Do these work in fire and CO detectors?

If the detectors use 9V batteries, they'll work. They're standard 9V batteries, just instead of NiMH or acid, they are lithium ion usb rechargeable.

Is the 9v battery good for voltage-sensitive devices? ie does it operate at 9v or closer to 8v?

This is a lithium ion battery and the chemical component of these batteries will outlast the alkaline batteries that we normally use. the charge will remain linear for a certain period in relation to what equipment we use. for high discharge rate equipments like motors depending on how we use,or use in continuous manner it will last for around 3 hours depending on my experience and you can recharge again.the voltage drop is sudden so do not overuse it.when you feel or sense something that is unusual recharge immediately.

Do the 9 volt batteries still charge from a 5 volt charger?

No, they will not. The specific charger for a 9volt steps up the 5v usb to aprox 10v to charge the batery. Much like a 12v car battery; charges at 13.8v giving a surface charge of 12.5v

Does the 9V battery have a boost circuit inside to provide a constant 9.0V output, or is the output just two cells in series for 8.40V maximum voltag?

I don't know what hardware is inside the batt as I did not open them. but I test the battery in a Multimeter seen to be giving out the correct amp & voltage, also the multi uses 9v seen to be working good and I use it a lot. Also, the stud finder is a hug, will kill a commercial 9v within hours, the 9v has worked wonderfully. now if they have the same hardware inside as the AAA yes I see then having issue delivering the same continuous current to your device especially if the device pulls a lot. I used the AAA on group of 3 on a headlamp and for the first 5-10 minutes I have a good light bright then dims down to low light, I left it on to fully drain the batteries for over 24hr still on so at least that is a plus.

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🔋 Power up fast, save the planet faster!

ECO WARRIOR BATTERY LIFE - Over 1000 recharge cycles means you’re ditching thousands of disposables.
2 IN 1 USB TO USB C CABLE - Charge up to 4 batteries simultaneously with one versatile cable.
ULTRA FAST USB C CHARGING - Get fully powered in under 1 hour—no more waiting around.
SMART LED CHARGE INDICATOR - Stay in the know with intuitive LED status—charge smarter, not harder.
SUPERIOR LITHIUM ION POWER - Stable voltage and current outperform traditional alkaline and NiMH batteries.

Pale Blue’s Rechargeable 9V Smart Batteries deliver 500mAh of reliable lithium-ion power with rapid USB-C charging under 1 hour. Designed to outlast and outperform alkaline batteries, each battery supports over 1000 recharge cycles, reducing waste and saving money. The included 2-in-1 USB to USB-C cable charges up to 4 batteries at once, while the LED indicator keeps you informed of charging status. Perfect for eco-conscious professionals seeking efficiency and sustainability.

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