My most handy tool for mucking around with electronics is a portable battery. I used to lug around a bike battery but lithium options got too cool to ignore.
this one's a bit of an overkill (up to 48vdc output) but it comes with a leather belt strap, weighs 600 grams and is 7500mAH, or 95Wh@12v. (12v out is on the other end )There's a bazillion options out there at all sorts of prices.
Handy when diagnosing in situ kit, you always have a known pos/neg source. Helps a lot in narrowing down the issue and quickly.