Maximum Safe Charging Voltage for RV Systems

Does anybody know what the absolute limit is for voltage applied to the 12V side of the RV systems? I ask because I have been considering adding some solar panels and associated charge controller, and one of the manufacturers has recently added a feature to their charge controller to limit the charging voltage to a user-selectable value due to reports from some RV owners that other systems were not tolerant of the nominal charging voltages recommended by battery manufacturers.

Specifically, my COSTCO (made by Interstate) batteries recommend a 15.6V equalization charge periodically, and the charge controller I am looking at will provide that. Is there risk of damaging the converter or other systems on the RV if that voltage is applied? Anybody know? I looked in several of the manuals and there is no mention of 12V specifications...



  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    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.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gsglengsglen Registered Users Posts: 5
    Thanks Bill, you've hit the nail on the head for my concerns. I have two 6V batteries that are rated at 208 Ah and I'd like to charge them correctly. The converter on my trailer is not optimal, but also serves as a "power supply" for all the 12V appliances and lights in my trailer when I plugged into shore power. They therefore have to choose the voltages differently I think than if they were purely interested in charging. At least that's my theory about why they are not optimized. My WFCO converter uses the following: 14.4V for bulk, 13.6V for absorption, and 13.2V for float mode. it does not have an equalization mode.

    Interstate recommends the following for my 6V batteries:
    Charging Rate: C/10 (20.8A)
    Bulk: 14.46 V
    Absorption: 15.3V
    Absorption Time: 2 - 4 hrs
    Float 13.4 V
    Equalize: 15.6 V for 2 hrs

    The WFCO manual states that it has "over-voltage protection" but I don't know what that is set at. I'd prefer not to have to switch my batteries out of the system to equalize periodically, but am not sure if everything can even handle the recommended absorption voltage of 15.3 V if I set the solar charger that way. I am worried primarily about hurting something with the higher voltages -- like the controller for the refrigerator, or the inverter, but I'm also concerned that the inverter will sense the higher charging voltages from the solar charger when I'm plugged into shore power, and might shut off, which could start a cyclic on-off scenario when I use the water pump, for example. It uses 8-9 A and without the converter would certainly consume all the solar current, dropping the voltage and causing the inverter to come back on.....and you get the picture.

    So I'm not sure what to do. The concern about damage is foremost in my mind, but I'm not sure how the converter/solar charger will work together given different charging algorithms....

    Any advice or experiences with RV installations would be appreciated.

  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    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:

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

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

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gsglengsglen Registered Users Posts: 5
    thanks for the outstanding info. I just installed a Trimetric 2030 battery monitor and have been considering the companion solar charger for use with 12 V panels. I'll dig in to HandyBob's blog in hopes of getting to a decision on what to do next.
  • HandyBobHandyBob Banned Posts: 31
    I'll just weigh in with a couple of things. When asked about the high limit, one of the RV water heater manufacturers said 14V on their controls. That is stupidity. Every motor home built today exceeds that even if it doesn't have solar power.

    My own RV system has withstood up 15.5V for MANY years. Two different brands of water heaters have both worked just fine. I know the Dinosaur control board in the Norcold fridge is over 12 years old because that is how long we have owned it. Equalizing GC batteries does not need to be a regular thing, but when I have done it, unless the temperature was low so I could expect to see over that 15.5V, I never shut the fridge off. Yet, it still works. What does that make me? Stupid or lucky?

    As I understand it the components on those boards will take 20V. Not that I want to do that, but asking any manufacturer of electrical appliances their recommendations will usually get you lawyer driven answers. This is a different issue, but to hear most companies talk, inverter power cannot be used for anything, even if it is sine wave. Bottom line? They don't live this life and they simply do not know. If you ask the wrong guy any question, you'll very likely get the wrong answer or at the very least one you don't know you can trust.

    I am now using a 2030. I have the high limit set at 15.4V just so our Morningstar Sure Sine will stay on. When that shuts off, stopping the sewing machine in the middle of something, the quilter gets very upset. Happy wife = happy life.
  • gsglengsglen Registered Users Posts: 5
    I certainly understand wanting to keep the quilter happy! As well as what the manufacturers will say. Of course if they don't know for sure they will gravitate to the "safe" answer that adds no risk for their warranty or legal departments.I will do as you suggest and see what happens and will plan to equalize only manually. I suppose I can wire my battery switch so that I can disconnect the load when I do that, but would prefer not to. Anyway, I'll concentrate now on the solar install.

    Many thanks for the info.

  • Mustang65Mustang65 Solar Expert Posts: 42 ✭✭
    When I installed SOLAR on the RV, I called the Fridge and Radio mfr. Both said 15 volts was the max. So I pull the fuse when I Equalize the batteries. Has worked for the last 2 years with no issues.
    Attachment not found.
  • scrubjaysnestscrubjaysnest Solar Expert Posts: 175 ✭✭✭
    I've never had a problem from the solar cc voltages getting to high. Our 400 watt inverter doesn't care, the cell phones and tablet don't care and the leds, most sensitive to high voltages haven't given a problem except for a very poorly designed china brand. My tests on the WFCO converter indicates it never goes above 13.6vdc so you could do like I did. Take it out and leave it home. Many converters will not go above 13.6 to prevent "boiling the batteries" as some advertize. Let to solar system maintain your batteries even on shore power.
  • dssampdssamp Registered Users Posts: 1
    I guess that was my question. Why worry about the on board converter anyway. When running from pv thru the mppt or whatever CC you pick, would you not set up the cc to run direct from the cc to the batteries? Or am I misunderstanding what your trying to accomplish. The whole point is simply keep the to batteries up.
  • gsglengsglen Registered Users Posts: 5
    The point in worrying about it is that everything that runs on 12V in the trailer sees the output of the CC (and the converter) so when the voltage is high for an equalization phase charge there could be components damaged. The alternative is to pull the fuse and disconnect the load to isolate it when doing the equalization phase. That will require a wiring change for me, but I'm considering it....

    I agree that the WFCO converter is lousy (I've never seen more than 13.6V yet) and will consider turning it off when I get solar running and rely on the panels for charging, at least some of the time. The catch there is that I'm going to build a sun shade to keep the sun off it, so I'll probably have to charge occasionally with the converter or some other method because the panels will not be working when the trailer is stored. In addition, since the converter acts as the 12V power supply when connected to shore power, I can imagine I'll want to use it in that case as well unless boondocking. Fortunately my converter is on its own breaker so it's very simple to shut it off.

    Fulltiming in the RV or being completely off-grid all the time bring different considerations than I'm worried about currently.

    Thanks all for the info.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Just to note: you can control when you do an EQ... so just throw the PV/battery to system breaker when you do an EQ and manually protect your sensitive 12V appliances...
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
Sign In or Register to comment.