Battery Question

no, doesn't it have a wall plug? otherwise, you will need a 12v lithium battery charger

 

different charging cycle. most of the compact jumpers are lithium, but you want to know for sure.

 

Often the old lead acid jump boxes have a ruined 12v battery from a long period of non use. Trying to directly charge this battery in parallel with a 100 amp vehicle charging system could result in smoke or at least fail to charge. A separate 12v dc to dc charger or a more sophisticated microprocessor based four stage ac charger is recommended. A battery tender jr for $35 might save the old lead acid jump box. However the newer lithium jump boxes are much more reliable, last longer in storage and can do useful things like charging your cell phone. The lithium units also ensure a reverse polarity jump won't accidentally strand your Prius with a repair costing hundreds.

You can charge a lead acid jump battery off a car charging system. The car charges by raising the charging voltage from 12.x to 14.x. The jump box itself probably came with an accessory to use in the car's cigarette lighter charging port which would be safer. Normally the accessory or the jump box port has a voltage regulator and fuse to reduce the lead acid charging voltage to a float voltage around 13.5v. Not fast but much safer.

In most cases the best use of a lead acid jump box is for ballast in the trash bin.

 

You need a regular ac input battery charger. A battery discharged to 6v is not going to hold a charge for long even if it comes up to 12v eventually. I would not put it on the cars charging system. There are cheap chargers at Harbor Freight if your neighbor does not have one. The Battery Tender Jr at Walmart or Amazon has the best chance.

As an option you could buy a small agm replacement battery for $20-$30 just as easily. Agms are "dry" lead acid batteries. Any lead acid battery will self discharge much faster than a lithium. Usually in months versus years.

Conventional lead acid batteries are considered "wet" batteries and will make a mess if they leak during an excessive charge attempt.

Cheap float charger
Automatic Battery Float Charger

 

I agree with @rjparker. Trying to charge a battery that's down to 6V with your car could prove to be the most expensive way to fail to get it charged. Just not worth the risk to save $30-40 for a regular battery charger. And, as he said, if it's down to 6V, it's most likely beyond help.

 

pretty tough to charge a battery from a battery under most circumstances, unless it is well designed

 

Biggest problem with this is there's no way to control the amperage flow at the initial connection. I would be very leery about doing it while the 'good' 12v battery was installed in my car and planning to jump a "2 year unused" battery of unknown condition. Slightly less if both batteries were on the ground with a heavy duty set of jumper cables. Slightly more if it were a little bitty prius battery trying to charge a 'dead' battery 2x or more its size.