Where do you wire the the power into the back of the plate amp? I bought the plate amp about a month ago and have been back and forth with support trying to get answers to some questions, but after many emails back and forth they just now told me that I can’t connect a battery to the plate amp, which I know isn’t true. I have the dayton LBB-5v2 (5x 26650 just like yours) which will have a 21v DC output.
Hi Churuhh and welcome to the forum.
I don’t own a plate amp 2.1 to check, but looking at a photo of the back of the unit, I would be pretty confident in saying that the two solder pads on the bottom LH side (one marked with +) would be the power input.
You could confirm this with a multimeter
Hi,
maybe we described wrongly. You sure could connect battery to this port to power this board, but the difficulty is how you charge the battery. I guess you need to use the DC socket to do the charging. You need to make sure the battery pack support this kind of connection, maybe a picture is more clear 
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That makes sense and I haven’t tried it yet but am sure that I could power the board by connecting the 21v output from the battery board to those two solder pads on the plate amp after checking which is +/- with a multimeter. But with this setup I don’t think i could use the DC input on the plate amp because this would back-feed power to the output of the battery pack. I’d have to install a separate DC port connected to the power input terminal of the battery board to charge it. Do you see any way around this? I suppose I could just put a piece of gaff tape over the input on the plate amp so that it doesn’t get used but I’d really like to make that the input for charging the battery if at all possible.
Yes. So the point is, you could find a battery pack which has this output switch internally. It has 2 wires, when feed power, it will charge and no output. And when no power feed, it will output with the same wires.
I guess there’s such kind of battery pack on market, it’s a common requirement.
Good to know, I’ll keep that in mind for my next project. Hoping if I unscrew the board that there will be room to connect the input port to the power input on the battery and then the output of the battery to the two solder points noted before. Not sure what is there between the input and where it connects to the main board but I’ll figure it out