Lithium Batteries have limited Shelf Life from Date of Manufacture, even unused. The _original_ Li Battery on my backup-to-the-backup Powerbook G4 just died after ~8 years, even though it was plugged in to the Charger 99% of the time.
You probably only lost a single cell, which kills the whole pack. I see this quite often working in the IT world.
Or ~850 hours out of a 100 watt Solar Panel= ~110 days, assuming good conversion efficiencies and ~8 hours of Sunlight a day. Say three months in reality to recharge, assuming no other Electrical load.
That's an awfully small solar panel for an EP boat. If I had a blue water cruiser that was EP, I'd want a 500W set of panels over the dinghy davits, and maybe a wind generator besides. On top of that, any EP boat has regen, so you'd have more than enough power for everything on a long passage. A recent test of a 48v 10kW system (which is only for a ~30' boat, not a blue water cruiser) yielded ~500W of regen at ~7 knots.
At least with Lithium batteries you have no chance of charging them too fast, even if you could magically combine shore power with your solar and regen. Most can be completely recharged in an hour with sufficient current. Some can be charged even faster than that.
The _only_ reason that Electric Cars on the road, like the Tesla, makes sense now is that the Charging Infrastructure mostly already exists. (Formula E is making inroads in off-road Motorsports, and I think that it's really cool. Still, only a handful of Tracks are equipped so far.)
Such Infrastructure does _not_ exist for Sailboats, and it is very unlikely, given the diminishing market, that it ever will.
I agree. A car is much more like a power boat, though, and I think we're a very long way away from seeing a battery equivalent of a 200 gallon tank of diesel, even if some of the recent breakthroughs make it into production right away.
If we're honest about our boat usage, I think it would be a pretty rare instance that you would need to cruise under power long enough to drain the batteries. Of course, if you found that you were doing that regularly, you could always install a generator.