I have an O2 sim in my tracker. I have selected "auto topup" on their web site, & you can select at what "money left" level you wish it to be topped up. I selected £10.
They don't necessarily add £10 every 6 months that way.
Re battery drain: If you allow the tracker to "sleep" then drain is very low (6 months to flatten your battery) However, if you set up any kind of regular update then it does not go to sleep, & battery will last (in my case) 1 month. For the first few years of having it, I did not select any regular updates, but just tried to remember to interrogate it every so often (with a phone call) I used to forget to do this, & so O2 disconnected the sim on two occasions ! I was able to convince them to reinstate it. I then thought it would be a good idea to set up a regular position update (text message from device) I chose the longest period available which was about every 3 weeks. Over Christmas I did not use my TR for around 4 weeks, but when I went to - flat battery. Puzzled, I did some checks on current being used, & realised the tracker was to blame. have now reverted to no regular updates, and all is well again.