OK Tom,
One more shot at this based on your info. Sounds like you do have the Parallax 5300 converter unit, so I suspect you're correct about your 2018 being a fairly early build. I also suspect that your fundamental problem with the dead coach batteries tracks to the loss of electrolyte from overcharging during a protracted period of storage. For that to be the case, you would have had to be plugged in to shore power (the 15-amp connection), and the main battery disconnect (or power switch) would have to be "ON". I believe those are the conditions that you outlined in your initial post. You saw the other reports in the other thread you posted to about similar problems under similar circumstances. That set of conditions that we just outlined is the way you have to have things set to allow battery charging during storage (as you yourself said), but unfortunately, with the Parallax converter-charger, during a protracted period of charging (especially without attention to battery water), the single stage charge system of the Parallax unit eventually contributes to excessive out-gassing which gradually depletes the water in the cells. Once the electrolyte level drops below the cell plates, the capacity of the battery to produce current is increasingly reduced and as it gets lower and lower eventually the battery is dead. With water (electrolyte) replenishment, and a recharge effort (especially with a "smart charger") it is possible that the battery may recover either partially or largely, depending on age and general condition of the battery. That's something that will have to be established by trial.
In retrospect, a better course of action for storage with that unit would have been to put into storage, fully charged, with the main battery disconnect switch OFF. That should minimize any incidental current draws in storage. Then, on your periodic visits to run your generator, turn the batteries on and let them "top off" while the generator is exercising. I suspect that for a 4-5 month storage period, that approach would have kept your batteries at a reasonable level.
Some options that are available are to replace the single-stage Parallax converter-charger with a multi-stage unit from Progressive Dynamics or another manufacturer as was detailed by some of the posters in the other thread you used. That's not an expensive route, but might seem a little challenging if you aren't familiar with electrical work. Another consideration (especially if you end up having to replace your existing "flooded cell" type batteries is to move to AGM (absorbed glass mat) sealed units. That will help relieve the electrolyte maintenance challenge. The other route that some PC owners have used (Jatrax has some good input on this in forum posts) is to install a TempAssure component from Parallax that they provide to help address this problem. This is an add-on item that attaches to and senses battery temperature, and sends that reading output back to the Parallax 5300, which then adjust charge rate to reduce output to prevent overheating the battery. The battery overheating from the continuous charge from the single stage charger is the mechanism that causes the electrolyte to out-gas. It's a pretty simple install that only requires a single wire to be run from the battery compartment to the 5300.
At the risk of cluttering this post with still more "verbiage", I'm going to copy a couple of excerpts from the Parallax site below. They provide a general description of the TempAssure option, but in the second paragraph please note their "disclaimer" regarding electrolyte level on flooded-type batteries. They are well aware that their unit will potentially result in loss of electrolyte if the user doesn't keep a close eye on it!!
"Your 5300 system may include the (optional) TempAssure™ module and sensor cable. When properly installed, the TempAssure™ system employs technology that allows the output voltage of the converter to be varied by the temperature of the RV’s “house” battery system. The benefit of this feature helps to provide a more appropriate charging voltage for the “house” battery system exposed to varying ambient temperatures. For example, in cold climates the output voltage of the converter will be increased, which results in a more fully charged and maintained battery. In hot climates the output voltage of the converter will automatically lower, which will aid in reducing excessive out-gassing of the battery caused by the battery requiring a lower “float” voltage."
"While the 5300 series converter with the addition of the (optional) TempAssure™ module is designed to provide a better means of charging and maintaining the “house” battery, Parallax Power Supply strongly recommends proper battery maintenance be performed by the owner on non-maintenance free batteries following the battery manufacturer’s recommendations.
DO NOT allow the battery cell plates on non-maintenance free batteries to become exposed to the air. Poor battery performance will result.
Parallax Power Supply will not be responsible for battery failure resulting from improper battery maintenance."
All the best, Mike
