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So somewhere along the line I made an idiots mistake that I'd like to avoid making again. I have a large 51.2V battery that's wrapped in shrink wrap and has wire leads soldered into a charging circuit that is soldered onto the terminals. The connections recently got crushed past being usable because one of our team members wasn't paying attention to where he was putting things. I thought that I would be safe if I only touched one terminal (i.e. one wire lead) of the battery at a time, but when I went to clip off the crushed positive connection I got a shower of sparks and a small heart attack. Thankfully I didn't get shocked, but I'd really like to figure out why it sparked so badly and what's going on.
We very carefully cut off the shrink wrap to take some pictures. Here's a folder of the pictures we took. Battery2.JPG is a picture of how each individual cell is hooked to the next one. The little red wire leads back to the charging circuit and connects as shown in Battery3.JPG. Battery4.JPG is an image of the battery as a whole. I suspect the spark shower has something to do with the charging circuit but I'm really not sure. Does anyone know how I can change the connections without killing myself?

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  • \$\begingroup\$ Was the battery on charge while you were doing this? \$\endgroup\$
    – Transistor
    Oct 3, 2016 at 17:54
  • \$\begingroup\$ It arced. That happens. You just tend to not notice it with lower voltages because the arcing distance is much shorter. \$\endgroup\$ Oct 3, 2016 at 17:57
  • \$\begingroup\$ The battery was not on the charger. It wasn't connected to anything. \$\endgroup\$
    – kpsgf7
    Oct 3, 2016 at 18:32
  • \$\begingroup\$ @TheMan_TheMyth Your claim that it was not charging or under load is not supported by the evidence that there was arcing. You need to be very careful with this battery - Short a ring across it, and it will take your finger. \$\endgroup\$ Oct 3, 2016 at 18:44

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Looking at those connections, I do not know if this will be practicable.

First, you need to know what current will be flowing. Then you need a switch which is rated at that current and that voltage for DC currents. The rating is important because you do not want the switch contacts welded together. When looking for one, you will notice that the DC rating for switches is much less than the AC rating: an AC arc will self-extinguish as the voltage goes to zero.

1) Switch the switch to the "off" position.

2) Wire the switch in parallel with the connection you want to break. Use insulated wire rated correctly for the current.

3) Switch it on.

4) Break the connection without interrupting the switch connections.

5) Switch the switch off.

6) Remove the switch and its wiring.

Reverse the sequence to reconnect.

The idea is to have the current switched by a device which can do so safely.

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  • \$\begingroup\$ I should have noted to use insulated wire. You want absolutely no chance of an accidental short circuit. Do the cutting of wires away from the batteries so that no little bits of wire can ping onto the battery or nearby circuits. \$\endgroup\$ Oct 3, 2016 at 18:54
  • \$\begingroup\$ Oh, and it's usually best to wait a while, say 24 hours, before accepting an answer so that there is more chance of someone else coming along with a better answer. If people see that there is an accepted answer they might not have a look. \$\endgroup\$ Oct 3, 2016 at 19:11
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An inductor, such as those used in chargers and switched mode regulators have nice ramp currents between switched voltages. But if you interrupt the current, the inductor resists the change in current flow by trying to bridge the gap with increasing voltage created by your disconnection. But since the gaps starts small, the voltage is low ( < 100V) and the arc can stretch with distance sustaining a wide arc but still at a low voltage but continuing the same current. As long as you weren't touching both terminals or nearby ground wire when it extinguished , no high voltage is transferred to you. ( There are exceptions )

Normally Air begins to arcs at 3kV/mm and sometimes 1kV or less with sharp points or salt spray even less. But a 10 mm arc starting from zero gap does not mean you are generating 10kV to 30kV because the arc can be stretched with low voltage drop across the arc once ignition has started.

But nevertheless when the current decays and the arc extinguishes, the voltage will still spike pretty high. ( Having fun yet?)

You can keep clear of the metal terminals and exposed conductors with 1~3 cm gap, holding the clean insulation or wear gloves (pref. rubber) or turn off loads before disconnection.

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  • \$\begingroup\$ So I'm going to be safe as long as I'm not grounded and not contacting both terminals? Will the arcing damage the battery? \$\endgroup\$
    – kpsgf7
    Oct 3, 2016 at 18:30
  • \$\begingroup\$ correct, and no damage, but shorting the battery will damage it. \$\endgroup\$ Oct 3, 2016 at 18:49

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