Battery condition question

[MikenOH]

I usually have our 06 on a trickle charger when not being driven on a regular basis. Lately, the trickle charger light seems to be on all the time, indicating it is charging the battery. This is the porsche plug in charger.
Had the car out today and drove it at highway speeds for 40 miles and when I returned and plugged the charger back in the light came on and was still on 3 hours later.

Since the battery is 5 years old I'm wondering if it's on it's way out. FWIW, the car started without drama and no lights came on.

Can anyone recommend an inexpensive battery test tool?

thanks.