[time-nuts] Low noise power supplies?

gary lists at lazygranch.com
Thu Jan 31 07:11:44 UTC 2013


I've measured AA nicad impedance at a few kHz, but it was years ago. My 
recollection is it was in tens of milliohms.

ESR does have a frequency dependency. In the same time frame, I was 
trying to find a simple non-instrusive way to detect alkaline cells 
versus nicads. The key was in impedance versus frequency curve. It 
looked more than a bit hair brained, and given that there are no 
consumer devices that do automatic detection today, I gather no one else 
found an elegant solution.

But low ESR may not translate to low noise. Yeah, I know everyone is 
thinking thermal noise, but a battery is a chemical device. You can 
detect gas bubbles from electrical artifacts in some types of batteries. 
At least under high discharge. Also the impedance of nicads increases 
when they get old. I spent a ridiculous amount of time dealing with a 
customer that found some old batteries that were not being detected by 
the safety scheme built into a particular chip.

Lastly, the chemistry in these batteries isn't constant. Once you have 
one battery technology nailed down, they increase the capacity and it is 
a new ballgame. That lead to "chemistry independent" charging schemes 
where the chargers had uP interface so that the charging could be 
tweaked by the customer.

Personally, if the device you are using needs a power supply from god, 
it wasn't designed well. The manufacturer should incorporate the 
required regulation in the product so that it meets spec with run of the 
mill supplies. Nobody likes to spend time debugging some customer 
complaint due to a component outside your product.


On 1/30/2013 9:44 PM, Arthur Dent wrote:
>> But the only NiCd I know about are the AA sized ones.
>
> I have some of the wet NiCd batteries that are capable of
> putting out 200A continuously. I'm assuming the internal
> resistance is pretty low. ;-)
>
> Arthur
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