I recently dug out one of my old tablets because I wanted to use it again, but something must have fallen on it because there’s a dent about 2-3 mm deep on the back metal casing of the tablet, kind of near the middle, definitely over the battery. Plus the battery was left completely drained for well over a year (or more, can’t remember how long) which is also not great for it.
The tablet boots up and seems to still work okay, but now I’m kind of paranoid. How high is the risk that whatever dented the back of the tablet damaged the battery and now it’s just waiting to catch on fire now that I’ve started using it and subjecting it to charge-discharge cycles? Should I also be worried about the deformed back case exerting pressure on the battery and damaging it over time as the tablet moves around? Are there any red flags I should look out for that something is not right with the battery?
Basically, can I just assume it’s nothing more than cosmetic damage? Or is there a good chance that it will burn the house down?
A tablet is mostly battery. If you can take it apart, do so and just bend it back in place.
Inside of most lithium batteries, it is basically a long set of ribbons that form a stack. They are wet like a clay and kinda oily (but still contained) on a ribbon like paper that is the width of the battery case/pocket. Then there are some layers of thin plastic that insulate the lithium ribbon.
It is not impossible that damage could occur to the center of a cell, but it is less likely unless the dent is sharp. The primary place that a cell gets damaged and where it causes problems is in the ends of the roll. If the end of the roll gets mashed, it is much more likely that layers can shorted out.
The thing to keep in mind is that something like a gasoline powered car uses a fuel mix of around 14 parts of air to 1 part of fuel. That means the atmosphere of Earth is providing a lot of your fuel requirements and it makes gasoline effectively like a super dense energy source. A lithium battery is proving all of the total energy in a single container. You don’t get to remove oxygen from the equation if things go south. You need a way to contain the situation if things go wrong.
Over discharging reduces the life and maybe some capacity, but the main issue is if it will charge at all. Most lithium batteries have a specification for charging them from fully discharged, but not all charge controllers implement the circuit block that is required. All lithium chargers (should) have a duel mode where it is current limited then voltage limited. The fully discharged state requires a very low current trickle charge until the cell hits a certain voltage before raising the current.
The main concern is localized thermal run away. If it starts getting unusually hot or expanding, you’re likely in trouble.
I’ve built robots and cat toys with lithium batteries and things like battle bots have them too. If you’re always supervising and have a container and a plan if things go wrong, you can be fine. What you can’t do is charge overnight or leave it unsupervised at all.