DIY 6x12 Cargo Camper with Solar


MASSIVE restock of EV Batteries at www.batteryhookup.com 5% coupon "POWERWALLS"

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
2,125
This link - https://www.solarreviews.com/manufacturers/grape-solar/solar-panels/grape24121gsstargsstar100w - shows
Short Circuit Current (ISC)6.13
Open Circuit Voltage (VOC)21.9

At 2s, the VOC = 43.8v. At max charge, your battery could reach 29.2v.

BTW - Can you share the exact Charge Controller you're using? and its specs?

The way MPPT works is that every x seconds or minutes it checks voltages to find the maximum power (watts) it can put into the battery. The MPPT algorithm will typically settle on a lower voltage than VoC. The ambient temperature affects this as the when the panels get hot they have a lower voltage (lower than 43.8) and if it's winter / freezing it will be higher.

That's what the "Temperature Coefficient" is used for - you apply this coefficient to the base VoC to see what the VoC would be for different temperatures higher and lower than the base VoC. You're panels show this:
Temperature Coefficient0.32


Bottom line - You're charge controller's will probably (just a guess) not go lower than 80% of Voc. 43.8v * 0.8 = 35v. 35v is still higher than 29.2v max of your battery - so it should work but it's close enough that I'd verify things once you get it working, especially on a hot summer day.

TMI - If you can find the graph of your charge controller's MPPT it will show you the curve of maximum power and you could get a little more detailed on all this. There may even be something written in the user manual that will help clarify this topic.
 
Last edited:
Joined
May 25, 2017
Messages
432
Here are all the specs on the solar panels:
https://grapesolar.com/wp-content/uploads/GS-STAR-100W.pdf

Before anyone picks on me for buying a cheap solar controller, let me tell you why I got it. Or just skip the next paragraph, LOL.
A while back, I calculated the total voltage of the panel array incorrectly, and came up with something like 115V. Not knowing any better, I looked around for a controller that could handle that much, and everything I found was just short, or very expensive. Except for this PowMr 60A. The reviews looked ok, so I got one. Now, a while later, way to late to send it back, I realized my math error, and don't need that ridiculously high voltage. The owners manual even warns to match the PV voltage with the battery voltage. But I have it now, so may as well use it. I'll probably upgrade later.

With that expensive learning curve out of the way, what I have is a PowMr 60A controller. I got it at Amazon, and I'll try to link the spec sheet here:
https://m.media-amazon.com/images/I/91LCCV7v7lL._AC_SL1500_.jpg
There's no charge curve or specific behavior explanations that I can find. I've been watching a lot of YouTube reviews on it. Both good and bad ones. It seems like it will work ok. Though I may upgrade it later on.
 

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
2,125
1646028004474.png

They list MPP at 19.12v which is lower than VoC of 23.1 - as discussed above. So back to you're original info - you can expect 38.24v at 2s.

Before anyone picks on me for buying a cheap solar controller, let me tell you why I got it. Or just skip the next paragraph, LOL.
No way - no one is poking fun! Just trying to explain / help you understand in case things don't work. You should see the 1st controller I bought, it was $17 and I was thinking I was so smart finding the deal of a century - never did use it :)

A while back, I calculated the total voltage of the panel array incorrectly, and came up with something like 115V. Not knowing any better, I looked around for a
That would be right for 5s (5 x 23.1).


controller that could handle that much, and everything I found was just short, or very expensive. Except for this PowMr 60A. The reviews looked ok, so I got one. Now, a while later, way to late to send it back, I realized my math error, and don't need that ridiculously high voltage. The owners manual even warns to match the PV voltage with the battery voltage. But I have it now, so may as well use it. I'll probably upgrade later.

With that expensive learning curve out of the way, what I have is a PowMr 60A controller. I got it at Amazon, and I'll try to link the spec sheet here:
https://m.media-amazon.com/images/I/91LCCV7v7lL._AC_SL1500_.jpg
There's no charge curve or specific behavior explanations that I can find. I've been watching a lot of YouTube reviews on it. Both good and bad ones. It seems like it will work ok. Though I may upgrade it later on.
Looks like you got the 60a model. 8 x 100w = 800w of panels @ 24v is good because it's <1440w max.
1646029255243.png


However, I also see this....
1646027803168.png

2s = 38.24v (per above) and is 'very close' to the 37v minimum - e.g. 1.24v is pretty slim margin and within the range of simple equipment measuring error + ambient (hot) temps could drag 38.24v down a bit - so if you go this way and it doesn't work be prepared to

0) Go down to a 12v battery - 800w is a little > 720w max (above) but probably OK and 38v PV comfortably higher than 20v - 80v.

