I have a spare car battery sitting in the shell................

My questions is, will the battery just die slowly/lose power over time or will it be like a Duracell battery and be ready to go when needed?

they'e normally ok if you keep them on a maintenance charge (solar chargers are pretty good)

If you just leave them though they gradually die to the point where they wont take a charge at all.

Yes, it will self discharge. Typically around 5% per month for a lead acid battery, but it depends on temperature, age etc. For the best life, keep it between 0 and 10 deg C and the charge topped up as best possible.  Visiting from DISCO3.CO.UK
Discovery 3 TDV6 Auto HSE Zambezi Silver

thank you.

Worth looking at some search results on the CTEK chargers. well worth the money.  405 AB exec seats Baltic and Cirrus
93 RR Classic efi

Remember it's easier to get forgiveness than permission!

Gone in order:
4.4 TDV8 SE - gone to a good home
93 Classic hard dash Plymouth Blue
03 L322 Oslo Blue
2000 Disco TD5 ES Epsom Green
98 P38 Rioja Red
89 Classic Cairngorm Brown

or the lidl one that they have on offer from time to time.

On offer this weekend I think.

interestingly, I had to pull an older 100ah bosch s5 out of the garage today to jump the mrs skoda after i flattened the battery listening to the radio whilst changing the suspension stops this morning.

It had been fully charged last winter, on the ctek, started the car no probs as it should being 100ah, then took the ctek only about 1/2 hour to fully recharge it, so must have been pretty close to fully charged  There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
Fiat 500x 1.4 multiair Lounge 2015
2010 LR D4 Commercial 2.7 TDV6