I have the Kobalt mower and string trimmer, which use the same battery and charger as the chain saw. I have 3 batteries, one 4 Ah and one 2 Ah. After learning about how storing Li-ion batteries at full charge can reduce their lifetime, I now charge them only to 3 bars out of 4 (they have an LED charge indicator on each battery) and then store them off the charger. It means rotating through batteries a bit more, and doing the 3/4 charge involves setting a timer to check on them before they charge fully, but I figure that what will eventually render the tools useless will be dead, obsolete, unreplaceable batteries, so I'm trying to make them last. This 3/4 charge thing may be unnecessary, if the designers arranged for the charger to not charge to the theoretical maximum anyway, but it can't hurt.
BTW, if you want a second battery, consider buying the string trimmer, hedge trimmer or leaf blower. They come with a battery for only $50 more than the battery alone.
And the batteries don't seem to lose charge on the shelf.