Hi David Nicholls,
Thank you for writing in.
Here is a recommendation for you:
DROK® Mini OLED USB Charger Detector Current Voltage Power Capacity Multimeter Amp Volt Measuring Watt Meter DC 5V 10V 2A with Power-off Storage Function Tester Tool Compatible with USB 3.0 2.0 1.0

FEATURES:
✓ Can display voltage, current, power, capacity and time(every 2 seconds switching capacity and time)
✓ Highlight intervals, which increases the life of OLED and prevents aging
✓ Can store 10 sets of data
✓ Store data when power off
✓ 20 milliohms sampling
✓ Range: 10V 2.0A
✓ Resolution: 10 mV, 2mA

OPERATION INSTRUCTION:
• Plug it into mobile hard drives, computers and other equipment to detect electric parameters, compatible with usb1.0, usb2.0, usb3.0.
• There is a multifunctional button on its back: short press it, the readouts can be flipped; long press it to switch among 10 sets of memory capacity data.

Hope this can help you.
Best regards