9-volt batteries you can recharge via USB

Originally published at: 9-volt batteries you can recharge via USB | Boing Boing


What do people use 9 v batteries for these days. I think I have one smoke alarm (others now use AA) a stud finder and one of those laser measuring things. I used to make sure I had spares because I used them. These days I should probably see how old the spares are.


This is an awkward design

Not a problem.

Amazon: USB Extension Cable 1FT $6.99.



Guitar pedals, of which I will never have enough.


For us it’s smoke detectors (I think we’ve replaced most of those with 10 year battery ones),and the kitchen scale.


There are plenty with less awkward designs. EBL do some, for instance.

Multimeters, IR thermometers, bat detectors.


1200 mAH in a 9V form factor seems rather impressive. Most NiMH 9Vs are far less. Some quick Amazon review checking seems to indicate that inflation of Li-ion battery capacity with your more questionable vendors is rather common.

I’m also fairly certain that smoke detectors just like to have their battery changed every so often. I can usually take the battery out and continue to use for a good while in a multimeter or scale with no issues. I’ve even swapped batteries around in smoke detectors and had them happy again for more than a year.


This. Highly recommended.



Yeah, it’s really weird that they have it be the male end of the USB. Also it makes the whole design more complicated – a flip top with embedded connections, rather than just a hole that could be covered by a rubber flap.

Everyone already has short USB cables for charging things, many companies have already worked out to assume this and let you supply your own.

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No good for me. I only charge my batteries with a generator that I made from a discarded lawn mower engine, two mechanical pencil sharpeners, and half a gallon of Dr Flerg’s Wonder Glue.

Regardless, who the hell uses 9v batteries in this day and age…?

I’d be hesitant to use any of these lithium replacements for alkaline batteries. Alkaline batteries have a very different discharge and current profile than lithium chemistries, and you have to design your electronics for that. Devices designed for alkaline are typically going to draw more current, and draw the voltage down lower than tiny lithiums like. Furthermore the lithiums are going to be drained too low to avoid damage unless these replacements have clever electronics built into them to show the attached device a lower voltage when the battery is at when drained to a safe level.

Furthermore, since lithium chemistry produces a different voltage than alkaline, there has to be a buck booster or other form of DC DC conversion in there, especially for the 9V. That’s going to introduce another layer of EM noise and other potential issues. I’m sure they’re fine for simple devices like flashlights, but I wouldn’t be surprised if TV remotes, radios, or other more complex devices don’t like them.

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bat detector?


They convert the ultrasonic frequencies used by bats into audible frequencies. Those more expert than I are able to use them to distinguish different species, and they’re used by ecologists to confirm presence of bats in environmental impact assessments.


If you were hoping for something a bit more Batman, me too, dude.


But this is still cool.


For me, stud finders, moisture meters, metal detectors. Nowhere near as common as once were, but still useful.


Do they not just use two lithium cells to give 8.4V at full charge? This is the same as 6 NiMH cells, which is what the NiMH PP3s usually contain.

Lots of lithium batteries are packaged with battery protection (e.g. DW01 chips) to prevent this, and it’s possible that these are too.

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I certainly wouldn’t be first in line to use these on anything I was particularly attached to(especially when cheap li-ions don’t necessarily bother to meet the low bar of not being a fire hazard); but the somewhat checkered history of rechargeable 9v batteries might help them out a bit.

With Ni-Cd and NiMH “9v” batteries I found it was most common to just get 7.2v. Swap out the 6 1.5v alkaline cells for 1.2v rechargeable cells; call it good. Apparently some who cared more sacrificed a bit of capacity and some additional cost in order to add the extra one or two cells and hit either 8.4 or 9.6; though I don’t think I’ve ever run across those in person; since that approach costs more and alkaline voltage tends to slump over time so devices were often fairly tolerant of voltages on the low side.

You could hit much the same range with just two li-ions in series; so it wouldn’t be a huge surprise to find that being done in the cheap and cheerful models.

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It’s not entirely reliable(some brands use little stacked rectangular cells or not-well-sealed cylindrical ones); but some 9v batteries are a convenient source of 6x cylindrical cells that are almost exactly the right dimensions to be used as AAAAs, which have few uses but typically way less availability, especially in a pinch.

I’m not sure that I have any remaining needs for unbutchered 9v batteries; but I’ve definitely chopped a few open to feed the hunger of a powered stylus in the not too distant past.

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If you have multiple cells, then you need a charge balancing charger which is a lot of complexity and cost. Lithium cells can’t be trivially chained together the way alkalines can.

Not at this price point they aren’t.

Those battery chemistries have always had lower voltage than alkaline, but it was often close enough to work anyway. However this is why lots of older electronics say “do not use rechargeable batteries” on them. Those more primitive rechargeables were always a problem and didn’t work in a lot of things. More importantly, the current rates and discharge profile of those chemistries are a close match to alkaline. Lithium is not. It’s a totally different animal.

See above. You can’t simply chain lithiums like this. They have to be charge balanced.

And as I said, the current and discharge profiles of lithium are totally different. You have to design the electronics around this.

I love how I have to say everything twice on technical topics on this board.

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