So I purchased a mostly converted short blue school bus 3 months ago. It had 4 interstate costco batteries 6v golfcart 210AH each for a total of 420 AH at 12V. I had it connected to a 2000 Watt microsolar inverter and that to my electronic system which normally sits between 60 and 120watts, occasionally going up to 240 watts for brief periods of time. This was connected to solar controllar I believe from Renology which is hooked up to 400 watt solar panels. This worked GREAT for a month. The batteries lasted up to 2 days when cloudy and getting no charge from the sun.
Eventually the batteries died due to me not knowing I needed to water them. I got new ones and hooked up a battery monitor when I installed them. Everything appeared to work fine for awhile. The battery monitor never dropped below 12.1V with load on.
Then one week it was very rainy and my batteries dropped pretty low 11.4V and I went to an RV park to charge the batteries. I hooked up my battery charger to the RV park AC and charged the batteries overnight. After that the batteries seemed cooked, after only 20-40 AH they would drop down to 11.4V effectively dead. I figured perhaps I had overcharged them so I went and got them replaced with 4 more NEW costco interstate 6v 210AH golf cart batteries.
When I hooked these up, they where showing 12.3 volts so I figured they just needed to charge up to full. (I also checked them before I installed and they read 6.15V each). I charged them up to full with solar. That night, first of all they immediatly go down to 11.8 so I know something is up. I am thinking perhaps the problem is my invereter, so I replace the 2000watt inverter with a brand new 1000watt renology inverter. Now the batteries go down to 12V immediately under load after supposedly being full. Then after only using 50 AH the batteries again die, go down to 11.6 volts (I just call it quits here this time since I do not want to damage the batteries and I have read that is a good stopping voltage for golf cart batteries.).
So what I am asking is, what on earth could be going on? It seems highly unlikely that the new batteries are bad. But why on earth are my batteries dying so fast? I am hitting them with the same load as before, and I have manually calculated my supposed wattage and it matches up with what is being displayed by my battery monitor. I have also used a voltmeter to check that my battery monitor is accurate, and the beeps from my inverter seem to agree. What can I try to debug this? Is it possible that these “new” costco batteries are just bad?