I bought a Black & Decker 700amp portable jump charger for my car, and did not use it for a couple of months (not that long really), and then when I went to jump start a vehicle it had no charge. So, I plugged it in, and charged it back to full, but then I unplugged it (once it was fully charged), and then took it out to my car. except that by the time I walked out to my driveway, to put it in my car, it had lost about half of its charge somehow. This meant I had to take it back in to plug it back in to charge again. For some reason, it does not seem to hold a charge for any time at all, and as soon as you unplug it from the outlet, it starts to lose charge very quickly, as if something is draining it. I am not certain if it is a lithium battery in the thing or not, but does this mean it is trash, and I need to throw it away?? Because that would be a complete scam if so!!
Most of these use sealed lead acid batteries. Unless its the small ones the size of a couple of smartphones put together.
The symptoms your describing sounds like the battery is no good and it does happen to these units a lot when they are hooked up in reverse or not maintained properly. I use them daily for my work and often need to replace them every couple years.