Here are some photos from today's effort with Coulomb and Newton. They show the MX-ϟ's eight battery boxes, containing all 228 40 Ah cells. They are all clamped up and have Battery Monitoring Units (BMUs) on them.
The BMUs and high current links are held onto the cells with M6 stainless bolts with a belleville washer and a flat washer. The belleville replaces the usual spring washer. The force required to bottom out the conical belleville washer is 3 to 4 times that of the split spring washer. The lowest online price I could find for the bellevilles was from Blackwoods. Even when ordering 500 they were nearly 30 cents each, so I hope they're worth it.
The box of 60 cells on the trolley on the floor have been fully charged and top balanced. The set on the round table (54 + 28) are being charged at present.
The charge is being controlled by a modified Tritium Driver Controls Unit (DCU). We have added four Industrial Fibre Optic (IFO) ports to it (2 in, 2 out) and modified the DCU software. One pair of IFOs connect to the BMS, the other to the Elcon/TC Charger.
The BMS sends regular status bytes to the DCU. These contain a "stress level" between 0 and 15 and an "all-in-bypass" bit. The stress level is that of the most stressed cell in the pack. Stress can be due to voltage or temperature or even the voltage drop across the links between cells. But in normal charging it will depend only on voltage. One stress level for every 10 mV above 3.500 V.
Only stress levels of 8 or more are considered "dis-stress". And only levels of 11 or more are considered "alarming distress" which means the BMU turns on it red light and beeps. Stress 7 or less can be called "eustress".
A lot of time in the past few weeks has been spent developing and debugging the new charging software (which doesn't photograph well).
To control the charger, we've implemented a version of a suggestion by Tritium_James, to run a PID loop
on the highest cell's voltage, with a set-point just above the voltage at which bypass turns on, and terminating when the lowest cell's voltage reaches the bypass level. However we don't run the loop on max voltage per se, but the 4 bit stress level, and we don't terminate on min voltage per se, but the single all-in-bypass bit.
Controlling stress, rather than voltage, will automatically cause the charger to back off the current if any cell or BMU gets hot, or if there's a high resistance joint between cells.
Bypass turns on at stress 6 (> 3.56 V). The set-point is at stress 7 and so has a dead-band from 3.57 to 3.58 V.
The same PID controller (with different parameters) will be used to automatically back off the drive or regen current from the motor controller (Tritium WaveSculptor 200) when any cell is distressed.
One of the fathers of MeXy the electric MX-5, along with Coulomb and Newton (Jeff Owen).