Jump packs are limited in the ammount of power, like nicads. But they are seriously convenient.
Most of them have an AC plug, so you can charge them off the
110 VAC power at home. Many have a "car charger" cord. So you can charge off the lighter socket in your vehicle while the car is running. If you are electric minded, you can make a cord with a lighter plug on both ends. Back feed power into the jump pack battery, so you can charge on the fly.You can also charge with a solar panel, that sits in the window of your car.
With any where from 9 to 22 amp hours gel cell, they give you some power to work with. Jump packs are designed to supply enough power to spin the starter for a moment, and get the engine going. Clipping a jump pack on will not charge the battery under the hood. But, after the engine starts, you can leave the jump pack clipped on. If the alternator is working, that should charge both your jump pack, and the starting battery.
Some jumper packs are strong enough to start van or sport ute vehicles. Depending on the size of the battery, how new the pack is, and if the pack is charged. Many people don't know that you have to charge the internal cell now and again. A smaller unit, or an older one. might not jump a 6 or 8 cylinder vehicle. I have jumped my own Blazer (6 cylinder, 4.3 engine) at least four or five times, when something was draining the power. The starter would go "garowf, click clilck click" and fail to start. I'd clip on the Winchester pack, and it would start right up.
I've also jumped at least three other peoples cars with my jump packs. One in the church parking lot during sunday school. One time at Kmart, the fellow offered me a $20 tip. Helped me pay for the unit. For me, it's a "don't leave home without it" item.
Both my jumper packs have a small inverter, so I can provide small ammounts of 110 VAC house power. Modified sine, but pretty good for some uses. Runs a drill, or a small light. I've used mine to run my microfiche machine. Walmart had for a while, a battery pack with an inverter, in either sporting goods, or automotive. They started out about $80, and have come down. I suspect they are discontinued.
I've seen at least a couple Harbor Freight units that won't hold charge any more. I avoid that brand of jumper pack. The one time I wanted to jump my Blazer, the HF pack wouldn't do it, but a Winchester pack would. Yes, the HF pack was fully charged.