I always plug my battery in to the battery tender. However, this past week I tried starting it on several occassions and it wouldn't turn over fast enough to make it start. I thought the thing was getting weak earlier this summer, but I thought I could limp it along by keeping it plugged in and 'charged'. Is this a good sign that I need a new battery or could something be draining the battery? It's an '03 with 20,000+ miles and original battery.