Upgrade Headlamp to Rechargeable Lithium Battery with Very Little Cost - No More AAA Batteries 

Took it on the first test and no issues so far. Thanks for the comment and subscribing!

Qeb

I've been fixing/modifying/building my own electronics for years but only ever with cheap tools. After so many mistakes you eventually learn a thing or two.

this is very true

Only thing I can think of (since it's below the +/- 10%of 5v) the thing is speccd for is would be reversed polarity by accident? If you only touched it for long enough to fry a resistor it might be fixable.

lol, okay, so if you put it all together and it immediately fried it, then yes, you connected the battery wrong and didn't pay attention to the polarity before connecting wires(so the negative terminal of the battery would've been connected to the spot on the board that was supposed to get the positive terminal of the battery, and vice versa for the positive terminal of the battery, while some devices can work alright regardless of the polarity, its imperative that you force a habit of always making sure the positive terminals are connected to the spot with a "+" and the negative terminals always connect to the spot marked with a "-" as connecting wires without paying attention to this can very easily cause things to go very wrong, very fast, and the next thing you know your house is on fire and you just killed your neighbors pet salamander because you mixed up the wires and caused a battery to go into thermal runaway and explode on you... lithium batteries arent like AAA or AA. they wont just "pop" or leak a bit of battery acid, when they go off, even the smallest ones violently explode with a pillar of fire... no bueno) anyways, if it was working fine for you, but slowly started getting hot, and after a while started to produce the magic smoke then completely frying on you, then you, again, didn't pay attention to the batteries. only this time, you didn't pay attention to the positioning of the original AAA batteries and gave your device more power than it was made to handle. so, just because something uses 3 AAA batteries, does NOT mean the device will always use 4.5v. so, there are 2 different ways to wire multiple batteries, and both methods provide different end results and give different numbers: 1. Sequence: when you see the batteries joined in sequence(so the wiring connects the batteries with the terminals like this: + - + - + - +-.... so 3 batteries in parallel would be connected with the first battery having the positive connected to the board, the negative connected to the positive of the next battery, the negative of that battery to the positive of the next battery, and the negative of the last battery to the negative of the devices circuit board.) the batteries voltage would increase, but the current remains the same. 3x 3.7v 5000mah joined in sequence would result in a voltage(3.7vx3)= 11.1v(though most 3.7v batteries actually have about 4.2v fully charged, so you're more likely to get 12v from 3 3.7 li ion batteries FYI) and the current would not change and would remain at 5000mah. the battery would be rated at roughly 12v=5000mah. try to remember, joining in sequence will increase the power, but you will not gain any increase in the charge time of the battery. so a headlamp that gets 2 hours of use with 1 battery will still get 2 hours of use with 3 batteries joined in sequence but will be brighter 2. Parallel: when you see the batteries joined in parallel(it is exactly as the name describes. all batteries are connected with each terminals connected to the same terminal of the other battery. so the positive of all batteries will connect to each other while the negatives of all batteries will connect, and the wires that connect to the board come out of each terminal as if all batteries were 1 big battery. so 4 AAA batteries connected in parallel would look like this: all 4 positive terminals are joined together with just 1 wire coming off from anywhere on that connection. as well as all 4 negative terminals connected together in whatever way with 1` wire coming off of those connections at any point. the wire coming off the positives are connected to the positive end of the board the wire coming off he negative the same with the negative end of the board. as long as only the same polarites are connected, it doesn't matter how its done. you could connect them all side by side with a wire coming out of 1 side to be used as the terminal, or you could bundle them together and have the connections form a loop with a wire coming out of the middle for the terminal, as long as you do not have the negative tough a positive or vice versa, it doesn't matter.) when connected in parallel, youll get an increase in current, but will have no increase in power. so a headlamp with, again, 4 AAA batteries at 1.5v 1000mah each connected in parallel will result in a voltage that's still 1.5v, but the current will go from 1000mah to 3000mah.. so the headlamp would be just as bright as it would with 1 aaa, but it would last 3x as long. joining in parallel will increase the amount of charge time a device can have, but it will not make it anymore powerful. so with that info, if your headlamp worked at first, even if just for 1 minute, and then just stopped working, then my best guess is that your specific device used 3 AAA batteries joined in parallel and NOT in sequence, meaning you put a 3.7v battery in a device that would've been made to only be able to handle 1.5v...... and im sure you know enough to realize why that would be a mistake and would result in the magic smoke making an appearance hope that helps, and if you've already stopped caring in the year since you posted this, already found the answer, or really just dont want to read all that text, then i hope this helps someone else with a similar issue that happens to find themselves reading this message, that's all i got for you.