Lipo USB rechargeable batteries in 9v form factor

[lars]

I didn't know these things even existed, but it's about time! These will be a slam dunk for guitar pedal use. They're the same size as a standard 9v battery, but have much higher mAh ratings (up to 800!). You can say goodbye to noisy/cheap wall warts. Just plug in a standard micro-usb cord to charge it. Brilliant!

[somnif]

That 800mAh is REALLY misleading. The thing has 2 420mAh lithium cells in it. Therefore, they said "well 2 times 420 is like 800!"

But LiPo cells are 3.7V. So to get 9V out you need a bit of a boost. At 9V the capacity is only about 345mAh. Which is fine for pedals, but it means you will be recharging fairly often.

The bigger problem is the boost converter will insert some noise into the system, not the dead steady output of a true 9V.

[lars]

That 800mAh is REALLY misleading. The thing has 2 420mAh lithium cells in it. Therefore, they said "well 2 times 420 is like 800!"

But LiPo cells are 3.7V. So to get 9V out you need a bit of a boost. At 9V the capacity is only about 345mAh. Which is fine for pedals, but it means you will be recharging fairly often.

The bigger problem is the boost converter will insert some noise into the system, not the dead steady output of a true 9V.

That is all disappointing (I watched the YouTube video about the tear down of one of these batteries). It has all the drawbacks you mention. It still might be a great option for a low-drain fuzz pedal, since it's capacity is about twice that of a standard alkaline 9v. It looks like "IpowerUS" lipo's don't have a booster circuit in them, so they would be a better option for noise-critical applications. Unfortunately, you can't charge those with a standard USB cord though.

[reddesert]

Red Witch makes a line of micro pedals with an internal rechargeable battery. They are kind of clever - you can use either the internal battery or run off a standard 9V DC pedal power supply. I opened one up and traced it for freestompboxes. The internal battery is a standard 3.7V lithium battery, similar form factor to what you might find in a small digicam. The pedal uses a charge pump to double this, although it only gets to ~5.8 V due to the two diode drops. (One might ask if they could have used Schottky diodes to get closer to 7 V.)

The overdrive pedal that I traced was basically just a tube screamer, but the supply / charging circuit is the more interesting part, as is cramming that all into an enclosure smaller than a 1590A. It's SMD of course. You can see the trace and gut shots at http://freestompboxes.org/viewtopic.php?f=7&t=28299