Some questions to clarify,
Does the battery go dead from being charged in 24 hours? You state 1 full day and I'm just checking that it goes dead from full charge in 1 day. That is a lot.
You said charging with a generator, is this using the 12 volt option off the generator, using the camper converter or using a stand along 120 VAC battery charger? If so what brand and size genny, converter or battery charger is it? Need to figure out how many amps it is putting out.
From once it is dead, how long (how many hours) do you charge it for?
Is it a group 24, 27, or 31 battery? Grp 24 is the standard Sunline size.
Do you have a digital volt meter? Need the voltage at the battery 8 hours after charging with no load at the toime of measuring and then the voltage when you say it is dead?
We are assuming this is a lead acid battery and is it a marine or deep cycle battery?
Sorry for so many questions. Need more info to try and help sort this out. Trying to figure out if your battery was ever fully 100% charged or even 90% and then how far it drains down over 1 day. Did you mesaure it or just run the genny a long time?
For a group 24 (standard Sunline size) or a group 27 or larger battery to go from 100% down to 20 % dead in one day it points to a very high current draw or possibly a bad cell in the battery. That is if the battery was ever back to 100% to start with. If the battery is really dead, down to 10.5 volts, then on a standard charger of only 13.6 volts could take 50 to 70 hours to get it back to 100% state of charge on a 126 AH battery. That may be bigger than yours but you said a HD battery so it may be.
A high end high current 3 stage charger with the right heavy sized cables to the battery may bring it to 90% state of charge in 3 hours pumping 60 amps into an approx 125 AH battery and then take up to 15 hours at standard charge to get the last 10% in. On my genny setup with a 4 stage charger I can only bring the battery to 90% in the genny run time hours at the parks I visit. And that is when the battery was never drained very far down. The size of the wire from the charger to battery on the big high amp chargers affects the charger rate. I suffer from the standard RV wire size and have not made it yet to putting the big cables in and moving the converter right next to the battery.
You may be suffering from not really getting the battery back up to a very high state of charge. If you are not measuring voltage at a resting state (8 hrs or more after charging) or using a hydrometer you may not know how charged up it is. Battery charging can be tough to jam all the power in there on a genny setup.
PS. If you are near an Advance Auto or Napa store they have battery testing devices that can use to tell if you have a bad cell. Or if you by chance happen to have a hydrometer you can check the electrolyte in each cell and tell.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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