1) Rewire as 3s3p (and loose 1 of the panels) to raise the PV voltage for 24v battery.
1b) A little messy but do 3s3p and an MPT7210A on the 10th panel. This off-beat unit will boost low volt PV to your battery voltage range and let you use that dangling panel. https://www.ebay.com/itm/393618274509?_trkparms=ispr=1&hash=item5ba57a38cd:g:W9AAAOSw079g7-Gz&amdata=enc:AQAGAAACkPYe5NmHp%2B2JMhMi7yxGiTJkPrKr5t53CooMSQt2orsSU8qUIAeVPdz29zRlje3Lg4nSFgc8zlSJiQvxL8g50Skor0od4C%2Fg8wB5x0yl1hyteGyY1kDaY%2FEyUayPN67 This is one of the very few 'boost' charge controllers and is limited to 10a on output.

2) Rewire as 5s2p with a different controller as 5s will be too high @ 24v for this controller.

3) Rewire as 5s2p and go to a 48v battery with this controller.

4) Go with a different controller - **BUT 38.24v is pretty low for any 24v battery situation so you'd want a 24v that can do 120-150v max.

5) Go with different panels.


There's no danger with trying 2s - just don't back yourself into a corner where you can't change things :)

TMI - I have 4 x 300w panels on my trailer with a 48v battery bank. I went 2s2p and at 2s my MPPT voltage is 62-60v. Which is 'very low/close' to 58.8v max of my battery bank. It works - but is very very close - e.g. similar issue as your case. That's why I'm biased to say try it but be prepared.
1646062083686.png
 
Last edited:
Joined
May 25, 2017
Messages
432
I actually have an MPT7210A. I bought it to charge a small battery in my shed to run a light and a fan. I only just got the shed built this last fall, so I haven't got to any of the electrical yet.

The stuff you said makes sense to me. I think the most feasible alternative in my case would be a 3s3p configuration. But as you said, it may not come to that. I think on that table where they say the open circuit voltage needs to be between 37V and 105V, I'm guessing that they expect the actual voltage will be less under various circumstances.

Best I can give it right now, is 32V from my benchtop power supply. With the battery at 26.6V, I reduced the power supply voltage until it went into night mode. To get it back into charge mode, I had to give it at least 28.9V, for a difference of 2.3V. I'm currently running the battery down further so I can try it again. I'm guessing it just needs a 2-3V difference to work, but we'll see.
 
Joined
May 25, 2017
Messages
432
Well I bit the bullet as they say, and bought the solar panels. painful on the wallet, but this way I can be sure they all match. Final bill was $1,034.49 after tax. Shipping is free. so about $1.04 per Watt. As far as I can tell, that's pretty good for smaller panels.

I've been running my old mini fridge for almost a day from the battery and inverter. The battery voltage has barely moved. It may be a couple days before the battery gets low enough to test the charge controller again. I figure that's a good problem to have. :)

I've made the first video of my series for this project. It's not much; just an introduction. But if anyone is really bored, you can watch it:
 
Joined
May 25, 2017
Messages
432
My BMS has arrived! It's a Daly 8s 80A. It's my first "smart" BMS, so there has been quite the learning curve. I can monitor and change settings over Bluetooth, which is great. However, the app is in poorly translated english, and it's been difficult finding specific information and explanations about the settings. I think I'm getting it figured out though.

Apparently there are better BMSs out there that have similar features, but I wasn't aware of them at the time I bought this. It may work for the trailer battery, but it's got a couple characteristics that I'm not liking.

First, it has a "time out" feature, where if no current is going into or out of the battery for a settable amount of time, the BMS will not only shut off the Bluetooth, but also the whole battery connection. It wakes up when there is a lode or charge put on it, but in the case of a solar battery that could be potentially problematic. See, without power to the charge controller, the solar power can't get to the BMS to wake it up. However, you can put a very long amount of time into the setting so it won't go to sleep overnight.

The Bluetooth connection seems to be on an independent timer though. It will shut off after a minute or so of no battery current regardless of the timer setting. I've seen suggestions for a workaround, so I'll try that. But otherwise, you have to push the little button on the Bluetooth dongle or make sure the battery is experiencing some kind of current. The tiny amount of current needed to run the solar charge controller doesn't seem to be enough.

The second issue, is that the Daly seems pretty picky as to the conditions that need to be met before it will balance the cells. The minimum cell voltage can be set, as well as the voltage difference between the highest and lowest cells. Those are two of the conditions. The third though, is that balancing will only occur if the battery is being charged by a certain amount of current. Like an Amp or so, but I'm not sure. Also, balance current is limited to 30mA. Doesn't seem like enough to balance large capacity cells.

Good news is that it seems easy enough to physically hook up to the battery.
P3050024.JPG


The battery is still only temporarily put together. I'm expecting more parts in the mail tomorrow.
P3060027.JPG


More good news; the BMS seems to be measuring the voltage and current pretty accurately.
P3060026.JPG


I finally got the whole battery down to about 22V, so I could test the behavior of the solar charge controller. It still only needs about 2.2V difference to charge the battery. I'm glad I got the BMS when I did though, as the cells are getting quite out of balance as they approach their bottom voltage. I'm charging the whole battery back up now, and I'll see what the balance looks like back at the top.

I wonder if they have different internal resistances. I may try to measure that. They all have the same capacity though, and behaved the same during the discharge test. Anyway, back to tinkering...
 
