I had a car battery explode on me once when I turned the ignition. Had the bonnet open (hood for those using corrupted English…) and bits of it landed a good 100 yards away. Quite spectacular but potentially bad for your underwear.
I instantly thought of the shorting aspect of using metal calipers also when I saw the picture, but I just figured that the set up seen was just for the sake of the photo. Metal calipers could be used with an insulating spacer and then just subtract the thickness of the insulator from the measurement. The insulator could be as simple as a piece of card stock.
Wow all this talk about my picture using metal calipers. I’ll admit I almost measured the cell bare but then realized why that would be bad and put a small piece of paper on one side. The picture you see is after measurement with the caliper resting on the table (not reaching the positive terminal)
Apart from the safety issues, a fat spark could ruin the precision ground surface of the callipers, spoiling them for precision use. Put a piece of e.g Kapton tape, or ordinary Sellotape on one end first.
And do treat car batteries with respect, inside they contain an explosive atmosphere of hydrogen and oxygen. If the battery has been over-charged and “boiled” the electrolyte (acid) heavily, there may little (but highly concentrated) acid left, and most of the volume filled with explosive gas. Abuse one with e.g. a short and if an internal spark happens e.g. when a bit of the lead inside melts, or an external spark ignites lingering hydrogen, kaboom ! A substantial explosion, happened to a neighbour, car bonnet blown off and conc. sulphuric acid sprayed everywhere. Before attaching leads, ventilate thoroughly in case there is any hydrogen lingering around.
Modern “sealed” ones are safer, they have a catalyst inside to recombine the H2 and O2 to water, but it is a slow process which does not keep up with heavy charging currents, e.g. after a battery has run low, then been recharged on a drive by the full output of the alternator.
Hydrogen has one of the widest range of flammability limits, so is an extremely dangerous gas to have lingering around. And when generated by electrolysis inside a lead-acid battery the perfect explosive mixture is produced.
Anyone remember where to find the post where a 3d printed lantern diffuser was featured, with a link to the program? A friend of mine just got a printer today and I can’t seem to find the post with thread searches.