A way I sometimes test is to charge to 100% then let sit half to one day for the voltage to stabilize. Watch the voltage while you pull the kill switch and crank the motor over for 3 seconds. The voltage should drop substantially and then slowly rebound to a couple 10ths below where it was in a few minutes (starting at 12.4, it may fall to 11-11.5 while cranking and then slowly go back up to 12.2-12.4) . If it drops to 10-11v and stays there, battery is toast.
Youll know if the battery is bad, it can sometimes be difficult to know if it is weak. An automotive shop will tell you its weak and they suggest you replace it to be safe, every time. I once took a return to AutoZone because i accidentally grabbed a 27 instead of a 29 and didnt notice until i went to install. I was standing by the counter waiting for help, and before i could open my mouth the "associate" said set it on that cart. I did so and he wheeled it to the tester and started hooking it up, by now im interested in what it will say so I kept my mouth shut. "Your battery is showing weak, but its within warranty so we can replace it for free today or upgrade you to a better battery for a fee". I said im good, i was just going to exchange it for the next size bigger, since i bought it last night and never used it (while handing the receipt over). The look on his face was hilarious, and the manager who overheard it was FUMING.
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