Four values, no driver, no software
Retoo USB tester, 4–20 V, 0–3 A, capacity to 99999 mAh
A USB 2.0 power meter measuring 4 to 20 V and 0 to 3 A with a capacity counter to 99999 mAh, showing voltage, current, time and capacity on an LCD.
Specifications
- Voltage and current range
- DC 4 V to 20 V and 0 A to 3 A, first bullet
- Resolution
- Nowhere in the listing
- Protocol identification
- Nowhere in the listing — it measures voltage and current without identifying a charging protocol
- Trigger function
- Nowhere in the listing
- Data pass-through
- USB 2.0, plugged between the port and the device with no driver or software, third bullet
- Display
- An LCD showing four values at once: voltage, current, elapsed measuring time and capacity, second bullet
- Logging
- A memory function for running or completed measurements, with a reset, fourth bullet
- Amazon reference
- B09QXLS5JP (same reference on 5 Amazon marketplaces)
- EAN
- 5904094227396
Four numbers between the charger and the phone.
What it shows
Voltage, current, elapsed time and accumulated capacity, all four at once. Not a single reading you cycle through — everything on screen together, which is what makes a charge legible while it happens.
The capacity counter
To 99999 mAh, with a memory and a reset. Run a power bank flat through it and you have a real number for what it holds, against the number printed on the case. That comparison is why most people buy one.
No software
No driver, nothing to install. It goes between the port and the device and works. For a tool you use twice a year, that is the right design.
The range
4 V to 20 V, 0 A to 3 A. Sixty watts at the top, which covers most phone and laptop charging and stops short of what the largest USB-C supplies now deliver.
The lines this listing leaves blank
The resolution — the smallest current it can read, which decides whether a trickle charge shows up at all.
And any protocol identification. It measures what happened; it does not say which negotiation produced it.
Availability
German, French and Spanish Amazon stores quoted 5 € to 7 € when this page was compiled.
British Amazon store carries it too, priced in its own currency, which the euro range above deliberately omits.
Italian Amazon store wanted more than 1.6× what the cheapest country wanted. A margin that wide usually means stock shipped in from elsewhere rather than a seller on the ground, so that quote stays out of the range above.
American Amazon store returned nothing at all, so the buy button there runs a search instead of landing on an empty page.
These figures are what Keepa reported on the day this page was last updated. Availability and pricing move without notice.
Prices quoted in this text were read on 16 September 2026. The live price is at the top of the page.
