Portable laboratory power supply battery  AC under the 20
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Portable laboratory power supply (battery & AC) under the $20

Assuming you have old laptop power bricks andor Ryobi battery to spare XYS3590 is affordable buck converter up to models ...

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Portable laboratory power supply (battery & AC) under the $20 alternatives
Power Supply Holder
MR_sportline
Power Supply
CreativeElectronics