Ihave never really gotten ahold of a bad battery tester, Ihave the type from tower that Ken showed and an older ACE tester. My biggest problem was finding a good Tach, Itried three of the cheap ones then spent the money on the TNC Tach, a bit over kill for most people. Ihave tested my Tach with the combo Tach/Volt Tester sold through Horizon and found it to be right on with mine, a bit high priced but you get two good testers for the price.
Istill have my little tower volt meter in my flight box in case of a dead battery in my ACE, it really works pretty good!! And the price is right!!