For the last 10 years or so I’ve had one of those permanently attached ( and wired directly to battery ) on my Tacoma bed tool box. I do take them apart and waterproof the wiring and diode better with silicone caulking. Harbor Freight sells them also.
P.S….I never disconnect either charger. They have diodes ( one way electrical check valves) built in.
If you plug it into a cigarette lighter outlet make sure it’s a permanently on outlet.
Also unless your car sits for long periods it’s probably not even necessary to use a trickle charger. My vehicles sit for a week or more sometime that’s why I use them.
I recommend against the cheap ones.
I had one last 2 years and the next lasted 2 months.
Neither met specs when new.
I’ve gone down a different path, but if I were to go back to solar I’d get a major brand like Coleman or “Battery Tender”.
Those run about 2x the cost of the Chinese odd-name stuff.
True, you get what you pay for……mostly.
I never use mine behind automobile glass always outside. I’ve left the Harbor Freight chargers on camper batteries and farm tractor batteries (fully charged first) all winter long ( western MD & WV ) and come Spring the batteries are still fully charged.
This is the simple diy’er battery/alternator test I use: Before first start of the day (car unused overnight), the battery should measure about 12.6 volts. Then immediately after starting engine, 13.5 -15.5 volts. Suggest to ask your shop to do that test for you, and report measurement here.
On my Corolla, when it is driven 15-20 miles daily, the first test is always in the 12.5-12.7 range. And the second is in the 13.5-15.0 range. I’m guessing your volt meter may not be perfectly accurate. Test some known-to-be-correct voltage maybe. Or same test with different volt meter. New & unused alkaline batteries usually measure around 1.65 volts for example. The range-switch on DVM can corrode/wear out and cause inaccurate voltage measurements. Try switching it back and forth several times. Does that cause the voltage reading to change?
Does the battery have removable caps or lids over 2 sets of 3 cells? If so, you can remove those caps/lids and starting at one end; probe 2 adjacent cells with a voltmeter. You should see 2.1 volts on a good battery. Move each probe down one hole and repeat. You should see 2.1 volts on all of them. If you have a cell showing say sub 2 volts then it may have a cell dying.