I also made this mod, slightly different “batteries” used. Batteries on the nuki app were lasting a week or 2. And the last thing I ever want to do is replace batteries when I’m in a rush to leave the house. Fitted to a pvc door, so now to get creative with hiding the wires 
Hi Nuki forum users,
I am also trying to eliminate the battery dependency for the Nuki 
I have an older (big block - non stabilised) adapter with 3, 6, 9 and 12 volts max 2 amp. I tried the Nuki with the 6 volt setting.
What occurred to me was that the led ring on the Nuki (that i have set to be on the lowest brightness setting) was suddenly very bright. Regardless, the bright led, the locking and opening operations went very well.
When I went into the battery settings and checked for the debug power details i saw the big power block almost delivers 9,2 volts (see screenshot) in idle mode to the Nuki. Quite high…
For now I disconnected the power supply and went back to batteries.
Would it be be OK to power the Nuki for a prolonged time with 9 volts idle power or would i slowly but surely kill the Nuki??
Any recommendations are highly appreciated.
As i m not allowed to post more then 1 image in my first post i post a pic of the power adapter in a follow up post
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This is the simples not stabilized power adaptor. Idle voltage can be much higher than you have set. It achieves around 6V only under nominal load (2000mA in this case) and still it will be very unstable.
You cah kill the nuki and any other device easily with it.
Try to use 2A 5V usb adapter (4x rechargeable batteries has 4,8V).
Deadlifter, why don’t you use rechargeable batteries with a charging adapter. In case of a power outage the doorlock will still work.
I also recently replaced the batteries with a 5v powered “dummy battery pack” which seems to work really well.
It fits into the existing battery cradle without any modifications needed. The wire from the battery is a flat cable, so it fits snugly within the battery cradle and main housing joint . If your mains power supply is not close to the door, then you may need to extend the power cable extension (1,8M) that comes with the pack (or just buy a longer extension cable). Even though the power supply rating is only 6V-1000mA it seems to work fine. Nevertheless, I probably need to load test it for a few more weeks to be totally sure.
https://www.amazon.de/gp/product/B07Q1KTXFR/ref=ppx_yo_dt_b_asin_title_o00_s01?ie=UTF8&psc=1
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.