I bought an Optima commercial battery for Liz's car, a diesel that needs a lot of cranking in the cold, for $208 on Amazon, with free shipping. Terrific deal. Next day I looked again to get some dimensions, and it was $285 plus $40 shipping. Hmm.
I bought a bluetooth interface for the hifi (for a lot more money), and it works both directions so I can play the cellphone over the hifi (why, I don't know) or use a set of bluetooth headphones so that I can watch movies without disturbing anyone else.
There is a slight delay in the signal to the headphones. It is a problem when someone wants to watch with the volume turned up a bit, but not up enough for me. The girls have young ears, I do not. The signal reaching the phones clashes with the sound from the set because they are not in sync. When I watch by myself I turn the set volume all the way down, and no problem. The delay in the headphone sound doesn't bother me, it is slightly out of sync with the picture but not horribly so. But one CANNOT listen to the bluetooth-ed signal at the same time as the original. So be careful how you plan to use it.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida