Marien Starting Battery =/= Deep cell battery

If you are not getting up to full charge you will obviously see seemingly poor performance. You could have a NOT fully charged battery load tested and have it check ok. A load test is usually going to show you if the battery falls flat on it's face because of a bad cell but it doesn't mean it's fully charged.

Again, if you don't know what voltages you are working at you will kill any battery. You don't want to go by the monitor panel either

It's also best to know how to use a hydrometer when running wet batteries. A voltmeter is a must but a hydrometer tells the rest of the story