I have 12 V5 kits. In the first week I had 2 batteries just stop working--no lights at all. I do have the new firmware. I rec'd 2 replacement batteries. Now I have 2 more that are dead the same as the first. We are using them just to learn basic programming. We use them 40 minutes a day. What is the recommended charge protocol? I want to be sure we are doing everything we can to prolong the life. Any help appreciated.