# Running 9v battery in parallel with Power Supply

I am putting a 9v powered circuit into an electric guitar. It will be powered by a 9v power supply through a cable, and also have a backup 9v battery on board the guitar. I am wondering if I could run the power supply and battery simultaneously in parallel, to accomplish 3 main goals:

1. Although the power supply has filters, it needs to be as ripple-free as absolutely possible, as any ripple will be audible as noise. will the battery help to smooth the power supply's delivery? This is the main goal.

2. I am hoping the power supply will keep the battery charged, will this be the case?

3. I want to simplify switching and avoid having to manually switch power supplies.

Thanks

Edit: essentially, I am trying to replicate a car charging system, where the alternator and battery run in parallel - the alternator float charges the battery, while the battery smooths any voltage spikes from the alternator, and if you disconnect the alternator the battery will keep the system running.

• No, no, and no. Battery backup circuits are far more complicated than that. Jun 11, 2018 at 4:30
• I would not use a battery as a ripple remover, Jun 11, 2018 at 4:42
• @Kendric yes, we do it that way in our factory during the tests. We put two series-connected car batteries right across the output of the lab-type linear power supply (27.8VDC output) when we test especially audio circuits and MCU-based products. We also use some extra circuitry to generate spikes (up tot 0.5 times the supply voltage) on supply rail for testing and the battery is always connected across the output. We've never encountered any problems for years. I'm also sure that a car battery is connected in that way in a car (i.e. alternator -> regulator -> battery). Jun 11, 2018 at 5:18
• Also car batteries are rechargeable and your average 9v battery isn't. Jun 11, 2018 at 5:22
• Every time you charge a battery you loose something. If you look at a simple 9V rechargable battery you also have the memory effect: en.wikipedia.org/wiki/Memory_effect . When using the battery as ripple remover then every ripple causes a charging cycle and your battery suffers. Jun 11, 2018 at 13:30

While this is not the answer to your explicit question, I must address the larger question: how can I minimize noise in my circuit? Yours is a good example of an xy problem. You need to do x, you've settled on y as a solution, and want help in doing y. What you really need is to address x.

And the most basic answer I can give is: get somebody who knows how to design amplifiers (or whatever your circuit does) have a go at fixing yours. There is simply no good reason nowadays for power supply ripple to show up in an output.

As an alternative answer: Get somebody who knows something about power supplies provide you with a new 9-volt power supply. One with negligeable ripple. Trust me, it's not hard to do.

Forget the recharge nonsense. Use 4 each 9 volt batteries and sum them together (in parallel) with 1N5817 Schottky diodes with low Vdrop.

Keep plenty of fresh batteries on hand and replace weak ones before each gig.

Buy a dirt cheap voltmeter to check each battery. The diodes will isolate each battery from the other, but sum their currents for a long life about 4 times normal.

• This is an interesting suggestion and I may use this setup if the power supply is too noisy. However, I am using a very power-hungry circuit and want to minimise the expense of batteries if possible. Jun 11, 2018 at 5:45
• You can recharge the batteries. Having multiple (packs) if batteries that you can switch quickly and recharge externally should be a plus. Jun 11, 2018 at 13:37

I would recommended a circuit like this: If you plug in a DC adapter, the power will be drawn from the adapter. If no adapter is connected, the power will be drawn from the battery.

To answer you other questions, you can't just charge any battery with another adapter. To filter noise, use suitable capacitors. Regarding the filtering, take a look for example at this: link