I have recently lost a bet to a friend. It was a $1, e-handshake bet, and I lost.
He bought some new fancy Motorola cell phone on Ebay with the seller warning that it didn’t charge correctly. He buys it convinced he could find a fuse, replace, and be in business. I say no fuse. I lose bet.
Do they really factory service these types of things? I’d have thought for sure they just toss bad cell phones. Repairing a surface mount fuse must expensive enough to not make a phone repair feasible, no?
Then I think of safety issues. The lithium-ion battery is pretty high powered…you wouldn’t want a short in that while it was pressed against your head! Protect against short circuit with a fuse! But where best in the circuit to place such a fuse? As close to the battery as possible, right? How about IN the battery? Put a fuse or fusible link in the battery…though expensive, people already consider them disposable. Plus, if someone, for example, puts just a bare battery in their pocket and it shorts against some keys, the burning-battery-in-pocket dance is avoided!
The question:
Are modern mobile phones ever really designed to be serviced as far as being taken apart and a soldering iron applied?
and
Are the batteries also fused?
With the proper equipment it’s possible to strip and repair a phone PCB surface mount fuse without too much hassle. I would think (WAG) the main reason the fuse is there is to prevent battery charging mishaps which can be dangerous in some rare cases.
Modern cell phones (and most other devices) are not designed to be serviced. If they had expected it to be serviced they would have probably used a picofuse instead (this is what a picofuse looks like: http://www.orvem.net/pagine/WICKMANN/picofuse.htm).
If you are really handy with a soldering iron though, surface mount devices are not impossible to deal with. All it takes is a bit of skill and a LOT of practice. All of the boards I’ve designed in the past few years had all of their components hand soldered on for the prototypes, and we have a person here who can remove and re-solder surface mount components fairly easily. I personally know many of the techniques but haven’t practiced at them, so I would be extremely likely to butcher a board if I tried to replace surface mount stuff.
I suspect a lot of the fusing of cell phone batteries comes from the threat of cell phones causing a gas pump fire. A shorted battery is one of the few ways that a cell phone actually could cause a pump fire, so they probably put a fuse there to prevent a catastrophic failure from making flames.
I should have mentioned that we are both fairly advanced electronics hobbiests and the actual disassembly and repair of the phone, including the SMT fuse swap, is fairly trivial. I’ve done plenty of these, for example. The pico fuses are through-hole. I have some devices (two-way radios) that use those too. The bet was to the existance of a fuse at all.
So why wouldn’t they instead fuse the battery itself? If the worst case happens (intentionally shorted while pumping fuel near a leaky propane tank at an explosives factory), the battery would just shut off. The further into the power circuit the fuse is, the more chances for non-fused shorts. Also, it can’t be uncommon for someone to carry a spare battery. A loose battery is begging to have its contacts shorted. The fuse or fusible link in the battery covers all the possibilities while protecting the phone and the user.