In some cases not even that. Modern Lithium-ion batteries are quite good, but self-discharge still is 2-3%/month. So, if you want to store for months (say charging a battery using the summer sun to heat a house in winter), you easily lose 10% to self-discharge, in addition to what you lose between charging and discharging.
https://en.wikipedia.org/wiki/Self-discharge#Typical_self-di... says NiMH batteries even lose 30% per month.
70-85% retained after a year is a lot better. Charge in the same charger. I used to use regular NiMH infrequently, the worst way to use them. Low sd way better for that application. Also NiMH doesn't lose capacity delivering high currents like AA alkaline does.
Yes, but this is changing and there are a number of very interesting options.
EDIT: These batteries seem to be even less energy dense than Li-ion which is one of the weaknesses of batteries in comparison to hydrogen.
Batteries are great for storing electricity efficiently as you say, but that only matters when electricity is marginally expensive (sourced from fossil fuels for example). However, if you have a cheap enough marginal cost of electricity, hydrogen is a better option because of the much cheaper fixed cost of hydrogen storage per kWH.
A good way to see this: what option would you choose for storage if electricity on demand was free but only during certain parts of the day? A hydrogen tank or batteries? A hydrogen tank is cheaper. And clean energy is essentially free besides the fixed cost of the windmill/solar panel.