Solar-charging RV battery when not in use

solarkris

I solar-charge my RV battery whenever the RV is not in use.  There is no load on the RV battery while doing this.

The setup:
- RV battery: group 27 Marine deep cycle hybrid, 12 volt
- 3-stage solar charge controller
- 40w solar panel on dashboard, gets sunlight through the windshield for approximately 5 hours a day

Q: Which of the following is healthier for the battery over time?

(1) solar-charging every day for several hours to keep it at or near full-charge

(2) charging it once every few weeks to bring it back up to a full-charge due to minimal self-discharge over time

IF the correct answer is #2, then what would be the lowest I would want the battery to go.  Maybe around 12.5 volts?

Thanks so much!

Estragon

IMHO, it depends.

If "minimal self-discharge" means this is an AGM battery, the best solution may be to remove it and store it in a cool location. Constant charging/floating, especially if the sun warms the RV and the charging voltage isn't temp compensated, could be problematic. An AGM should be fine in a cool location with months between top-up charging.

If the batteries are flooded, I would still lean towards #2, but with more frequent charging. If warm (and with no loads - not even small loads for things like stereo standby or alarms) maybe once a month. If cool/cold maybe once every 2-3 months. 12.5v is about as low as I'd ever want to let it sit for any length of time.

In cold months, I don't charge unused AGM at all, and flooded in the boat maybe once over the winter. I had a small panel for floating boat batteries, but ended up not using it. I suspect it had no or failed diodes as in less than full sun it looked to be draining the battery rather than floating/charging.

solarkris

Very good, thanks for the input.  Forgot to state battery-type as flooded.  The AGM info is useful as well as this will probably be my next new-battery type.

scrubjaysnest

I use a 25 watt panel with CC for 6 to 8 months to keep a second vehicle cranking battery up. Been doing this since 2012 with no problems.
For the RV I keep 2 100 watt panels feeding the cranking battery and both house batteries via the CC. So far no problems.
All batteries are flooded.