Since most of this batteries are little more then a bunch
of "D" cells, I wouldn't expect too much in the way of a
recharge.
A freind has a 1999? Toyota Corola and he bought one
and when he left his lights on, the magic battery would not
charge the Lead Acid to the point where it would even
light up the headlamps.
I have used a Panasonic LC-L1224PU 12V 24Amp/hour
gell cell to start my 1985 Celica and my 1991 Honda Civic.
I bought 2 Radio Shack #23-119 6V gell cells designed
to power those little kid cars.  While I bought them mainly
for emergency  use to power my ham and SW radios.
I wired the to 6V batteries in series for 12V. So I decided
to see if it would start a car. I had no trouble starting my
1985 Celica with this battery.  My Civic is much easier to
start. I got them on close out for 5$ each. I intend to put them
in a mil surplus 223 ammo case and wire them into the Civics
electrical system to they will stay charged. I had to remove
the supplied 25A fuse and I intend to add some heavy duty
(5/8" insulated bolts) to allow emergency jumping to  my Civic.
Unless you are willing and able to to something like this, I
suspect those little emergency batteries are going to leave
you stranded.
I intend to use a 10Amp, 100PIV Si diode, in series with a 5
Amp fuse to keep the battery charged.
The slight (0.7V) drop across the diode will leave plenty of
voltage to keep the Radio Sahck battery cahrged.
My Civic produces something like14.6V  at any engine speed
above idle. That gives 14.6-0.7V=13.9V for charging/.
In a perfect world I would want ~13.69V to chanrge a gellcell.
But 13.9V will work great, tough it might shorten the life of the
gellcell. The last gellcell I bought before the Panasonic lasted
for over 25 years. It might have lasted longer, but my homebuilt
charger failed and applied 15V and the battery failed.
GellCells are exactly cheap, but with some carefull shoping
you can sometimes find them for pretty cheap.
I got the Panasonic from a friend who works for an alarm
company. "High Value Sites"( liquer stores etc) get new batteries
every 18~24 months. The batteries are cheap and thier insurance
demands no failures. WheelCahir rental and medical supply
companies are also good places for older, but still functional
batteries. You will have to roll your own case, charger etc,
but you will have a much better product.
Terry