Everything the machine does reduces battery life some, exactly how much is pretty much specific to a given battery. TM's initial backup can take some time depending on how much data you have, that will certainly use more of the battery than a periodic backup of only the changes. And, all usages hasten end of life but you can't prevent that.
The decision you have to make is battery life vs data security. I've never felt the usage caused by TM to be significant enough to sacrifice security. Generally, when TM runs I have an external drive attached as well as the charger. As to it hastening battery replacement, well ... I guess it does somewhat, but everything else does too.
This is just the nature of the machine. Personally, I bought mine to use it until it breaks; I'll deal with breakages when they occur. But I need my machine now and have a recovery plan for that time when I can't use it any more.