I have one to extend the capacity of my ham rig battery. It does the job to some degree. I decided to refrain from using it for the car though.
IMHO itīs not worth the hassle. It needs a well lit place and it must be packed away for driving. If you decide to use one, make sure that it has a diode (which will cost you about 0.7V) in it to prevent reverse flow of current. Otherwise it may actually drain your battery.
Itīs better to regularly check the the battery and replace it on time.
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If it isnīt broken, it doesnīt have enough features yet.