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[Lipo423]

Colopaul,

The new cells will "work harder" than the old ones...for this reason battery soldering/configuration/cells position/consistent temperature amongst them is a key element for a successful battery pack performance...
Since the cells will discharge at different rates, the first cell to reach the minimum voltage will trigger the detection circuit in the BMS, which shuts off power. The BMS has a balancing circuit but it does not have the ability to bring up a cell that is a lot lower than the others
This is one of the reasons to balance the pack all the time (and from time to time leave the charger plugged for a few days)

[Electric Terry]

After looking at all the data closer, I am positive the reason for the reduced range and cutouts is a problem with the fuel guage and not that one cell is empty when others still have 50-75% SOC remaining.  While a cell in bank 14 and 15 may have slightly reduced capacity, the others are at 3.4v while the lowest cells are 3.0v to 3.2.  However looking at this graph, the energy left below 3.4 is minimal.  There should not be 6-7 bars left when the highest cell is 3.4v. 

[protomech]

There should not be 6-7 bars left when the highest cell is 3.4v.

Just a thought. The battery voltage will sag a bit under heavy discharge, and it may take a while to recover. I believe the older (2010 and before?) Zero bikes used just the battery voltage to determine capacity .. I think the newer bikes use a combination of coloumb counting and voltage monitoring to determine capacity. (I know I've come back to the bike after work and seen an extra bar remaining).

Full throttle on a ZF9 is probably 3-4C discharge, which could (per that chart) drop you to 3.4V at 60% DOD. Capacity bars don't really correspond well to DOD (or SOC), but they should at least be in the general ballpark.

I think when I did a range test of my bike a while back, it was reading 3.5V per cell at around 5% SOC. The last 10% SOC discharge was under very light usage, basically lapping at 25 mph to fully discharge the pack. So 3.4V does seem to be very low, especially if you have avoided significant discharge.

[ColoPaul]

After looking at all the data closer, I am positive the reason for the reduced range and cutouts is a problem with the fuel guage and not that one cell is empty when others still have 50-75% SOC remaining.  While a cell in bank 14 and 15 may have slightly reduced capacity, the others are at 3.4v while the lowest cells are 3.0v to 3.2.  However looking at this graph, the energy left below 3.4 is minimal.  There should not be 6-7 bars left when the highest cell is 3.4v. 

   But you started this thread with the following quote.   

So I was confused the past few weeks when I seem to be losing range.  A lot.  Tonight from a full charge, the bike quit with only 26 miles on the odometer with city riding (no highway or speeds over 55).  A ZF9 remember.

So if there isn't one or two bad cells, are you saying the pack as a whole has lost 75% (26 miles city riding versus 100 miles expected) of it's storage capability?

[Electric Terry]

No I think the bike wasn't fully charged or even close, yet the fuel guage read full.  I have been confused that the green lights never go solid or even flash on the chargers when the fuel guage reads full and charges for an extra 15-20 minutes even.  I also thought it was charging too quickly at 1 kw the other day.  It read full after 3 hours and I was expecting 4-5 bars.  I'll bet there really was only 4-5 bars and somehow the fuel guage calibration is confused somehow.

I'm just gonna take a volt meter and use the anderson connector port on the side (negative has the "T") to look for at least 74 volts, full is 74.7, regardless of how many bars show.   I'll do this for a bit and report back.

[Doctorbass]

IF YOUR PACK WARRANTY IS EXPIRED OR THAT THE COST OF A NEW PACK IS TOO HIGH FOR YOU, THEN HERE IS SOME GREAT SOLUTIONS FOR YOU:

From my opinion and my 6 years experience with lithium pack building, when it happen to have low bank you have two choices:
-Get  replacement bank ( the age diff of the bank will not matter so much) and the pack will balance better and get closer to his supposed capacity anyway

or 

-Solder one or multiple parallel single cells of the same chemistry to the low bank from the proportion of the missing Ah your low bank is missing to be equal to the rest.
I tested that method and it work well.

The Internal resistance and age of the cell dont really matter since it's only a little % of the total cell bank.. but you'll recover a certainly better pack health than keeping the low bank as is!

Let say your average voltage of the rest of the good cell bank is at 3.4V while the low cell bank are at 3.0V, you can determine the equivalent capacity is missing from 3.0V to 3.4V. Then find the single cell that have a capacity closer to that calculated capacity, parallel it to the low bank and it will be ok!

The total energy recovered  you will get will be the added energy of cell time the number of total bank..

ex:  if you need to add two parallel cell of 2.9Ah ( 5.8Ah total) to the bank no 15, you will get the Ah of the total pack voltage wich mean 5.8Ah x 67Vnom ( 18s)   = 104Wh energy. That's the gain you would get in these conditions from repairing the low cell.

So recovering the missing Ah of a single bank mean recovering the total pack Ah, so adding few cells in parallel will do a big job and is not so difficult to acheive... determine the missing Ah, solder the cells to the low bank ,  fully charge and let balance your pack and it's done!

Doc

[Lipo423]

From my opinion/experience with Lithium I would support Doctorbass option 1 -worst scenario- however, I still would advice you to let ZERO repair the pack.

Negotiate the shipment cost/repairing timing with ZERO and get it back repaired in 3-4 days (this is the cost of a potential $8-9.000 headache)

[protomech]

Agree. If the bike was out of warranty, certainly there would be options to tinker with it and explore different repair routes.

It's not out of warranty. Get Zero involved, if they can't reach a solution remotely then they should either ship the pack back or the bike itself - either way it will be going via ground transport.

[Doctorbass]

I Just corrected my suggestions about  pack under warranty

People famillar with RC hobby and lipo  or with a well knowledge in elecrtonic or physics will have more chance to succed as well.

Doc

[Electric Terry]

Great suggestions Doc!  I'm running a few tests now and will report back after the weekend.  I have to ride 500+ miles on the bike to the Southeastern Renewable Energy Conference this weekend.  I'm measuring energy used, pack voltage, and individual cell voltages, every leg of the trip very carefully.

[Electric Terry]

After resetting the BMS, the fuel gauge is more accurate and not leaving me stranded anymore.  It does appear at least one cell in banks 14 and 15 are a little weaker than the others, but not significant.  The range is back up again close to what i remember.  Something with the BMS was letting me ride with 2 or 3 bars until a cell hit 2.5 volts, the lowest ever according to my log is 2.3 volts, and then the bike would instantly cut out without warning and wherever I was I had to push it to the nearest 110 plug.

But since the BMS reset the fuel gauge drops accordingly so when the low cell hits 3.3-3.4 volts or so, the bars are all gone just like they used to be.

[ColoPaul]

After resetting the BMS, the fuel gauge is more accurate and not leaving me stranded anymore. 

Which method did you use to do the BMS reset?

[Electric Terry]

I pressed the button to the left of the serial port behind the rubber plug