Time to get back into some technical work
I have been working on a way to detect if a cell goes low in a LIFEPO4 pack without monitoring every cell. The method works only if the cells are top balanced. The pack I did the development work with is a 42 cell 40AH GBS pack from Elite power solutions using there basic balancer board that sits on top of the cell. This is a stand alone board with no commutation to any readout board or computer. So I have no wires coming off the cells for monitoring.

The method I proposed is based on the principle that if you top balance then when the pack starts to be drained one cell, the cell with the lowest capacity will drop in voltage first. This being said if one broke the pack into two half for measurement purposes  

Positive














Center tap














Negative


Now if we  measure the balance between the two half's when the pack is near drained one cell is going to drop in voltage first causing a measured imbalance between the two half's secondly the voltage difference between the two half's will become noisy meaning the voltage will bounce around as the cell hit there knee and drop in voltage. This setup can also detect which cell is fully charged first as it's voltage rises above the other cells again causing a pack imbalance.

This is how I set up to measure the balance between the two half's

All the resistors are .1% 100ppm drift very high precision wire wound resistors
Observations:

1. The balance between the two half of the pack has never deviated more than 20mv
2. A one volt drop in a single cell is 200 mv
3. When the pack is full charged and in balance mode I see no more than 70mv
4. I I intentionally unbalance the two half's .1watt drain on one half of the pack for 48 hours, when the pack started top balancing I observed 1 volt of unbalance, The balances did there job and the pack settled out at 70mv

For a first test to manually see if this is a viable method to detect a first cell or a series of cells at there knee voltage I concluded it is a good method to detect the point in which one should stop using a battery pack and call it drained. This method is very poor at determining state of charge and can only detect when to stop using a pack or risk cell damage. In combination with counting Watt-hr per mile to determine range I feel a almost fool proof method for a battery gauge and  protection can be developed without running wires to monitor every cell or communication links daisy chaining from cell to cell. 

Time for new batteries

I have been struggling with thundersky batteries for way to long and have decided to try a Lipo pack in the bike

Here is a picture compilation of the process from cells to pack in the bike. As of this writing she still isn't back together but will be in a few days.
The cells come from Hobby King Turngy four cell 5AH hard case packs. I'm building this up as a 8P 28S pack or 115 volts @ 40AH. We'll see how these fair over time and hopefully without any incidents

A second side benefit is the bike is losing over 50 pound of weight!

Lipo and LIFEPO4

I'm doing a interesting mixed chemistry setup that uses the best of LIFEPO4 and Lipo in a diode summed system.
My motivation in doing this stems from the fact that the 40AH Thundersky cells just don't put out enough voltage under load (Sag). For EnerTrac this makes it difficult for my customers to get a true understanding of the power and speed of the 600 series motor, when test riding my bikes. So I need a way to boost voltage under load, without replacing the complete battery pack.

 In the above picture you'll see the CA meter (lower) and a second smaller meter up top. The top meter monitors the Lipo pack current while the CA meter monitors the total system. Up to 1.5C or 60 amps, the LIFEPO4 batteries run the bike, then as current exceeds 60 amps or beyond 2C the LIPO kick in. At 100 amps the LIFEPO4 pack maintain 1.5C and the Lipo pack does the remaining 40 amps. My 40AH LIFEPO4 cells now never see much over 60 amps while my 20C rated Lipo see as much as 200 amps peak.
The Lipo are bulk charged with a separate Meanwell type power supply. Separate balance units are used as needed, they are connected in parallel with the display units.

I accomplished this by exploiting the fact that Thundersky cell have much higher IR than the Lipo cells and setting the Thundersky pack to be at a higher no load voltage. The diodes are 150 volt schocky diodes.
Testing was done on my Dyno. The box under the bike is the Lipo pack 25S 2P

 

I have only done 4 miles of road testing before the snow hit. I had the Lipo up to 100 amps while the LIFEPO4 stayed under 80 amps during this on road test.
My Dyno Currently can only do 100 amps continuous, measured at controller input. My plan is to do a video record of the display system so I can record in real time the speed, current of both batteries and then put the data in a spreadsheet.
My data so far shows temperature not to be a big problem I've had the batteries on test from 45F to 70F
My big concern is
The 4 to 1 AH ratio - is it correct, only on road testing can determine that.
Battery pack

I think the battery box weights more than the batteries.
The big challenge was getting all the hardware into the bike and still be able to get the skins on.

The bike with the skins back on

I have under belly skins that will mostly hide the Lipo battery box

Conclusion:
More data is needed but my testing to date, shows it's possible to mix two chemistry types and use the best characteristics of each respective battery type.

Mark