Hi,
I thinking about buying a rechargeable 9V battery for my MTR. I read that these type of batteries are injecting noise into the radio, because they have some kind of voltage amplifier.
Is there someone who is using this type of batteries and have you noticed additional noise from the battery?
Ivo
LZ1CLM
I have used both the Nimh and lithium 9v rechargeable batteries on my MTR 3b, homebrew QRPP transmitters and shortwave receivers and never noticed any additional noise.
They don’t last long though! Just not enough current reserve in that small package. I use 8 2800 mah “AA” cells, and they can last up to 10 hours.
Rick N8TGQ
A rechargeable battery wont generate noise. A power pack may well do so as it will typically have either some kind of buck or boost voltage regulator.
Do you have a link for the item you are talking about?
If you are talking about the “Paleblue” type usb rechargable cells, they are almost certain to use one 3.7V lipo cell and a flyback step up convertor which will be extremely noisy. As they are both cheap and small, they will be minimal i.e. have no filtering.
They might work when you try them, by luck, but that doesn’t tell you that on another day, the frequencies won’t interfere.
I wouldn’t touch them.
Update: I found a few teardowns.
A simple 2S lithium and another, with no usb charger, another was a step-up regulated 2S design, and the third a 1S step-up design
So the 2 without usb charging were simple (no noise) 2S lithium. The two with usb charge were step up (noisy) type.
One giveaway would be if the output voltage is >8.6V when fully charged e.g 9.0V, then it must have a switchmode step-up regulator.
Big Clive also notes that NiMH type “9V” batteries can have 6,7 or 8 cells, and rather different voltage.
I would assume that if well designed, a keyer draws very low current when idle. Until you draw power, there won’t be much noise from a switchmode. Most switchmode chips go discontinuous (i.e. spend most of the time OFF, when current drain is very low.
Probably putting a 220 ohm resistor across it (40mA) would be a better test for noise.
I updated my previous post. The no load voltage is probably a good test of whether it has a step up switchmode .
It is in Bulgarian language, but you can use google translate
There’s no way it can be 9v 6800mAh in a package that small. It will have a switched mode boost converter built in and it will be noisy.
I’m using this one from Amazon. No noise that I can tell on transmit or receive. I also don’t believe the 1300 mah claim. It’s more like 600 mah.
The normal Voc from a 2-cell Li-ion “9V” battery varies from 7-8.4 V depending on the state of charge. Some new brands are using a step-up converter within the package to bring the voltage up to 9V - I’m sure that some of these cause RFI. If a manufacturer claims “true 9V” on their battery, that’s a clue they’re stepping it up. Probably also bad for shelf life - to be avoided.
Thank you all for you comments.
Finally I decided to go for another type of battery with almost the same dimensions. It is 450 mA and it last like forever with my MTR
Got 2 for a QRP Pixie I was building out, they caused terrible RFI on the Pixie and a nearby AM MW radio! This was just sat on the desk, not even attached to the radio. I switched back to NiMH already. 
M0RWV
You can’t make a 9V Lithium battery, that’s not a terminal voltage the chemistry produces.
It’s a 3.7V battery with an inbuilt inverter.
