I am sure someone will also check in on this. As an automotive technician for over thirty years, that tester is really only good for non deep cycling batteries. A deep cycling battery gives out its voltage over a long period of time. As a starting type gives off a lot of current in a short time. That tester is for starting type batteries. They are a very good tester for that type of battery. I have one in my tool box. But for warranty purposes I have to use a digital battery tester. The newer battery testers such as the GM GR 8 tester will do all types of batteries including deep cycle, glass mat. Hope this helps you out, sounds like you are losing your battery.
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