First, the lower charge rate is quite safe although obviously takes longer to charge the phone than you might like.
2nd, I suggest that you install an internal wireless charger and not worry about charging any more.
I am using this one:
* Qi Standard Wireless Charger kit 'mine' for $18.99 and includes the Charger Pad
and for a very reasonable price it has the charging pad with it. It has been in the phone now for about 4 months and never missed a beat at charging up the phone. I am using the original 2.1 amp Samsung charger module that came with the S5 phone to provide the power to the charging pad.
You mention a car charger. I should think there shouldn't be too much need for that unless you are away from home for a very long time. I leave my phone on the wireless charging pad until I go out the door. That way it is always at 100 percent and ready for a long day's use.
There are other ways to enjoy a long life battery away from home, if you are willing to switch the internet OFF when the phone is idle. I use MacroDroid to sense the screen status.
When the screen is Locked, MacroDroid turns off the internet (DATA and WIFI )
When I unlock the screen, MacroDroid turns it back ON again.
I can go for days at a time w/o having to recharge the battery if need be. The phone at idle with go 24 hours on about 5% or less battery use.
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