Building a car battery recharger

A friend brought this over and asked me to test its voltage to see if its charged, he figured i would know how much voltage it should have after recharging
It measured 18V after about an hour of charging, and it was quite warm, almost hot when he brought it in.
I took it apart, it has 20 AA alkaline primary cells (no name brand) that are not listed as rechargeable even though it says on it the package to recharge for 45-90 mins after use. The batteries ranged from 0.75-1.47V, all connected in series and no charging circuit (save some resistors and certainly no balance charging)
The cigarette lighter plug that comes with it says operates up to 30V, and 20 alkaline would put out about 30V. Am i correct in assuming this may fry a car’s electronics when plugged in for 10 mins to ‘recharge’ a car battery?

Also, i assume i could make something superior from NiMH battereis, i figure about 13 batteries for a bit over 14V under load in series, and i can recharge them on my bc700.

Your thoughts?

A car battery has a much higher capacity than 30 AA alkaline batteries, so it is not possible to overcharge the car battery with that “Jump-Emergency-Starter”.
The voltage of the car battery won’t get too high, the AA batteries will be empty long before that.

But if the cigarette lighter is disconnected from the battery (contact off in most cars) the “Jump…starter” can damage all the other things that are connected to the same circuit because the voltage is much too high.
It is not safe to use, the current in the AA batteries will be very high, it is even possible that they will explode.
If you try to recharge the AA alkaline batteries in the battery pack, it is also possible that you get an explosion because alkaline batteries are not rechargeable.

I don’t even want to try that thing on my 12 year old car…

its not capacity i fear, its voltage, how high is a 12V system designed to handle
he already ‘recharged’ it before i saw the thing, my advice is to throw it away but a successful non destruction in one use probably trumps my sage advice.