Yes, it was some concern. I have owned a 2003 Civic Hybrid for the past nine years, and had to replace the battery to the tune of $1700, and therefore have been highly motivated to research the topic. The short answer is that you should not be greatly concerned. There's tons of data now, and it says that you have roughly a 1 in 20 chance of ever needing to replace the battery. Both NiMH (as found in the Prius) and lithium ion (as found in the C Max) both have routinely logged over 200,000 miles without failure. Some efficiency loss, yes, but not enough to affect vehicle performance much, and not enough to need replacement. If you want extreme examples, Google for hybrid fleet use. The taxi studies jump right out, but there are others as well, and they have hybrid vehicles with 300 & 400 k miles on them. If you are one of the unlucky 5%, current replacement costs generally run $3000 or so, and can sometimes be gotten cheaper. (Think pulling a pack from a wrecked vehicle.) I've also seen reports where they are expecting lithium ion batteries to drop by 75% by 2025, but I wouldn't depend on it. (Great for all of us if it happens, though.)