charged, or excessively discharged, the battery cell should dramatically be deteriorated. For this reason, the BMU controls the battery cell not to excessively be charged or excessively be discharged. When the main battery is charged, the BMU should control the battery cell having the highest voltage not to be in excessively charged range. When the main battery is discharged, the BMU should control the battery cell having the lowest voltage not to be in excessively discharged range. When the main battery is repeatedly charged and discharged, the voltage should vary among the battery cells. If the voltage varies, some battery cells can reach the excessively charged range or the excessively discharged range faster than other battery cells. This causes the available capacity of the main battery to decrease. For this reason, the BMU drives the balancer installed in the CMU under the specified conditions to reduce the difference in the voltage among the battery cells. This allows the available capacity of the main battery to increase. The difference among the cell voltage may exceed the specified value during the regular charge. In such a case, the BMU should send the balancer drive enable signal and the target cell voltage to the CMU through the Bat-CAN. When receiving the signal, the CMU should drive the balancer so that the target cell voltage can be obtained.

 The cell monitoring unit (CMU) is mounted in each module. The battery cell voltage sensor measuring the voltage of each battery cell and the module temperature sensor measuring the temperature of the module cell are mounted in the module. There are three module temperature sensors where four battery cells are connected in series and six module temperature sensors where eight battery cells are connected in series. The signal is input into the CMU. This allows the CMU to monitor each cell conditions. The CMU has the balancer that smooths voltage variations among the battery cells

### CELL MONITORING UNIT (CMU) POWER SUPPLY CONTROL​

When the EV-ECU turns on the EV control relay, the voltage from the auxiliary battery should be supplied to each CMU to activate the CMU.

Thank you very much for your expalanations. I better understand the charge and the balancing. I will try to do the balancing by myself.

The calendar aging never quite makes sense in my feeble mind. Do other EV BMS have this slow artificial capacity reduction ? In fact, is it the reason why the iMiev requires occasional capacity calibration, fully charge from 2 bars ?

how can we disable this calendar aging "feature" ?

Unfortunately battery capacity reduction due to age and usage is a real thing especially for 1st gen EVs, fortunately it’s not as bad as Mitsu engineering had anticipated (at least for those operating in an ‘EV friendly’ climate).

While you can’t disable the model, you’re right you can correct it by running the battery calibration on a regular basis.