SureFlap Microchip Cat Flap - Smart Entry for Cats with Microchip ID, White, 4.75" H x 5.625" W Opening - Perfect for Indoor/Outdoor Access & Multi-Pet Homes

I have two cats - one is a bit of a bully and the other is the victim of his bullying. I use this door to give my female cat access to my utility room without my male cat being allowed in. This results in a better quality of life for her by giving her a private place with food, water, litter, and a bed where she can escape from the other cat for a while if he tries to start a fight.The door does a great job at opening only for the cats it's supposed to open for. When she approaches, you hear it unlock, and it locks behind her just a second or two after she gets through. It would conceivably be possible for another cat to follow in the cat who is supposed to be allowed in the door, however the timing window would be tight and while it is technically possible, it's unlikely the other animal would act quickly enough to get in before it locks again. Installation into an interior wall was easy, I can't speak to installation into glass or an exterior door or wall - any of these are likely much harder.I can't speak to battery life - I've had it for a couple months now and the batteries are still going strong. But the product works and works well. If you have a space that you want your cat to be able to access and no other critters, this will do the job. There are cheaper devices on the market which use a magnet that you put on their collar. However, that would mean that you have to put a collar on your cat and if it ever lost its collar, the door wouldn't work. This solution is much mor elegant as long as your cat is chipped. It doesn't depend on anything that the cat could possibly lose. All in all, it's a solid product. Expensive, but it does what it says. Think about whether you really need this or whether a similar model that uses a magnet on the collar will do. If you do need the feature that this offers, it's great and you won't be disappointed.