A lot has to do with what kind of batteries they are.
Are they NiCad, Lithium Ion, or Gell Cell? I have heard
that the life span of Lithium Ion batts. is shortened
by totally draining them. I don't think it does any
battery any good by letting them go flat for any period
of time. I would mark my calander and once a month
give them each a charge. Of course a trickle charger
would be ideal, but to find one for 18 volts would be
dificult. Also, keep them somewhere out of freezing
temperatures.

They are B&D slide pack nickel cad.

So letting them sit around (indoors) with a monthly "top up" charge is my best bet? I can do that.

NiCads present another problem, and that is 'memory effect'. Battery experts say you should occasionally
drain the batteries completely and then charge fully
to avoid the 'memory effect'. In my power tools with
nicads I always exhaust the charge on one batt. and
then use the other while the exhausted one is charging.
However, I don't believe a few months with an occasional
top off charge will harm them. I have let some small
cell nicads sit for a couple of years and couldn't bring
them back to life.

I think no matter what yo do they going to go bad. may take 1 year or 3 three. Keeping them full charged and out of cold weather IMO can't be a bad idea, but that still don't mean they won't go dead next spring.

I agree, rcmoser. The problem is that inside those
plastic shells are a whole bunch of little batteries,
(mostly sub-c cells), and all it takes is just one of
the cells to go bad and others soon follow, (sort of like
one bad apple in a bushel). I have a 12volt Dewalt drill
that one of the batteries would not charge. So, I just
had to tear it apart. Inside are 10 sub-c nicads in
series. Only 2 of the 10 were bad, but one of the two
had actually reversed its voltage, and the other one
was totally dead. And like you say, they can go bad
in a week, or last many years, but eventually they
will go bad. And what really iritates me is the
price they command for new batteries. They gotcha
by the short ones, (if you bought their drill, then
you gotta buy their batteries, no matter how expensive).
The Gillette Principle.

FYI... it occurred to me to look on B&D support for their recommendation. Which follows.

It is safe to leave slide pack batteries charging for the entire cutting season, because they have an internal time and cannot be overcharged.

After the cutting season, our recommendation for NiCad packs is to store them indoors in a location between five degrees Celsius and 25 degrees Celsius, or between 41 and 77 degrees Fahrenheit.

After the last use of the product of the season, take the batteries off the product and store them off the charger in the temperature range mentioned above. The batteries do not have to be charged up before storage, and can be stored at any level of charge. Simply store them after the last usage.