Is a clamp on resistance style battery tester valid for deep cycle batteries?

My deep cycle battery is 2 years old but i have always recharged after use and kept on trickle charger between extended periods of non use. Last trip I noticed it reported around 12.2v on my Garmin display after just a few minutes of moderate use of the trolling motor. In the past the voltage stayed higher after such light use so i was thinking maybe it was weak and I put the load test on it after recharging again.

The tester showed it as good. I’m still unsure of its health. Is this a viable test?

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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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Before I condemned battery, I’d for sure go back and check all my connections. What kind of battery?

Yes, those are accurate testers. As long as you're "loading" the battery with the red toggle in the center after hooking it up. Leave it "loaded" for 30 seconds or so. If the needle doesn't hold steady, the battery has issues. It will have an initial drop of voltage, but should hold relatively steady after the initial drop. If after 30 seconds or so the needle is still in the green range, start checking for other issues.

I would say, you have a battery issue. I always keep a battery jump start pack in my boat, just in case. I have never had to use it for myself, but i have helped others with it, car or boat.