My wife need a new hedge trimmer and I can get the EGO without the battery for $99 here in my neck of the wood so one battery is good enough for all of our need and the few times I use the chainsaw.
About batteries life:
As long you used and charge them frequently Li-Ion batteries can last a lot more then those sitting for months. Here a good link to learn the science of batteries charging:
http://batteryuniversity.com/learn/a...ased_batteries
Have you ever open a Li-Ion battery pack before? I rebuild some old laptop batteries pack with new cells. You just need to be careful breaking the case and even if you damage it a bit, you test the pack and seal it with epoxy for good!
PS: I even replace the cells by bigger capacity one (mAh)