This one can be used for it! It is an replacement power cable:
https://www.amazon.ca/MyVolts-Supply-Adaptor-Compatible-AX-Edge/dp/B07J6BD5R3
Although it does appear a bit shorter in length than the one that comes originally with the Ax-Edge. However, should work exactly the same
The NiMH are a fine option, but Lithium ion are a clearly better option. There are multiple reasons why that's pretty much all electronics (laptops, cell phones, etc.) use.
They are also more convenient to charge since they are just USB and they are higher MaH.
Here an example: https://smile.amazon.com/EBL-Battery-Batteries-Capacity-Rechargeable/dp/B08RZ5NDMM/
Have you tried something like this?
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I have only used it to connect to a laptop and a phone so far, so sorry about that, I can't speak to how effective it is with 5-pin midi at both ends, but I understand that it works.
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Sorry that I didn't answer your question completely, but as there were no other answers, I can at least try contribute something that might be halfway helpful.
I read that someone successfully used this inexpensive gun case for the V2! https://www.amazon.com/Flambeau-Outdoors-6500AR-Tactical-Large/dp/B00C3MFDKK/ref=pd\_lpo\_sbs\_469\_img\_1?\_encoding=UTF8&psc=1&refRID=QPVVZ6GQGK1TN3Y2BJX4&utm\_source=pocket\_mylist
If he's interested in keytar and not in piano, don't force him into piano. Kids at that age get bored and move on to new things really fast, so don't assume that he'll be so into keytar that he's ready to learn piano.
There's a really terrible keytar available on Amazon for like $40 if you think he just wants a toy. I bought it for my collection because I MUST own every keytar and hang them on my wall (it's becoming a problem). The keytar's not great, but it definitely makes noise and you can play it like a keytar. If he wants to actually learn music, I'd recommend the Yamaha shs-300 since it's cheap and does a nice job at what it aims to do. Also 8-year-olds might be the only audience who thinks that Jam Mode is fun. If he's young and has small hands, then playing on 37 mini-keys can still teach him a lot about piano if he decides that he wants to go that route.
Just whatever you get him, make sure it has a standard headphone jack so that you can tell him that you love that he's learning an instrument but he has to go learn his instrument quietly.
If you want live manipulation Ableton Live is the best software out there. I personally use and recommend this software. Youtube is your best resource to understand and learn Ableton
If you plan on using more than two keyboards look into getting a MIDI interface like the MOTU micro lite. These kinds of interfaces help your computer choose between keyboards and it allows everything to work together better so you don't have a keyboard that is falling behind.
The products I have listed are not the only products available. Feel free to look up some alternative products.
Im going to order them now. So sick of my strap coming off and I tried to install straplocks before with opening the keytar up and It didnt go well.