My neighbor who does not work on cars just had his 12 volt battery replaced. He does not look forward to doing this every 12-18 months. He asked me if I could hook up a Battery Tender to his 12 volt battery. It looks pretty simple to me to do this. He would like to top off his 12 volt battery for 8 hours once a month. He intends to do this while not charging the main battery. Any one have any concerns on why he should not do this ?