Maximum Safe Charging Voltage for RV Systems

Welcome to the forum Greg!

It depends to a great deal on what is connected to your 12 volt battery bus on the RV.

In general, many RV/Automotive (such as 12 VDC computer adapters, etc.) are designed for automotive use--And ~14.4 is a typical maximum voltage.

Many AC inverters are designed for ~15.0-15.5 volts before they shut down (and some of the 12 VDC adapters have been reported to fail at these voltages). And you can find some AC inverters that have ~16.5 volt shutdown.

Lastly, and firstly, the battery bank... Check what the battery vendor recommends. Some typical values:

GEL ~ 14.2 volts charging
AGM ~ 14.4 volts charging
Flooded Cell Deep Cycle ~ 14.5 to 14.8 volts or so
Float ~ 13.2 to 13.6 volts
Equalization for Flooded Cell batteries ONLY ~ 15.-15.5 volts (really the voltage needed to reach ~2.5 to 5% +/- charging current -- 100 AH battery bank ~ 2.5 to 5.0 amps during EQ).

One problem that many of our friends to the North (and in the moutains) find is that many (good) solar charge controllers have an ~5 mV per C increase in charging votlage for every 1C below ~25F (1.8 volts below ~75F).

So, get down to -5C (30 C below room temperature) on a 12 volt (6 cell battery):

-.0.005 volts per cell * 6 cells * -30C below room temperature = +0.9 volt increase for low battery temperature charging...

You pick 14.6 volts nominal for charging + 0.9 degree = 15.5 volts charging (at -5C)--That means for many (most) of the typical RV/12 volt automotive type gear, you are well into the range they will shut down and/or self destruct.

Some solar charge controllers can be set with a maximum voltage--Other folks will disconnect their loads in high charging voltage conditions. Insulated battery box/batteries in a heated room can help prevent low battery bank temperatures (and a remote battery temperature sensor that plugs into the charge controller).

There is also another charging type called equalization for flooded cell lead acid batteries... These voltage may be in the 15.0 to 15.5 Volt range. Disconnecting DC loads many be needed here to prevent damage/AC inverter shutdown.

And a few "industrial" type lead acid battery types may need ~15.0 to 15.5+ volts for charging (certain Rolls/Surrette and other industrial batteries).

Deep Cycle battery system typically (widest range) run from ~10.5 to ~15.0+ volts -- Surprising how few 12 VDC devices really work well with this voltage range.

-Bill

When you are attached to utility power--You usually can get away with lower charging voltages (14.5 to 14.8 volts or so). With solar, particularly during winter, and deeply cycling batteries (75% to 50% state of charge), many times you need to hit the battery with much higher voltages to get it to charge quickly in the limited number of hours per day.

Since you are have flooded cell batteries, get a good Glass Hydrometer--Or this very nice temperature corrected hydrometer.

Fully charge your battery bank and log the specific gravity for each cell... Some vendors recommend equalizing once a month, others when SG from high to low cell to high cell varies 0.015 to 0.030 SG or more.

Equalization/Gassing is pretty hard on a battery bank. So--You want to not "excessively" "equalize" charge (at high voltages) your battery bank.

Besides voltage and current monitoring (making sure your settings are working correctly), more or less you should need to add water roughly every 2 months (never let the plates get exposed). If you "never" add water, you may be undercharging. If you are adding lots of water every month or more often, you are probably overcharging.

As a lead acid battery becomes nearly fully charged (and during equalization), the battery becomes less efficient and besides "gassing", the batteries also heat up... In some cases (like during equalization), the batteries can overheat--you want to keep the battery below the rated temperature (keeping below ~110 to 120F maximum).

There is a blogger named HandyBob that has worked on RVs and helped many others--He is a very good read about off grid power from an RV'ers point of view:

HandyBob's Blog

One of the reasons I tend to recommend for Off Grid cabin folks to use a good quality AC inverter for powering there loads/appliances/etc... Is that you can get an AC inverter that will run from 10.5 to 16+ volts--And these days, many of the AC appliances/devices are just about as efficient as their DC cousins, it is easier to send 120 VAC longer distances than 12 VDC, many AC devices are cheaper than DC versions (look at LED lighting), and you don't have to worry about the "12 VDC" battery voltage wiping out your laptop 12 volt brick during charging. The AC inverter, roughly, runs at ~85% efficiency--So a bit larger (~20% larger) solar array/battery bank is all that is needed to power the inverter tare losses (usually).

For a smaller RV system--An AC inverter may be a "bit much" for 100% of your power needs.

There are other tools you can use to see what is happening with your battery bank. A Battery Monitor (like the Trimetric) is nice. And for debugging, a DC Current Clamp DMM (digital multi-meter) such as this one from Sears (good enough for our needs) make working on DC power systems much easier.

Some light reading about battery care:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

After reading the above--I convinced myself that batteries do not work () -- But obviously they do.

A pair of golf cart batteries is a very good start. Pretty rugged and forgiving. And not too pricey.

-Bill