Solar Controller Wire Size

Basil48192

I have a 2021 T@B 320S with a 105W solar panel.  I purchased an additional 200W suitcase panel.  I'm debating about just mounting a second controller in my battery box, mounting a second controller under the seat, or upgrading my 75/10 controller to a 100/30 to handle both.  So, I have a number of questions and my camper is sealed up for the winter:
-Can I upgrade the existing controller to handle both panels?
-Will the existing wiring between the 75/10 and the battery handle 305 watts of solar?
-If not, how difficult would it be to pull new wiring?  I haven't looked at routing, cable clamps, etc.  
Thank you in advance!

bergger

I'm not sure if the existing wiring would handle that load or not.  That may be a question for NuCamp Tech or maybe someone here has already asked them.  I know several have upgraded solar so I'm sure they will pipe in.  Does your solar suitcase already have it's own controller?  If so I'd just keep it simple and run a separate wire and plug that connects directly to your battery as a separate redundant system.  That's what I did on my T@b 400.   I typically use the onboard solar but if needed can plug in the suitcase for additional charging.  

Basil48192

@bergger.  Thank you.  Yes, my suitcase came with a very basic, PWM controller mounted to the back of the panel.  Since I like to dry camp, I bought a 20' extension for it.  I'm sure it will work, but I'm reading a lot about using MPPT controllers as close to the battery as possible to maximize efficiency.  I also like the Bluetooth feature on the Victron controller.  I probably need to study a bit more to quantify the efficiency gains before spending the extra money.  Thanks again for the comments!

pthomas745

This thread from a few days ago discussed the same type of issues as you bring up as far as "combining" panels, etc. 

You can fall down the internet rabbit hole reading about MPPT efficiency gains.  If I had a much bigger system with multiple panels and batteries I would consider MPPT important...but with a 100 watt panel and 80AH battery.......so, I might get charged up five minutes faster?  I have an inexpensive MPPT controller, and the main advantage from my older PWM's is the form factor and build quality.

The advantage of the Victrons is the battery monitoring aspects with the Victron App.  Makes them well worth it. 

https://tab-rv.vanillacommunity.com/discussion/12438/solar-advice#latest

Denny16

If you wanted to upgrade the Victron MPPT, a 100/20 would be large enough, but both panels should be the same capacity to use a single controller.  That said it would be less $ to just buy a 75/20 to handle the 200 watt solar panel, you can network the two Victron controllers via BT to work together.  This way the two panels can be different size/capacity.  This is the solution I am using on out TaB 400, we have a 75/20 on the 103 watt factory sokar and added a second 75/20 for the 200 watt portable panel setup.

 Victron recommends using No 8 AWG wire to connect 209 watt solar panel to the controller and from the controller to the battery.  This is also the size wire used on our portable solar panel setup.

There are additional discussions on this topic at: https://tab-rv.vanillacommunity.com/discussion/12438/solar-advice#latest
cheers

Basil48192

Thank you @ Denny16.  I did buy #8 AWG wire to connect the panel so thanks again.  I think this is probably the way I will go.  

MarkAl

Having gone down the "rabbit hole" as @pthomas745 says is an interesting exercise but it's mostly not needed if you follow the above good advice. Brushing the dirt off, here's my thoughts: Large (small #) wire gauge is not the issue as long as you don't reduce the gauge to the point of feeling any heat at maximum usable current. The few watts you lose to heat is as mentioned above only results in a few minutes less charging of your battery. It's also  almost imposible because of solar conditions and loads to get the maximum output of the arrays for any length of time to worry about the losses. I also don't like the difficulty of managing long lengths of heavy wire for (practically) small current loads. Depending on the solar situation, I prefer using the panels in series, this limits the current and maximizes the power transfer. Note this series / parallal switch I built is not needed, only if in the rabbit hole.

I have found the 100 watt (320) installed solar panel is adaquate to recharge my (2-6V) 225 A*H battery if I get 3 or more 4 hrs in the sun a day. But this rarely happens on a daily basis so having a remote 100 watt panel is ideal to follow the sun. If the site / weather is really bad I pull out the 2nd 100 watt panel to capture as much light as I can. I specifically do not use a larger array as they get to big. I now store the 2 on my cargo carrier and pull them off (or not) as needed. Having the top one exposed I sometimes leave it and connect it into my external input. This acts as a front and rear panel on the 320...

Note I am learning that winter in the high northern latitudes w/ trees is not conducive to week-long dry camping and a generator is probably a smart move.

Enjoy.

Basil48192

Thank you MarkAl.  Agreed...there are a lot of factors to consider.  I will probably do the same, use my suitcase as needed to supplement the 105w roof-mount on my TAB.  Thanks again to everyone.  I appreciate all of the comments.

m_lewis

I disagree regarding wire size from the controller to the battery. It is not heating per se that is the problem, but voltage drop. Because these controllers use the same wires for charge current and measuring battery voltage,  the minimum voltage drop is recommended. This means larger wire, with #8 being correct for a short (<5 ft) run from controller to battery. 

As far as wiring from the panels to the controller, I’ll agree that series configuration of the panels enables a smaller wire size, but please don’t assume that as long as you don’t feel them getting warm they are large enough. Why waste what energy you have heating wires? The voltage drop may be quite significant well before noticeable heating occurs, depending on length. 

rfuss928

m_lewis 
Voltage drop in the wiring is only significant in the voltage controlled stages of charging.  Specifically absorption and float.  In these stages current is limited to one amp or less in these small portable systems and the resulting voltage loss is too small to have any practical effect on the resulting charge process.  As MarkAl stated, the gains made at the rare high current moments are very small and the inconvenience of handling long bundles of large gauge wire are just not justified.
A longer, detailed example can be viewed here:
https://tab-rv.vanillacommunity.com/discussion/comment/143974#Comment_143974

m_lewis

@rfuss928, I agree with you. My point was that using such a small wire that heat becomes noticeable is foolish and wasteful, just one size larger is still wasteful.  When trying to eke out whatever you can from the panels, scrimping on cables is counterproductive. 

No 10 for pushing 8 amps thru 100 feet is reasonable, but #18 or 20 is not. Whether they get warm to touch is not a good practice for sizing conductors in a solar application.