It is an 18 Amphour capacity AGM battery. Likely the UB12180 rebranded as Mightymax.
https://powerwerx.azureedge.net/productattachments/D5745.pdfIdeally, for maximum possible battery longevity, if it were discharged to 50%, or ~12.2 rested volts, one would apply about 5 amps until voltage reaches 14.5 to 14.9v then hold that voltage until amps taper to 0.09 amps at which point it would be considered fully charged.
Anything less than this, such as whatever voltage the alternator is allowed to maintain by the vehicles voltage regulator, likely into the mid 13's not too long after engine starting, is going to slow charging and be far from ideal for recharging of it, but any charging is better than no charging.
I have this mightmymax 18AH AGM battery, the same 35$ battery.
Not sure how much effort you want into perfectly recharging a 35$ battery for maximum longevity. Good enough is subjective opinion.
If you want a small sized charger. I'm not really sure what user friendly product to recommend.
These 18AH AGM batteries often come inside lead acid jumper packs and often these simply have single voltage 0.5 amp chargers inside of them that look like a wall wart. Very similar looking to some battery tender maintainers. These are far from ideal for recharging these AGMS, yet these jumper packs still seem to be able to do their job for a few years anyway, if regularly plugged and not allowed to self discharge too far.
Something like this could be more than 'good enough'. I have Zero experinece with this particular product, I just saw it is about 5 amps and 'should' be able to get a 50% depleted 18Ah agm battery back near the full range in about 4 to 5 hours.
https://www.amazon.com/dp/B07G98SN92/ref=sspa_dk_detail_3?psc=1&pd_rd_i=B07G98SN92&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyNTlQTkU0MzZCSTQxJmVuY3J5cHRlZElkPUEwNzkyMDczM0tLWFZOSUVIUU9WJmVuY3J5cHRlZEFkSWQ9QTAyNTU4NTIxODVVNTBWMDFUMURTJndpZGdldE5hbWU9c3BfZGV0YWlsJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==Again I am not vouching for the efficacy, function, or durability of the linked charger. It simply has a small physical size as requested, and is just under the maximum 'recommended' 5.4 amp charge rate of the 18Ah mightmax battery(Ub12180)