I'd like to create a small circuit that I can use to power my headphones off a USB port's 5V to avoid having to deal with AAA batteries (even rechargeable batteries can get lost, etc.). To do this, I need to know how much current I can expect my circuit to deal with to be within the specification that the headphones are made against, so I tried to find the maximum peak and sustained current of a AAA battery, and, to my surprise, I was unable to find it either here or on another website. What currents should I design my circuit to handle?
The max current of a standard AAA alkaline battery is... a few amps. At dead short. It does not last long. This is not a standard use, basically destructive, and is not tested for.
A standard rating of 1 amp draw is normally 1 hour or less. This is the far end of usable and you can find this on the Current vs Capacity or Lifespan graph on the datasheet for the battery you have. If it's very cheap you may not find one, so Google energizer AA alkaline datasheet. This will be a baseline "good" battery for your reference.
But its safe to say that your use case is not amps or even 1 amp unless your headphones only last 1 hour. It's probably a much lower 500 to 100 or even less mA draw.
I gather your headphones use AAA batteries, which means they operate/expect some multiple of 1.5 volts (for alkaline cells).
Your question is somewhat based on a false premise of worrying about the current that batteries supply versus a USB port. Instead you should be concerned with voltage. The load (your headphones) will draw whatever current they require at the moment, which will vary depending on what they are doing.
Power supplies and batteries (voltage sources) will try to maintain a given voltage, and the load will "pull" whatever current it needs. If the pull exceeds that of the voltage source, the voltage will drop (also called "droop" or "sag" etc.). Some power supplies, like a USB port, may shut off as a precaution, also known as "over current protection."
My suggestions for you are:
- Determine the voltage the headphones operate on. If they use two alkaline AAA cells, then the voltage is 3.0 V.
- Research how to drop/buck the 5 volts from USB to the required voltage. I wrote a Q&A on this here.
- Consider that by powering the headphones from a USB port, they will presumably become wired instead of wireless (I'm guessing they are wireless if they take batteries).
Once you've done this, post a new question that asks about a specific aspect of what you are attempting to accomplish. Hope that helps!
Procell AAA ALKALINE-MANGANESE DIOXIDE BATTERY 1200 mAh <=29 mA for 34h with load = 43 Ohms from 1.5 to 0.8V or you could same 1200 mAh/20h = 60 mA or much shorter durations with higher than suggested load currents.
Lesser than these professional AAA cells will not last as long.
If you headphone amplifier can be kept on float charge with 5V USB using a 3 terminal FET LDO, then your battery life on float voltage will last much longer and not drain much power from the USB 10W or 12W charger if the amplifier is say 1W per channel.
Your headphones probably don't use much power. I would say that you should design your circuit to supply 100 mA. I can't imagine headphones needing more than 100 mA. AAA batteries can supply 100 mA or maybe 500 mA. At higher currents they just drain too fast and are not practical.