
When will people learn!

The trouble is the ballast in the concrete and as every fool knows Ballast 
resistors are used to discharge batteries. Furthermore it is very silly to 
store the battery with the terminals downwards as you must have done to 
contact the ballast. 

Seriously: self discharge (the actual problem, as stated by others) does vary 
greatly with certain types and freaks show low self discharge. I have in 
fact seen ordinary automotive batteries which have effectively held full 
charge for > 2 years so it must be possible.

If your garage is heated, store the batteries somewhere cooler but above 
freezing (flat batteries freeze more easily). Occasionally charge it (once a 
month?) or even leave it on 'float' charge permanently (special charger, 
DON'T do this unless you know what you are doing, seriously dangerous).

Anouther point is the unsuitability of automotive batteries for things like 
electric mowers -- they are not generally designed to be repeatedly deep 
discharged and their life may be greatly shorted. Some early zero maintenance 
automotive batteries in fact responded to a full discharge with total failure 
shortly afterwards but modern ones are superb. (6yrs, 95000 miles and 
counting)