Theoretical understanding and practical application are very different.
Regardless of the exact cause, this reads a bit like a horror story. Beside the shock thing, the first 'accident' was spiking the cell above 12v. I can't ever see calculating charge times before hand to be a viable, long-term 'free lumens' plan. Maybe I'm too ignorant of this area to see the deeper implications, but i'm completely unconvinced by any of the 'safety' measures described.
Some places engineer for 4 levels of safety, each action in a sequence works to protect you during the subsequent steps. Even if a single step fails completely it is so contained by the previous actions that it is not possible for any harm to come. A person would have to fail three procedures before the next step even exposes them to personal risk. Ideally the system is designed that you may not proceed unless the previous safeguard has been successfuly initiated. In this ideology, you have to perform a complex set of actions to successfully put yourself at risk.
In your current set-up, you have to perform a complicated set of actions to remain safe.
Way too true... my setup was the result of rushed work and ignorance into how dangerous the setup was... no excuse there.
In an ideal setup (and what I will do from now on if I need to work with these voltages again (aka when I need to))
*Operate the supply with thermocouple attached to charging medium to prevent overcharge even in case operator is distracted. This would be level 1. If I had this the supply would have shut down once the cell started bleeding off the extra charge.
* wear electrician gloves... I have 1000volt ones siting here :shakehead If I wore them the whole thing minus the ruined cell would have been avoided... but then maybe I wouldn't have learned as quickly. The reason I didn't put them on (I was too embarrassed/ashamed to even say I had them) was that I looked at the voltage and just reacted, not looking behind me to grab the gloves, a pair of pliers with 2000v insulated grips...anything which would have helped). I have the equipment but I was scared I didn't want a lithium fire as I have no class D extinguisher and don't live alone. This would be the second line of defense (the gloves)
*put a quick-blow fuse inline so that even if the leads are shorted through something the power cuts instantly. Right now the supply shuts down if shorted, however, a very fast acting fuse would be safer, plus it's very easy to install such a fuse. That's level three.
*Use one hand... this is hardly a level but a great way not to risk your life
I found three steps I could have taken which wouldn't have required a heck of a lot of setup and would all have prevented this.
Also, I calculated the charge time (not that it really matters but interesting to look into my planning) assuming 100mah capacity left in cell, 750mah full capacity, charging efficiency of 99%, Vin of 4.00 volts average and Iin of 300ma. I guess I could have integrated the whole thing as a function of voltage (which would rise), however I was lazy and assumed 4 volts. This means the cell is receiving
1.188 watts / hr of charge
The capacity of the cell is 750ma at 3.7 nominal or 2.775 watt hours
assuming I need 650maH to fully charge that is 2.405wh needed / 1.188 watt/he = 2.02hrs of charge time. Total charge till accident was around 1hr 15 minutes... well within what I calculated
Where I went wrong is that I assumed 100maH left because I ran the cell to cutoff, however this was at a rate of 1c and thus the cell actually had around 20% capacity left (assuming 3.3 volts is dead, 3.8 is 50% charge and 4.2 is 100% SOC). Add to that the cells are 2 years old and probably don't have 750mah of capacity full and there is the problem.
... but what really did it was that as the cell warmed up the internal resistance of the cell decreased which consequently increased the charge voltage, which at constant current meant more watts which heated the cell more... positive feedback loop :-(
Anyways, that's my long explanation justifying my stupidity
TYDR: I'm going to use my 0-15 volt supply to only charge SLA's, the lithium batteries stick to the lithium chargers