I currently use aNikon Z5 camera. The Z5 is interesting in one aspect, it finally has USB charging using USB-PD. In my opinion, USB charging should have been present in all cameras since they started having a USB port, even if charging with the limited 2.5W will take a long time. But, nowhere in the manual or the specifications does Nikon say what parameters the USB PD charger is working at, so I decided to check it out.
Charging the Z5 battery in camera
First off, with any phone charger of USB hub which is not USB-PD that i tested the camera draws about 2.5W from the USB port. So the camera will treat it like a regular USB port, even if the charger is more capable, and it takes about 8 hours for a complete charge. This makes sense, since the battery is 16Wh.
Second, in the tests below, the battery has always been discharged using a video recording until the camera shut down because of low battery, before charging it.
When using a 90W USB-PD charger, there is a maximum and constant 10W power draw, then it drops to about 7.5W constant power and finally there is the continuous reduction of charging power. This is when the battery is topped up in the last 10-20% when the charging is done in constant voltage mode and the current is slowly decreaseing. I suspect the drop from 10W to 7.5W is due to the battery getting warm.
All in all, the charging needs about 3 hours and it looks like you only need a small USB-PD charger, as charging is limited to 10W only.
The camera sets the voltage on the USB bus to 5V and draws 1.81A. So it does not use the higher available voltages like 9V or 12V from the USB-PD standard.
Note: the yellow curve is traced over the green with a 1 minute averaging, useful for other measurements but not here when the process is really slow. The power is measured with a TP-link HS110, read by node-red and fed into Influx and displayed with Grafana.
Charging the battery in the dedicated MH25a charger
The charger starts at around 11W and slowly increases to 12.5W – probably operating in constant current mode. According to the label, the output is 8.4V, 1.2A so the first part is charging the battery at constant 1.2A, but as the battery charger the voltage rises and so does the consumed power. Then there are multiple alternations between a lower power 7.5W mode and the higher power 12.5W mode and even a drop to 0W. Probably this is due to battery temperature, I don’t know for sure. Finally there is the continuous slow reduction in charging power.
Charging from a regular USB port is slow, the 8 hours required make this an overnight process. So you would likely want to charge either through a USB PD charger of from the camera provided one.
In both cases the battery charges in about 2 hours 50 minutes. The charger gains some more speed at the beginning because of higher power, but ends up needing the same total time because of the phase where it just does not charge the battery anymore.
The first part of the charge (probably till around 80% charge) is slightly faster on the charger, about 1 hour 30 minutes, while the USB-PD charging needs 2 hours for that. So if you want to top up the battery as much as possible within some short time, the charger is better. But if you just care about getting the battery completely full, both seem to take the same amount of time.