Tír Na HiFi

"A battery filters down to DC. Even if the battery has .1 ohm resistance what value capacitor do you need to equal that? Answer infinite.

The definition of capacitance is C=Q/V. The important part of the first derivative is C x dV/dT = i. Now most folks get that F = 1/T. So they assume that if the lowest audio frequency is 20 hertz then dT = .05. As this is a power supply not a series pass use that is too simplistic. The power supply filter capacitor may be recharged 120 times a second but the is an effective series resistance in any power source before the filter capacitor. So instead we need to look at the envelope of the signal. IE when a drummer strikes the drum a surge of power is required for as much as 2 seconds as the note decays. So you would want your power supply not to sag a noticeable amount during that time. Not being noticeable would be around 3% or less. Just to make things a bit tougher on the filter capacitors, maximum low frequency energy in music is around 150 hertz.

So the beat frequency of 30 hertz will show up at some level. As a minimum that should be 30 dB down.

Now adding capacitors to add surge filtering to a battery power supply requires stupidly large capacitors to have any effect. However with virtually all cases the measurable effect will not have any audiable change.

So the only challenge is to make sure your power distribution traces or wiring doesn't screw things up.

As to where to place a rail to rail capacitor, if you are using an op-amp as the output, right across the + & - power pins. This is in addition to the normal capacitors to ground.

Conclusion is that with batteries you are not trying to get rid of power supply noise, you are trying to damp the circuit load noise. This is best done at the noise source. That is where the circuit connects to the rails.

I have some unused A123Systems batteries (ANR26650M1A) and got curious about their noise level from reading here in the forums that batteries apparently were noisy.

So I connected them to my AD7760 evaluation board through a capacitor and compared the noise level with the basic noise level of the ADC with inputs shorted at different currents. FYI I've attached two screendumps - one of the ADC's intrinsic noise level (first one), and one of the A123 battery at 0.48 amps. I tried other lower currents but the differences basically were unmeasurable in this setup.

A couple of remarks on my measurement setup:

- It seems there's always a spike at ~6.4 kHz and the very low frequencies in general are influenced by noise except to a lesser degree when the ADC's inputs are shorted. I know - not perfect - but sufficiently informative for my uses (and wallet ).

- The setup is not capable of indicating - 200 dB noise levels so it can't ultimately show the actual noise of the battery.

jkeny wrote:I picked this up on DIYAudio about LiFePO4 batteries which confirms what we have discovered
http://www.diyaudio.com/forums/power-su ... ost4565910

"A battery filters down to DC. Even if the battery has .1 ohm resistance what value capacitor do you need to equal that? Answer infinite.

The definition of capacitance is C=Q/V. The important part of the first derivative is C x dV/dT = i. Now most folks get that F = 1/T. So they assume that if the lowest audio frequency is 20 hertz then dT = .05. As this is a power supply not a series pass use that is too simplistic. The power supply filter capacitor may be recharged 120 times a second but the is an effective series resistance in any power source before the filter capacitor. So instead we need to look at the envelope of the signal. IE when a drummer strikes the drum a surge of power is required for as much as 2 seconds as the note decays. So you would want your power supply not to sag a noticeable amount during that time. Not being noticeable would be around 3% or less. Just to make things a bit tougher on the filter capacitors, maximum low frequency energy in music is around 150 hertz.

So the beat frequency of 30 hertz will show up at some level. As a minimum that should be 30 dB down.

Now adding capacitors to add surge filtering to a battery power supply requires stupidly large capacitors to have any effect. However with virtually all cases the measurable effect will not have any audiable change.

So the only challenge is to make sure your power distribution traces or wiring doesn't screw things up.

As to where to place a rail to rail capacitor, if you are using an op-amp as the output, right across the + & - power pins. This is in addition to the normal capacitors to ground.

Conclusion is that with batteries you are not trying to get rid of power supply noise, you are trying to damp the circuit load noise. This is best done at the noise source. That is where the circuit connects to the rails.

jkeny wrote:And later this measurement of A123 battery noise (not quite but you'll see what I mean - the noise is below his measuring system
http://www.diyaudio.com/forums/digital- ... 0134-150dB)

I have some unused A123Systems batteries (ANR26650M1A) and got curious about their noise level from reading here in the forums that batteries apparently were noisy.

