It's OK if the current rating is higher than what your phone can use or old charger can provide. The phone won't pull more current than it's capable of handling. You also don't want the voltage to be below. You want it to be the same. Considering that most all smartphones use USB for charging you won't have to worry about the voltage. If it's USB it's either 5V or counterfeit. There are no legitimate USB chargers that give anything except 5V. You'd have to buy your charger from a non-trusted source and it's pretty much universally recommended to get your charger from someplace you trust and to buy a brand you know or have researched.
FWIW, what actually happens is there's a circuit in your phone that takes the 5V from the USB, converts it to ~4.2V that the battery uses and then manages how the battery is charged to ensure it's charged safely. That circuit determines how much current is being drawn, not the charger.
I think most of you are missing the point and as usual, it seems in these blogs, I hear a lot of people talking but no one is actually answering the question. Garublador has it partially correct, there is a governer controlling the actual load your phone is taking on so the adapter is not generally speaking, going to, fry your battery, because most micro USB chargers are fairly standardized these days . However, there is a good reason the manufacturer recommends never using another charger. Its not because the charger can inherently overload your phone, but because In the event that the wrong charger is not up to par on the standard (or of a different one altogether), the adapter itself (which also has a small chip) may not send the correct return signal back to the phone (letting it know that "I'm only using my Samsung charger with you galaxy") which in some cases results in the governer not being activated, and the failsafe being bypassed. In which case there is the possibility that you're phone will instead try to "drink all the juice it can get" which on a 2a charger (because this is already set for very close to max load) would normally cause the battery to take on absolute maximum, get hot and lose permanent life (the memory of the battery). What It sounds like is going on here is that the bb adapter is not causing the governer to wake up resulting in the phone trying to take more juice than the adapters 750ma (Max load) rating (this is a rating just like you would see for say, an extension cord, NOT what it pushes to the phone), if this is indeed the case, then it will be the bb adapter that gets hot, not your battery. I also believe you have a second issue which is that, in my experience, it's usually the cord that is actually at fault, not the adapter, when s4 came out, everybody was getting on board finally with the USB 2.0 standard (which is what both the s4 and it's matching adapter were rated for). However, it seems Samsung (at least through Sprint) had a lot of leftover USB 1.0 cables to get rid of (or someone had a brain fart at the charger packaging meeting), I encountered this myself as I also got an optional tab3 with my s4 purchase, with an identical looking adapter and cable which I was to later find out when using them interchangeably was indeed the same adapter but only a USB 1.0 cord, which charges much more slowly. Don't quote me on the rest of this because I'm going mostly off of "if I remember correctly" statements but what I've already told you should answer most previous questions and this will partially answer some more. The way I understand it, when the USB 1 arrived, the USB cord was designed with 5 contacts, 4pin plus chassis (ground?), however only 3 contacts were actually being used, I believe #'s 1,2, and 5, with 1 and 5 used for charging and only one data cable, pin 2(I Don't know if the casing is considered pin 1 or 5 but I'm sure it's one of the two. So you had 3 working pins/wires out of 5 but the other 2 were non functional because unless you were buying really expensive cables, as a shortcut most manufacturers didn't even include the last 2 wires in the cables as there wasn't even a perceived use for them
on the horizon for a long time. When 2.0 came out There were now 3 contacts capable of being used for charging while also 2 or 3 we're used for data (the contacts being multi-functional at this point) but still had I think one unused contact (and they still managed to squeeze in MHL and a few other neat tricks!) Now we have 3.0 which I believe uses all 5 for both charging and data. So an older (or cheapo lt. Guage garbage wire) cable is most likely going to be the problem, not usually the adapter, although I do think it played a role here as far as the bb being the wrong one. There is one other possibility,I believe the s4 was already designed to accept 3.0 standard as it was getting ready to roll out anytime, so it's also possible that your bb charger is actually 3.0.
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