Should I get better than 13.5 volts?

We have a Starcraft 10RT that's kept plugged in in our garage all the time when not in use. The panel gives us a maximum charge of 13.5 volts.

We also have a big solar panel on top, wired directly to the batteries. When in the sun, we get up to 14.4 volts.

This is with a pair of new Trojan 12V AGM batteries (didn't have enough room for 6 volts).

Should the built-in charger be giving us better than a 13.5 charge?
Re: Should I get better than 13.5 volts?

Actually, current does the charging; voltage produces the current that charges your battery.

If you have a battery that just sits there without being used (no current draw), then to keep it "charged up" all you have to do is replace the charge that is "self discharged". That can be as little as 100 mAmps (very small). Now, that won't recharge the battery, but this "very small" amount of current will maintain the battery.

So, the answer to your question is not what "voltage" your battery is charging at while it is sitting there, but how much "current" is being pushed into it while it is sitting there. That built-in charger at 13.5 volts COULD be creating just the right amount of current to maintain your battery while it is not in use.

You don't want to push too much charge current at rest, or you'll evaporate the water in the cells. That's why alternators have "regulators" on them. High current is produced (with higher voltage) to recharge the battery and switch to lower current to maintain the charge.

I've had a car battery that I use for computer system backup that only allow 50 mAmps of maintenance current. I've load tested it once a week for 5 years now. "Still going ..."


Senior Member
Re: Should I get better than 13.5 volts?

Actualy, battery voltage falls as a battery is discharged and it rises as it is recharged. The voltage that the battery can be charged to is determined by the design of the battery but it can never be charged to any more than the maximum of the supply voltage. To fully charge the battery you must have a higher supply voltage than the expected voltage of the battery. The 14.4V will do an excellent job of fully charging the batteries, but if it is maintained at that level for too long, it will begin to boil off the water from the batteries. I don't have any experience with AGM, but it is my understanding that they will be damaged by high voltage above 14V. I suggest that you check that out.

It is normal for the converter of an RV to be set to between 13.5 and 13.7V because that level, while not quite high enough to fully charge a wet cell battery, will get it to about 90% and yet it will not boil most batteries when kept at that voltage for a very long period. AGM batteries are much more voltage sensitive, especially to over voltage. They are longer lived but the also need a lower voltage. I suggest that you talk to the place where you bought the batteries about this. Voltage definately does matter. There are voltage control systems that can be set up for controling solar charging for an AGM battery. I have a lot of experience with batteries and charging in my career, but we never used AGM's so all I really know about them is what I have read. I am a service tech. by trade so have quite a bit of knowledge of flooded cell batteries and power supplies, but very little on solar of AGM.
Re: Should I get better than 13.5 volts?

It depends on the age and manufacture of the converter. Some of the older ones (magnetek & Progressive Dynamics) would only charge at a maximum of 13.6 VDC. The step-mode converters will go higher (14-14.5 VDC). Some of the tent trailers are still using the small mechanical converters that won't put out the higher volts.
RE: Should I get better than 13.5 volts?

My (2) 130 solar panels show 14.4 all day, but I have an auto tracker and the panels follow the sun all day. After the sun sets it drops with whatever I use. I run (6) 6 volt batts. As my unit continues to track even at night it doesn't take long at first light in the morning to get it back to 14.4. When I run from shore power my converter is on and when I am dry camping I shut the converter off and run just from the panels to the inverter. I don't know a bunch about solar, but the stationary flat panels I have had in the past were a joke for extended dry camping. This unit does what I need it to do and that is all I ask. I still pack a 3000 watt generator for continuous cloudy days. Happy camping.
RE: Should I get better than 13.5 volts?

You should have something called a charge wizard inbetween that solar panel and the battery limiting the battery to 13.6vlt not 13.5, 14.+ constant charge will fry the plates in that battery in a short period of time. also causing boiling as the batteries heat up then the acid will corrode the tray and whatever else you have under the batteries, you need some type of 3 stage system/float mode. correct me if im wrong.
some batteries you are supose to give a super charge like 15vlts before storing them for winter but not a 6vlt.
I dont care what the Manuf. says every unit i had that had 6vlt batteries and the charging system was putting 14.2vlts into them they all went bad. and they were water acid batteries. I switched to Jell sealed.
RE: Should I get better than 13.5 volts?

The only time my batts go to 15 is when I am cleaning them. I have never had a boil over problem. I do however have to pump the pro-fill once or twice a month to make sure there is plenty of water in them. Mine works and has been working for over a year and a half. Mine has a cut off built in so they don't over charge or boil over. That's the way this system was designed, I should know as the inventer is a very good friend. There are dozens of these out there as they have only been out for about two years. Biggest problem people seem to have with it is they want to forget to lay the panels flat before traveling. Although the frame will stand up 60 mph winds. Of course most folks will pull over after a mile or two and lower it with the hand held remote. Anyway I love this system, don't plan on changing it, and I have used it so much I don't plan on worrying about it. Trailer life is going to do a write up on this unit in the near future. I can say that because I was with him when he got the phone call from the. when it does you can forward them the problems you feel this system has or will have. In fact there is a Sunseeker add in there now. Meanwhile I will be elk hunting. Happy Camping