/diy/ - Do It Yourself

TLDR Installed pic related recharagable battery into guitar; massive background screeching ensues. Why?
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Bought pic related 9V for an acoustic guitar with a piezo bridge pickup. These systems run on 9V alkeline batteries, so thought replacing it with rechargable would be convenient. Bought 2, one from Amazon and one from Ali.

Plugging guitar into the amp, there's background screeching. Sort of constant frequency, like 60 cycle hum but with a lot more trash harmonics thrown in. Quite loud. The Ali one is worse than the Amazon one, but neither are usable. Replacing with alkeline 9V fixes issue; dead quiet with no background noise. I don't have the circuit diagram but it would be a form of piezo pre-amp circuit, which is then plugged into a main amp. I could run it down if needed.

Why is it doing this, and is there a way to fix it? Assume the "9V" is lower capacity cell that's being stepped up using circuitry in the battery, or something, and it's passing through harmonics. Install filter circuit?