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It sounds like that product isn't going to solve your problem - it will prevent damage to your battery due to over discharging.

Your immobiliser should not discharge a healthy standard battery within 4 weeks. If you are away longer than 4 weeks connect a float charger as mentioned above. If your battery drains in less than 4 weeks and is good condition (note that severely discharging a battery reduces usable life significantly) you need to isolate the device that is draining your battery.

Why don't you just disconnect your battery when you are away?

Having an alarm switched on isn't going to do you much good if you only drive it every month or so.

Disconnecting your battery and not making it obvious that it is disconnected, will also be a good deterent to thieves as they won't be able to start it anyway, and being as dumb as most car thieves are, they won't even know where the battery is!

Trickle Charge is the answer.

Only if you leave your car in a garage with an electrical plug nearby, then you can buy a 12v car trickle charger. It'll keep your car battery fully charged everytime.

I have one for my motorbike where the clips screws on the battery terminals and then the rest can be remove through a quick release plug attached to the unit when not in used.