So I connected them to my AD7760 evaluation board through a capacitor and compared the noise level with the basic noise level of the ADC with inputs shorted at different currents. FYI I've attached two screendumps - one of the ADC's intrinsic noise level (first one), and one of the A123 battery at 0.48 amps. I tried other lower currents but the differences basically were unmeasurable in this setup.

A couple of remarks on my measurement setup:

- It seems there's always a spike at ~6.4 kHz and the very low frequencies in general are influenced by noise except to a lesser degree when the ADC's inputs are shorted. I know - not perfect - but sufficiently informative for my uses (and wallet ).

- The setup is not capable of indicating - 200 dB noise levels so it can't ultimately show the actual noise of the battery.

nige2000 wrote:

jkeny wrote:And later this measurement of A123 battery noise (not quite but you'll see what I mean - the noise is below his measuring system
http://www.diyaudio.com/forums/digital- ... 0134-150dB)

I have some unused A123Systems batteries (ANR26650M1A) and got curious about their noise level from reading here in the forums that batteries apparently were noisy.

So I connected them to my AD7760 evaluation board through a capacitor and compared the noise level with the basic noise level of the ADC with inputs shorted at different currents. FYI I've attached two screendumps - one of the ADC's intrinsic noise level (first one), and one of the A123 battery at 0.48 amps. I tried other lower currents but the differences basically were unmeasurable in this setup.

A couple of remarks on my measurement setup:

- It seems there's always a spike at ~6.4 kHz and the very low frequencies in general are influenced by noise except to a lesser degree when the ADC's inputs are shorted. I know - not perfect - but sufficiently informative for my uses (and wallet ).

- The setup is not capable of indicating - 200 dB noise levels so it can't ultimately show the actual noise of the battery.

yes this makes a bit of sense to me
lifepo4s are great but not a "magic cure all"
say youve got a 26650 cell it does seem to absorb noise well up to half an amp draw and starts to decline above that and you have to add in more cells in parallel to counteract

18650 cells arent that fantastic and dont like too much work to do but ok as a bypass cap on an xo or use 4 in series for the low draw on a mobo 24pin 12v rail
likely would a decent job as bypass caps on various spots on the soekris its the switching on/off that really complicates it

they are not fully immune to noise of poorer chargers if used as a capacitor as it uses up some of that flooding effect

and when youve substantially lowered circuit noise with use of lifepo4
you've revealed the next set of problems:)

jkeny wrote:
]I don't think that's what he is saying but yea, above a certain current draw the noise from the chemical reaction in the battery will start to make noise - not sure that .5A is the point of crossover - where di you get that from?

nige2000 wrote:

jkeny wrote:
]I don't think that's what he is saying but yea, above a certain current draw the noise from the chemical reaction in the battery will start to make noise - not sure that .5A is the point of crossover - where di you get that from?

its not a crossover by any means noise absorption is always good compared to alternatives however there is a audible decline in preformance around the .5 amp per cell

i noticed this on the 3.3v rail of a pc were i was at four cells before there was no audible benefit to adding more
amp draw is around 2a

lots of stuff operates at less than .5

jkeny wrote:

nige2000 wrote:

jkeny wrote:
]I don't think that's what he is saying but yea, above a certain current draw the noise from the chemical reaction in the battery will start to make noise - not sure that .5A is the point of crossover - where di you get that from?

its not a crossover by any means noise absorption is always good compared to alternatives however there is a audible decline in preformance around the .5 amp per cell

i noticed this on the 3.3v rail of a pc were i was at four cells before there was no audible benefit to adding more
amp draw is around 2a

lots of stuff operates at less than .5

That's what I was wondering - was it from your own test/experience?

Thanks for that - it's interesting - I wonder is there a transient current draw on this rail > 2Amps? What is the 3.3V rail powering?

The problem seemed to be that more power was drawn from the pos (or neg, I can't remember now) end of the battery pack & the cells at this end tended to become drained but the amp still played at <17V so the current draw on these cells continued & killed them over time.