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?
In: 11
They’re not soldered together, they’re welded together.
More specifically, the leads between batteries are spot welded on.
This makes removal difficult and reuse almost impossible, pair that with the volatility of lithium batteries and you get: too dangerous to bother.
It’s only really worth it when you have a very expensive battery(like in an electric car), and one of the sections fails prematurely making the whole pack unusable.
For the original manufacturer, you already have a heavily automated production line, why bother training people to manually fix things?
For third parties, what kind of warranty can you realistically provide, when your customers only start coming to you when the original warranty expires?
Cell balancing is also an issue. you won’t be getting the full cycle life out of the new cells unless you replace every cell in the whole battery.
As for why it’s dangerous, there are high voltages present with high currents available, lithium batteries are also a fire hazard, and the’re often welded together. The packs as a whole are often glued together. Not designed to be worked on.
See what can 100 car batteries connected in parallel and in combination (series + parallel) do. https://www.youtube.com/watch?v=ywaTX-nLm6Y
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….
Because cells in parallel groups are matched to each other by their internal resistance, so that all of them experience the same voltage sag under load and thus age together more or less equally.
When you replace some of the cells in the old battery with new ones they will naturally have lower internal resistance, and will be delivering more power than the aged cells around them. This will lead to their rapid aging and overload/overheating.
The physical process itself is also quite dangerous due to the energy densities involved and the damage that an accidental short circuit might cause.
In addition to what others have said:
While taking energy from the battery pack, every battery has to give the same amount of current (when they are in a serial pack) under load. If you replace just one battery out of a pack, than it could be that it doesn´t have the same amount of charge or the same capacity. So the replaced batterie, or the other old ones, will be “empty” before the others. If you don´t stop to take energy from the pack in this moment, the “more than empty” battery does get damaged.
Latest Answers