Joined
May 25, 2017
Messages
432
New video:
Just me talking again. This time I'm giving a brief overview of how I plan to do the electrical. I've updated my wiring diagram too, based on some new stuff I've learned lately about my battery and solar panels in general.
CargoCamperElectrical.jpg


I've been thinking about grounding lately. Specifically, what to do with the ground wires that lead up to my isolation switch, and the ground post on the body of the inverter. I've made a diagram showing much more detail of the AC side of things to maybe help me figure it out.
CargoCamperGrounding.jpg


So here's what I think I know:
-If the trailer is connected to a service connection somewhere, there should be a physical ground rod buried in the actual dirt somewhere.
-If the trailer is connected to a generator, there should be a bond between the ground and neutral. Either inside the generator or via an adapter.
-If the trailer is connected to nothing, there will be no ground connection to either neutral or earth.
-The neutral bus bar in the breaker panel needs to be isolated from ground.
-The trailer body needs to be connected to ground.

Here's what I'm not sure of:
-The solar panel frames (being aluminum) and mounting hardware should be grounded to the trailer body?
-The solar panel frames should also be grounded to the inverter via the ground connector on the inverter's body?
-The inverter body should be grounded to it's AC output ground?
-All the ground wires meeting up at the isolation switch should be connected together?

Vaguely related: I plan to use GFCI breakers, or maybe the GFCI/AFCI combo breakers.

Anyway, does it mater that sometimes the trailer will be connected to a ground rod, and sometimes just boded to neutral, and sometimes neither?
 
Joined
May 25, 2017
Messages
432
Well, I've done more digging around, and I still think that what I know is correct. As for the things I'm not sure of, it's looking like the solar panel frames and any mounting hardware should indeed be grounded to the trailer body. What's more, a lot of aluminum products are anodized, which causes high resistance, so any ground points should be connected to actual bare aluminum. It looks like the ground lug on the inverter body should also be grounded to the trailer body. And from what I've seen, all the ground wires meeting up at the transfer switch should be connected.

I guess there's some inverters that bond the neutral to ground on the AC output side to simulate a path to ground. Some appliances and electronics won't work if they detect an open ground. I think this is what my inverter does. So in my case, I definitely need to have both the hot and neutral wires isolated at the transfer switch. This will prevent having more than one neutral/ground bond in the system at a time.

I've also just learned that there is supposed to be a bond between the AC ground and the DC negative. But only one. When the trailer is hooked up to the truck, The AC ground will be bonded to the truck's negative through the chassis. But that goes away when it's unhitched. My battery is a different voltage than the truck's, so I've been planning on keeping the DC isolated from the AC. Not sure now if that is still the best way to go or not.
 

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
2,125
I have a 7 x 14 trailer -> camper with MPP Solar 3048 + 48v battery bank. I've never been 'clear' on grouding, even after reading and reading.

What I did
1) Hooked the external 120v ground thru to the 120v distribution panel to the sockets - and let it go at that. As someone pointed out, rubber tire vehicle does not have a 'path to ground' unless you do something unusually like drag a something that physically touches the ground.
1a) Disconnected from campground power or being powered the generator - there is no earth ground.
2a) Connected to campground power - the trailer ground is connected to campground power 'ground' - whatever that is or isn't.

2) 12v subsystem. I have a 120v -> 12vdc transformer and ran all the + and - as separate wires. There's no contact with the chassis.
*12v is not an electrical shock hazard as far as I understand.

Not saying what I did is right - just sharing what I did if it helps and/or prods further discussion :)
 
Joined
May 25, 2017
Messages
432
Sounds about right to me. Here is an updated diagram based on all the stuff I've learned:
CargoCamperGrounding.jpg


I think that's going to be all for my grounding research. In other news, I got my solar panels! I made a short video of my solar plan and unboxing and testing a few of the panels.

This weekend is the last cargo run before the trailer gets gutted and rebuilt. From there, progress will only be limited by funding, weather, and how much energy I have between this and other projects. :)
 
  • Like
Reactions: cak
Joined
May 25, 2017
Messages
432
Since my cells have a self-discharge going on, and they seem to discharge at different rates, I got one of those "active" balance boards to keep the battery top balanced.
20220802_161131.jpg


There's a spot on the board where a switch can be added. Otherwise the board is balancing all the time. No point in trying to balance the cells when they're under 3.4V each. So I found a programmable switch that turns on or off depending on voltage. There's a threshold separating the on and off function so it's not getting triggered a bunch while the cells are right at 3.4V. It even shuts off the display after a while so as to not use extra power.

Without the switch and balance board, it takes about 20mA to keep the battery at full voltage, and the difference between cells is always more than 100mV. With the switch and balance board, the battery takes 30mA to stay topped up, and the difference is only 12mV. I ran an air conditioner and a mini fridge for a few days, along with a power supply to simulate a bit of solar input. The battery periodically got low enough in voltage for the switch to shut off the balance board. With the fridge and A/C compressor cycled off, there was enough power to recharge the battery, and the balance board kept everything within 50mV as far as I saw.
 
Top