Your Nuki received >9V only because the power supply (psu) is not stabilized psu and deliver higher voltage than is marked on the selector. Just lower a step to 3V and you probably get 5 or 6V. If you’ll use a stabilized power supply, even a smartphone power battery charger (5V/2A) you’ll see a right value.
I’m android Nuki user and I’m wondering how you get such info about battery as shown in posted picture? Because my Nuki app doesn’t show such informations, that screen is not available. Maybe you make some light for me 
Thank you.
In Administration/Battery tap 7 times on the info text in the middle of the screen.
Thank you. I found also the thermometer feature:)
Hoe weet je wat de plus en de min is in de nuki?
gr
I don’t like to add a power adapter instead of 4xbatteries NiMh…
If the power supply 220V-120V/5V is interrupted for many hours from outside to a service team or power adapter fails you can’t not enter in the house. To avoid that situations you need a backup battery like UPS. Is not worth it.
I have from september 2019 until now (10 months), 4x Panasonic Eneloop Pro R6/AA 2500mAh (charged at 1A) and Nuki app show full battery.
For strong doors …
If a door is secured in 3 places when was locked, don’t need to lock 2 or 3 times. One times locked is enough to save batteries. You can find on internet … a door can be hacked even was locked 3 times from a key.
For safety I have a key to my office work and an additional alarm.
We wouldnt neither want to have a power adapter, since there is no plug nearby.
I wondering why it’s not possible for @Nuki / @MatthiasK make a new accessory that would allow to charge four AAA battery inside the Nuki Lock without removing them, just by attaching a powerbank every 1-4 months (or everytime you clean the room for your guest to be sure) at the door handle and plug an USB-C into the Nuki-Lock-Battery-Compartment-Version2.
Imagine someone renting out the Nuki locked room for several months: not practicable to change the battery that often. But very easy to change hang a powerbank of night at the door handle, plug it into the battery holder and remove it later. simple. easy. no worries.
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I connected my nuki with a net adapter as well. Much the way @fvhuizen did. His solution actually looks more simple. Didn’t see this thread until afterwards. I posted instructions on Instructables. https://www.instructables.com/id/Connect-Nuki-Dooropener-to-Ac-Outlet-No-Battery-Ch/#discuss
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My 4x Panasonic Eneloop Pro 2500mAh kept the power for Nuki SL approx. 10 months and 2 weeks only one time turn the key and open/and lock the door for 2 times/day.
Nuki listened to us (page is in Dutch):
Hallo,
den Grundgedanken eines Akkupacks finde ich nicht schlecht, allerdings finde ich die Umsetzung hier eher ungenügend und zu teuer. Der Ni-Mh Akkupack mit 2500 mAh bringt mir keinen Mehrwert gegenüber normalen Rundzellen. Bei meiner Suche bin ich auf Hixon (J 818) Li-Ion AA-Zellen mit einer Ausgangsspannung von 1,5 V und einer Kapazität von 3500 mAh gestoßen. Beim größten Online Auktionshaus habe ich hier für 12 Zellen inkl. Ladegerät, dass gleiche bezahlt wie für das Powerpack. Nuki läuft hiermit einwandfrei und der Mehrwert ist damit für mich erheblich gestiegen. Jetzt bleibt noch die Langzeiterfahrung abzuwarten.
Gruß
P.S. Leider muss ich feststellen, dass mein Dokument an manchen Stellen völlig falsch übertragen wurde. Zum Beispiel habe ich geschrieben…das der Wert für mich gestiegen ist. Ich werde das bei Nuki bemängeln.
Es liegt wohl am Google Übersetzer!!
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It’s mWh not mAh.
3500mWh : 1,5V = 2330mAh.
Ni-Mh Accumulators with 2500mAh * 1,2V = 3000mWh.
Wether they perform better or worse than Ni-Mhs most likely depends on the usage scenario. Where you need to be careful is that (mot likely) due to the integrated PCB with power stabilization the Smart Lock won’t show proper battery in % nor any warnings, until the batteries die off when trying to lock. The Smart Lock could even end up in a reboot loop if they recover and provide stabilized power again after that.
Hallo,
tatsächlich mein Fehler mWh und nicht mAh. Trotzdem steht mir mehr Leistung zur Verfügung. Die Restkapazitätsanzeige ist ja noch recht neu und daher habe ich damit auch noch keine Erfahrung. Auf die Warnung konnte ich mich noch nie verlassen. Ich habe es extra mal darauf ankommen lassen und wollte die Warnung abwarten, da hat der Antrieb die Falle nicht mehr zurück ziehen können. Da verlasse ich mich lieber auf mein Gehör. Das passte bisher immer. Lieber zu früh getauscht. Die Hixon werden vom Schloss als Lithium Batterien erkannt. Bisher habe ich immer Alkaline Batterien verwendet.
Gruß
Hi, could you tell me if your power supply 6v @ 1000mA is still working?.
I would like to do the same.
Hi Sergio, Yes, it’s still working like a dream 
I’m also surprised how easy it was to setup.
Thanks…Now I am using a cell phone battery charger (5v @ 2A) and it works well … But battery level is 68% and I do not like that. So, I will change because I have just bought a dummy battery pack…to try.
Greeting from Peru
I am very interested in this idea. Batteries in my smart lock 2.0 seem to last about a month (maybe 4 openings and lockings a day) which is rather disappointing.
I have a socket near the door so could set up a battery eliminator. However, if the power goes off and on, does the lock then turn all the way in both directions (full lock and full unlock?). If so, at the unlock step my door would open and not close without being pushed, so whenever there was a powercut the door would be unlocked and open.
Wow, don’t know about this, very cool! I have already accu packs for my locks, and they I can also connect permanently to a USB charger for loading, - and for convenience, I setup a automation what charges the accu packs in one night in a week.
Can the powerpack be connected to a USB power source all the time? This seems like the easiest option for people with a socket nearby. Is having the powerpack connected all the time bad for the batteries?
Google is your friend:
… It is not necessary to unplug the cable to use the Smart Lock. This is very suitable for short rental hosts as you don’t have to care about the battery level anymore.
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@Lumo I have a SL2.0 and use random rechargeable NiMh batteries. We have I guess a dozen openings a day, and usually the batteries last 2 months (my feeling), unless I have a bad battery as part of the four batteries, which might happen as I use them frequently in other devices, so some of them are worn out. I try to detect them by using a charger that shows their capacity, like this one
Therefore, I am surprised to hear that your batteries are gone in less then a month.