How do I determine solar capacity?

[HubFlyer]

My 2015 Roadtrek RS Adventurous (2014 Sprinter 3500) has one panel on the roof connected to a Go Power solar controller (gp-pwm-30-ul). It also has 4 6V 225AH AGM batteries (2 under hood and 2 external behind the left wheel).

When I purchased the vehicle 2 years ago the dealer did not provide a lot of detailed specs and I didn't know to ask all the right questions. Various Roadtrek brochures from the time period list 240W, 270W and 470W options.

I have been up on the roof but cannot find any ID markings on the panel. Short of disconnecting it is there a way to determine the capacity through the solar controller or other tools?

I'd also like to determine whether the 4 batteries are connected in series. The attached screenshot of the solar controller app is all the information I have at the moment.

Would happily take it to a trustworthy RV service provider who has electronics experts on staff (I'm in central Florida) but wasn't sure if there is some investigating I can do on my own.

TIA.

[Uncle_Rico]

Hi HubFlyer, solar panels are typically about 200W per square meter (a bit lower if its a polycrystalline panel rather than monocrystalline). A quick measurement should confirm what you've got. As far as the house batteries go, each pair will be connected in series to make 12V and then the two pairs are connected in parallel to double your capacity. So if each battery is labelled 225AH@6V, you've got a battery bank with 450AH@12V. Your screenshot shows the system voltage at 13.4 which is typical for a fully charged 12V AGM battery bank.

[Ken Allan]

I also had unknown (unmarked) solar panels. I was able to get an inspection camera underneath and found the Mfr lable. I was able to identify from that. I'm not sure if you have room to do that. Short of the camera I would have had to lift the panel off.

You could try one of the Roadtrek Facebook groups to see of anyone knows who the mfr was.

FWIW... My panels are 5 sq ft each(.47 sq m) and are rated at 100W.

[HubFlyer]

My measurement is 62 x 39 inches. That equals 1.55 sq meters. So I guess that's about 300w.

Appreciate your help!

Originally Posted by Uncle_Rico

As far as the house batteries go, each pair will be connected in series to make 12V and then the two pairs are connected in parallel to double your capacity. So if each battery is labelled 225AH@6V, you've got a battery bank with 450AH@12V. Your screenshot shows the system voltage at 13.4 which is typical for a fully charged 12V AGM battery bank.

Does this mean I have TWO battery banks with 450AH@12V since each bank has two batteries? Does the screenshot indicate that all 4 are connected to the solar controller or just 1 bank of 2?

Much of the wiring is hidden as it disappears from the 2 under hood into the chassis and from the two external to the left wheels. Could it be that the front bank is connecting the solar panel to the controller and the rear two are separately connected to the inverter?

Short of dismantling is there any way to tell?

[Ken Allan]

As U. Rico mentioned, and I would --think-- too: In effect you have a front 12v battery and a rear 12v battery. These are in parallel for a 12 system. Both the solar and the inverter positives should be connected to the 12v battery positive. Same for the negative leads.

This way the solar can charge the entire battery bank. The inverter can charge (presuming it has a charger built in) or draw from the entire battery bank.

Also it sounds like you have 270W of solar.

Edit: Just an after thought... Do you have switches to disconnect batteries? One or two?

[HubFlyer]

First off, thanks to you and Uncle Rico for being so patient with my basic questions.

Originally Posted by Ken Allan

Edit: Just an after thought... Do you have switches to disconnect batteries? One or two?

There is a battery disconnect switch on the monitor panel. Is this the one (or two) you are asking about?

[Ken Allan]

I was expecting one on-off switch. This looks like you turn ALL the batteries ON with the blue switch and OFF with the battery disconnect switch??

I am not familiar with how Road trek wires these vans. I was only curious if you had separate control of the front or rear battery. It appears not.

You might find a Road trek owners manual online if you don't already have one.

[Uncle_Rico]

The blue is a light for feedback showing when the battery is on.

Roadtrek manuals are available on https://roadtrek.com/owners/ Not much in the way of electrical design but definitely should be consulted if you haven't seen it.

Do you have the under-hood generator (i.e. second alternator) shown in the owners manual? That should be easy to spot.

[HubFlyer]

I do have the manual from Roadtrek. The vehicle does have an underhood generator (Nations Alternator Mercedes-280-XP, newly replaced in 2021).

The battery disconnect switch in the monitor panel is the only one AFAIK.

At the risk of pushing my luck I would still like to get an opinion on my battery configuration. I am contemplating installing a battery monitor similar to a Victron so that I can more closely identify battery input and output.

Would that be overkill since the solar controller has a SOC indicator?

Since the wiring is mostly buried I am not clear where I would put the shunt to monitor all 4 batteries in the bank. Any thoughts?

[Uncle_Rico]

A shunt based battery monitor is a great idea but they are far from perfect. That being said, I just bought one for my van. It will definitely provide more insight than the voltage based state of charge from the solar controller. There's some good reading here: https://marinehowto.com/programming-a-battery-monitor/

As far as where the shunt needs to go, I don't think I can be of much help. The shunt needs to see all of the return current for your entire battery bank but with your battery bank distributed between the front and back of your RV this might not be possible without some rewiring. Maybe a Roadtrek specific Facebook group can help with this.

[booster]

Originally Posted by Uncle_Rico

A shunt based battery monitor is a great idea but they are far from perfect. That being said, I just bought one for my van. It will definitely provide more insight than the voltage based state of charge from the solar controller. There's some good reading here: https://marinehowto.com/programming-a-battery-monitor/

As far as where the shunt needs to go, I don't think I can be of much help. The shunt needs to see all of the return current for your entire battery bank but with your battery bank distributed between the front and back of your RV this might not be possible without some rewiring. Maybe a Roadtrek specific Facebook group can help with this.

I think you are wise to do so, as I am a big fan of properly setup and used monitors.



That article isn't horrible but I do think it missed a lot of important points about the intricacies of setup and use and ignored what I consider the elephant in the room, which is charger/charging deficiencies that make your "full" not full most of the time or possibly overcharged. Without being able get a consistent full charge you have no starting point for capacity or consistencies. There are lots of discussions on it all on this forum that are much more detailed and will give better results than following that article's methods, IMO. Plus the threads here aren't full of links to paid for sales sites.