[dTal]

I also seem to recall that standard car batteries respond poorly and inefficiently to trickle-discharge, being designed mainly for providing a lot of amps over a short period. Providing long term off-grid power is what deep cycle marine batteries are for, and they are a separate product.

[makomk]

As far as I know standard car batteries are fine with this kind of trickle discharge. Where they fall short compared to deep cycle batteries is providing a more substantial current (think tens of amps) over a prolonged period of time.

[zaarn]

Trickle discharge up to a few dozen milliwatts is generally fine for car batteries. They don't like being closed circuit in general but if you draw more than about half a watt for a longer time period, the battery plates will start to desintegrate.