I may be weird but I first read the 1 star reviews on Amazon before jumping to the 5 stars. I would have a dead battery in something around the garage to test these things before I ordered one. That way if it doesn't work I am within the window to return it. Almost all the 1 star folks said it worked once or twice and never again. Or worked once and would not recharge. And it was too late to send it back for a refund. Just sayin....
I have had a bit to do with these mini jump starters (MJS) and I have quizzed a lot of family and friends and I have come to this conclusion.
If the voltage is allowed to get too low they are unlikely to recover and they become a paper weight, this maybe why some people get good results and some get poor results.
Just how low is too low I'm not sure.
If they are used to start a vehicle and that vehicle starts easily all will be good but if the load on the MJS is too great you can kiss it good bye.
I used one to start a car that was showing 10.2v and it fired right up and the MJS charged back up fine.
I used that same one to start a V6 car that had no dash lights showing, the car started but the MJS never recovered, in fact it nearly caught fire while on charge.
I kept one under the seat of my bike, after about 6 months I tested it and it was only showing 5v (should have still been full) when I put it on charge the charger never showed full and it too expanded and split open the cover and got so hot fire was eminent.
Talking to friends they all tell the same stories, if they have been worked hard they never recover.
Because I have been so close to having 2 catch fire I don't trust them but I do believe if they are treated & maintained properly they are a handy gadget but I would limit it to small loads & starting bikes only.