Finally managed to upload the binaries. It's the same under Linux as in Windows. esptool.py --chip esp32 --port /dev/ttyUSB0 --baud 921600 --before default_reset --after hard_reset write_flash -z --flash_mode dio --flash_freq 80m --flash_size detect  0xe000 boot_app0.bin 0x1000 bootloader_qio.bin 0x10000 vfd_clock_wifi.ino.bin 0x8000 vfd_clock_wifi.ino.partitions.bin Still have a problem with the heating voltage (T3) but the clock is running fine. Thanks for reading.

salon du laboratoire Paris 1987 ou 88 Pour illustrer mes premiers dispenseurs  de la boratoire piloter par un Psion LZ programmé en OPL j'ai réaliser un piano-cocktail inspiré par le livre l'Ecume des jours de Boris Vian, ce fut un grand succés, j'ai gagné le prix de la meilleur utilisation par la société Psion -France avec cette application qui ne fut jamais commercialisé. le monde se réinvente toujours... JP  

Well, to be honest, dealing with the Arduino IDE is a pain in the a##. I am running Arduino UDE 1.8.5 on Ubuntu 16.04 and added the ESP32 DevModule like instructed on GitHub. Everything looks fine - all libraries loaded. When trying to compile I get approx. 70 lines like this: /home/mto/software-inst/arduino-1.8.5/vfd_clock_wifi/clock.ino: In function 'void Clock_Read(CMD_PROC*)': /home/mto/software-inst/arduino-1.8.5/vfd_clock_wifi/clock.ino:80:38: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]      p->emit_str_fn(p,"Time not set",0); 1. Is this the problem you are referring to? 2. How can I install the October 2017 version of the ESP32 software? 3. As an alternative how can I upload the ready made binaries you provide?    There is a Windows how-to only... Appreciate your help on this. Michael

Side note. The above discussion does not account for any issues which may be caused by an imbalance in the stack caused by different end state capacities and voltages.  For non rechargable cells this is not likely to cause any significant problems but that is another discussion.

An out of the box refinement which takes note of the law of conservation of energy would be to stack a number of 'used' batteries to achieve a net voltage greater than or equal to 5V (most likely 5 or 6 cells) lets say 6 cells.  Instead of a boost regulator use a highly efficient buck boost device with a very low voltage cutoff.  6 new cells = 9V where as 6 used cells would have a net voltage of 5 to 6 volts. Lets say that the cells are 1 volt.  The rules of battery capacity say that if you stack a number of cells which have a specific capacity, then the stack will still have the same capacity but the voltage will be the sum of the voltages of each cell. Going back to the 'debunk' response, with six cells we would have a battery with 250mah at 6V.  With the input and output voltages being roughly the same then the 250mah would become availible.  This means that, at 50ma draw you would get roughly 4 hours from the reclaimed stack.  If you started with 6 new cells the buck-boost would be in buck mode for most of the stack's life but as it switched to boost mode, the difference in input to output would allow you to get the most out of the batteries. The net of this discussion is that it's best to match the battery voltage with the use voltage and that buck is preferred (eg input voltage > output voltage).