Finished testing charging EN EL 18ds with the Anker PowerCore+ 26800 PD 45W (97 WH capacity). Used the MH-33 batter charger attached to the power bank with an Anker USB-C braided power cord. Done inside at around 65 F.I am testing the Anker PowerCore+ 26800 PD 45W (97 WH capacity) and its ability to charge an EN EL 18d (36 WH capacity). It's the battery recommended by Nikon for hte Z9.
With the Anker battery fully charged to 100%, I attached it with a USB-C cable to an MH-33. Put on an EN EL 18d battery that had been used down to 14% (as shown by the Z9 battery menu). The Anker charged the EN EL 18d back to 100% (both as shown by the green lights on the MH-33 and by the Z9 battery menu) and went from displaying 100% power (10 small lights) to 70% power (7 small lights). So relatively efficient, it would seem, although I do not have a digital readout of the remaining power on the Anker.
I suspect that this means I could charge a second EN EL 18d back from 10-20% to full with the Anker. I will wait a couple of days and try another battery on it.
On Wednesday or Thursday, I charged one EN EL 18d from 14% to 100%, both as measured by the battery menu in the Z9.
On Saturday, I charged a second EN EL 18d from 8% to 100%, both as measured by the battery menu in the Z9.
After the second battery was charged, the power block was at something less than 10% power (the last light of the 10 lights was on for a while and then started to flash). So I think the power bank was getting pretty close to shutting off, but it did manage to fully charge the second EN EL 18d.
The power bank has 97 WH of capacity and was at 100% when I started. It added 31 WH to the first battery (31 = 36 x (100% –14%), rounded off). It added 33 WH to the second battery (33 = 36 x (100%-8%), rounded off). So the 97 WH power bank added 64 WH to two EN EL 18d batteries and was itself close to depleted. Since I generally do not run my batteries down to 0%, I conclude that I could likely charge 2 EN EL 18d batteries from the power bank on my trip if they were discharged to 15% or so.
It’s also the case that I would likely get more energy from the power bank if I used it to run the camera directly, using the appropriate cable. In that case, I could use all of the 97 WH of the power bank and not lose some to the inefficiency of charging other batteries. Assuming the power bank was at 5% when the second battery was fully charged, the power bank used 92 WH of energy (92 = 97 x (100% - 5%), rounded off) to provided 64 WH (31 WH + 33 WH) of energy to the two EN EL 18ds. Approximately 69% efficiency (69% = 64/92). Even with this loss of efficiency, if I can manage it, I’d prefer to just use the batteries after charging them with the bank. It seems more convenient/easier.
Note, these numbers are not as precise as they would be in good lab testing. But I think close enough for my purposes.