I often hear that it’s hard to find someone who’ll rebuild battery packs because it’s not worth it and can be dangerous, so it’s easier to just buy a new one. But from what I understand it’s just a series of smaller batteries all linked together?
Rebuilding seems even easier than building from scratch, just desolder the old batteries and swap them for new ones. Bing bang boom job done. …so how likely is the boom part? lol
What is it that can so easily go wrong that makes people say don’t attempt this on your own?
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An 18650 (the round batteries used in most power tools, older laptops,etc) can dump about 30A into a short. My quadcopter battery is rated at a safe discharge rate of 110A. God only knows what it would dump into a dead short. Luckily if I touch it then my bodies resistance is enough to safely limit the current.
But that’s dangerous “fire levels” of current. For comparison, the main fuse for my house is 60A.
In my house there are lots of layers of protection. Firstly everything is either double insulated or wrapped in a protective earth (You can’t protective earth a battery) then there are fuses in the equipment, fuses in the plugs, a breaker, a GFCI device, and finally that 60A fuse at the meter. With no exceptions, to work on anything, the power is removed first. This is done by unplugging the device, or throwing the breaker or pulling the main fuse. With a battery the power is there, there is no protective device that can remove power because you’re working upstream.
Furthermore, if you do manage to create a short while working the lack of protective device will mean potentially the full power stored in the battery gets let out. Everything turns to heat eventually, but if you dead short a battery the heat is only created in the parts that are resistive…. And the biggest resistor is the battery’s internal resistance. In other words, all the stored energy goes into heating the battery.
A li-ion battery is made out of combustible materials, if you get it hot enough it will set fire to itself, a lead acid battery is made from lead submerged in acid, if it gets hot enough it will boil, crack the case and spray boiling acid everywhere.
Now, that’s before you start building a battery.
The batteries charge up to 4.2v, 50 cells would be European mains voltage, just 25 would be US mains voltage. Below 50v is considered safe to work on (which is 12 cells), safe to work on means you can touch it and not risk death. It’s surprisingly easy to get a battery that can kill just by touching it.
And then we get onto arc flash….
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