Batteries are low noise, right? That's why people want to use them in audio systems. The most common assumption is that because they are a pure DC source they must have no AC noise. Unfortunately we live in a world where nothing is pure, so let's look at why batteries are not so great a source of power as some may want you to believe.
How a battery produces power
Batteries produce power through a chemical reaction. Every type of battery will have a slightly different chemical mixture that is used to generate power.
Batteries have a voltage rating and a current/time rating called Amp-Hours. Amp hours basically say how many hours the battery will last if it's drawing one amp of current. Because many systems draw variable loads, a 10Ah battery will not necessarily last 10 hours. It may last 20 or it may last 2 hours depending on the system it is connected to.
As a battery drains, the amount of chemical compound available to produce electricity drains. That means there's an exponential decay on the amount of power a battery can produce. So a battery at 100% will have far more capability of producing fast transients than a battery at low charge.
Battery Noise Generation
Batteries have a substantial amount of high frequency ripple. This means there is a large AC component to their DC output voltage.
What causes this ripple?
The battery's chemical reaction will generate different types and frequencies of noise. The chemical reaction changes as the battery drains. That means the spectrum of noise changes as well. The change in load also impacts the way the battery is draining, which will alter the chemical reaction and thus the noise produced.
As a battery drains so does the output current and the output voltage. As these elements change the voltage will become less stable and the transient response slower.
So ultimately you are creating a non-constant source that has varying spectrums of noise, that is constantly changing, and produces the exact noise we want to get rid of.
Regulating and filtering battery power
The first thought is to regulate and filter the battery power. This can help the battery's performance considerably, but it does not compensate for the variations that occur as the battery drains. Batteries in particular do not have a pump, so as current demands stay the same and their ability to produce that current diminishes the regulators and filters become progressively less effective.
Here's the catch. Regulators are very sensitive to input impedance. They like a stable input or they must regulate both Line and Load variations. A battery's output impedance changes with drain.
Different batteries sound different
Every battery will have a different chemical composition and thus different chemical reaction. This translates to different levels of noise and different resulting sounds.
The noise generated is usually very high frequency noise up into the mhz or ghz ranges. This is not only more difficult to filter, but it's the exact noise that pollutes digital audio the most. The noise generated may be less than the noise filtered out compared to a SMPS, but it is still generating noise that is a challenge to compensate for.
Recharging the Battery
For one, recharging is a hassle. Many companies have circumvented this by having micrcontroller charging circuits to automate the process.
Many chargers only begin trickle charging when a battery is below a certain charge level. Introducing AC noise into the battery's path means the system has to filter out both AC noise and battery noise. You also loose the benefit of being off the grid. If the AC noise is somehow isolated from the battery's DC output using dual batteries with a switching circuit or other method then the charging will still alter the chemical reaction of the battery.
When a battery charges it creates the opposite chemical reaction to return the battery to it's initial chemical components. In this event you have opposite chemical reactions occurring at the same time.
GROUNDING
This could be one of the single biggest issues with battery power. By using a battery you are no longer connected to ground in the same way. That means that shields become a noise "cage" rather than pulling noise away from the system.
In addition, you end up having ground loops along cables and your system must find an alternative path for ground noise.
Enter the Linear Power Supply
Linear power supplies are going to be larger than batteries. But unlike batteries they can be built to a much higher bandwidth that functions consistently. A well-designed linear power supply can be considerably lower noise, faster, and higher bandwidth than any battery. Linear power supplies can filter the incoming AC noise, eliminate ripple to exceptionally high frequencies, and have much higher slew rate and current output. This means their sound will always be consistent and their potential much greater.
Comparing a battery to a SMPS or entry-level power supply will put a battery ahead as the clear winner. It will be much lower noise than a SMPS. But a well-crafted
Linear power supply has several advantages and should be the only consideration when looking for the ultimate power solution.