Two 6v batteries in series charged by two separate solar panels

I have two solar panels - first 30 watt 2 amps second 40 watts 2.3 amps, and two 6v deep cycle batteries in series on my RV. My question is: can I simply use two separate charge controllers (max 7 amps each) and run the neg and pos connections from each to the same pos and neg terminals on the batteries? Or do I have splice the wires and use only one charge controller?

• $2 \cdot 6 \neq 30$ At which voltage are you panels specified? It seems it would be best to put the panels in parallel to each other to a regulator and the regulator to the batteries in series. Also, yes you can use two regulators, although I don't see why you would want to do it. May 20, 2013 at 19:52
• @Gunnish It doesn't matter what the panels are rated for, as long as they fit within the specified input range of the charge controller - it should be ensuring the proper voltage/current output for the battery. May 20, 2013 at 21:18
• @KurtE.Clothier yes it is possible, but suboptimal. May 24, 2013 at 15:44

I have an RV that I've done quite abit of electrical work, so I know where you are coming from. To start, let me make sure I understand what you are trying to do. Take a look at this block diagram:

I believe the only information I left off is the 7A max for the charge controller(s). To start, let me bring up the topic of this related question: Charging Lead Acid Batteries in Series. It is not the best idea to charge these batteries in series, but it is commonly done without much issue. However, individually charging the two separate batteries should be better for them. This charging technique is commonly incorporated in more advanced Lithium battery systems. You are almost doing that with your setup...

Your charging sources are not identical which could pose problems. Even though the charge controllers may be identical (6V output), the power sources are not. The two solar cells have different characteristics meaning they will output different amounts of power. It is possible that the charge controller could be set to output a constant current just so long as the input stays above a certain level, but you did't specify the type of charge controller. Most of the ones I have seen are cheap without much concern for proper battery charging.

Another bad point is that individual cell charging systems also handle the load output from the batteries. I don't imagine yours will be doing this.

This is a problem because if the batteries are not charged equally (at the same rate, to same level of charge) their performance will be severely deteriorated.

Another thing to think about is the internal RV converter/charger. This is the device that converts shore power (120V AC) to 12V DC to power your internal lights and fans as well as recharging the battery. The stock converters on most RVs are abysmal at recharging batteries. They basically pump a high amount of amps into the batteries nonstop, causing them to rapidly boil over requiring you to constantly add more distilled water. The best chargers use multiple states: high current, normal, and trickle. During charging times, the converter actually powers all of the 12V stuff, and the current I_RV is reversed through the batteries to charge them.

The reason I bring this up is that you will potentially have multiple charging sources if you have everything on at once: 1 12V charger trying to charge the 2 6V batteries in series, and the other 6V chargers trying to charge the two 6V batteries individually. This will most likely damage the batteries, chargers, or both.

I don't quite know what you mean by this:

Or do I have splice the wires and use only one charge controller?

But it sounds like you mean charging the two 6V batteries in parallel with one charger while they are connected in series to create 12V. This doesn't make the slightest bit of sense. It is possible that you could connect the two solar panels in parallel and feed them both into one 12V charge controller to charge the two batteries in series, taking note of my previous comments about doing so.

Here is some information about the work I did to my RV electrical system.

• Thank you so much for your detailed and knowledgable response. Your drawing is almost correct except the positive from each crtl goes to the outside negative on the left side 6v and the negative from each crtl goes to the outside positive on the right 6v. The inside positive and negative battery terminals are connected as shown. May 20, 2013 at 23:47
• So you are using 2 parallel 12V charge controllers? That is quite a bit different from my drawing. In that case, no, you should not do that... You might get away with using multiple solar panels in parallel fed into one controller, but you cannot use two chargers in parallel. Also, what you are describing has reversed polarity - when charging, the positive goes to positive and the negative goes to negative, unless the charge controller is connecting the batteries to the load as well. You might want to add a drawing of your own to your question for clarification. May 21, 2013 at 0:39
• I wrote that wrong the polarity is ok. It is the same as your drawing except the positive orange line from crtl 1 goes to positive on battery 2 and the negative red line from crtl 2 goes to negative on battery 1. Basically I have two solar panel kits and I wanted to use both right out of the box without any serious modifications. Thanks for all your input. May 21, 2013 at 23:22
• Your thoughts? Wouldn't hooking them up as discussed simply increase the charge to the batteries? May 25, 2013 at 15:06
• Hello? I could really use your input. Thanks for your time. May 31, 2013 at 0:23