What to do with a lead-acid car battery?

[Piwoslaw]

I recently bought a new car, but it came with a cheap, undersized battery. For the engine Start-Stop function to work, I had to get a larger AGM battery. Now I have the old one as a spare.

What can I use a 12V, 510A, 62Ah lead-acid battery for?

• I could keep it in the car, wired parallel, and delete the alternator, but the alternator doubles as a starter. And the ECU already turns off the alt field when the battery is topped off.
• I would love to use it to catch solar or wind, but our house is shaded year-round under tall trees.
• I could charge it at off-peak hours and power the house at peak hours, but our utility does not offer such a tariff, only flat rate.
• I could just leave it in the garage, topping it off every weekend with a trickle charger, and use it for testing random electronics projects, but that seems like a waste of a nearly new battery.

Any other ideas?

EDIT: One more idea would be some kind of e-assist project for a bicycle or bike trailer.