On my MP3/250, I have a Dual USB Power Port directly opposite to the other side where you would check your Coolant. Easy to tie into the keyed Replays.
There are some nice ones know that provide a LED Readout of your Voltage.
I ordered one from Amazon to do a swap the next time I have the front removed.
My 2016 500 Sport has one just under the windshield in the glove box which has worked well.
On my Vespa/300 I have a Dual USB installed in one of the knee pads that are held in place by one screw. Very easy to install as well.
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