Posted on 08/29/2023 3:36:17 PM PDT by Rebelbase
Hello, I have a technical question for 12V solar folks.
In the process of building an off-grid solar set up for my camper trailer. Just need enough juice to keep the fan running in the propane fridge and to power the blower on the propane heater plus the lights and water pump.
Going to do a 200 amp hour gel battery and two 100 watt panels. A couple of months ago I bought a new-in-box 100 watt panel at an estate sale that I was told was for a 12V system.
Reading the back of the panel I don't see 12V anywhere on the spec decal, which is below.
Here is the question, when I buy the second panel does it matter if the voltage specs do not match up perfectly?
I looked at the Renogy website and do not see the exact specs on their current inventory. I suspect this panel is several years old as it came from a well stocked prepper who passed a few months ago.
Perfect panel for your application.
I’ve had those on my boat for years.
12v is just the familiar name for a lead acid battery system. A fully charged 12v battery is actually 13.2v and most charging systems run around 14.5v. The panel will put out more voltage when not being loaded. Just get a regulator designed for solar charging. A vendor of such chargers can help you pick the right one for your needs. I did this with a solar system for my cabin. We used a golf cart battery.
“I may bite the bullet and go with LiFePo4.”
You can completely discharge them and still get thousands of cycles.
I put together a system similar to yours. 8 panels, 300Ah LiFePo4 battery. I can charge via genset as well if there isn’t enough light.
My basic goal was to be able to run a fridge and freezer indefinitely, and I am able to do that, no problem.
My six 12v deep cycle bank charges to 12.7 typically.
I usually buy HQST panels but these have a good rating and are 8 dollars cheaper per panel so I thought I'd take a chance.
Generally you use something called a solar regulator to connect panels to a battery.. they are inexpensive for a smallish setup..
I would NOT get a gel cell battery. I would get a 12V 100Ah LiFePO4
The LiFePO4 will have a much longer service life and they are NOT the same as the batteries in EVs ... I wouldn't have those burst-into-flames things in my home :-/
I recommend these, about 270.00 after the coupon and free ship.
We’ve 660w of solar on our boat with 300ah of Lithium (Battleborn). We DO have a 2200 Honda generator for cloudy days, but, for the most part, the solar works great.
As was said before, your solar controler is what is important. We have Victron and they have GREAT customer support and are very easy to use and adjust.
We have our stuff set to Absorb at 14.4, Float at 13.6 and and we usually get down to 12.8 over night.
Right now, we are in a marina with shore power, so, we run the DC items on the solar, and AC on shore power. With the storm in the gulf, we might lose power, but, the batteries are full. Tanks are full, and the fridge is well stocked.
We’re in the Treasure Coast, so, we aren’t expecting too much.,
A fully charged 12v battery is actually 13.2v
My six 12v deep cycle bank charges to 12.7 typically.
Actually you are both right. When new they will maintain 13.2 but as they age or if you have one weak one in the bank they drop to around 12.7.
One thing to remember though, do NOT charge a LiFePO4 if its temperature drops below freezing!!
I'm very happy with my 18 lifepo4 batteries totaling 92kWh storage. But that's for a 2,300 sq ft all-electric home and charging an EV. LOL Which is way more discharge, and more frequently, than you probably intend.
Those panels are ridiculously cheap. When I first got into solar over 20 years ago they were running $5 a watt for cheap ones. lol
Generally the voltage of a new lead acid battery runs 2.2 volts per cell. This degrades over time as the cells build sulphate. At least that’s what the Navy taught me and I’m sticking with it!
I’ve got a Bluetti AC300 3000 watt inverter with B300 3072 Wh (250 AH) LiFePo4 pack in my Transit. I’m going to drive up to Renogy and Rich Solar in Ontario, CA in the near future and pickup around 600 watts of panels (2x200 and 2x100) for the roof rack. I’ll also run off of the second alternator or shore power. : )
” “charge controller” regulates the voltage down to just under 14 volts”
Does that regulation reduce the voltage by wasting the voltage excess through a resistor?
“Does that regulation reduce the voltage by wasting the voltage excess through a resistor?”
I think with the newer controllers they basically have a “Buck Convertor” incorporated to step down the voltage while stepping up the current. So there is not much actual loss of total input.
https://en.wikipedia.org/wiki/Buck_converter
https://www.amazon.com/gp/product/B09D7BMBWP
Your charge controller does the rest.
I would highly recommend Will Prouse's book: https://www.amazon.com/Mobile-Solar-Power-Made-yourself/dp/1546567119
If you want to skip the book, he has lots of videos on YouTube.
Here is a recent system we put in a friend’s work van. Much more than Rebelbase is wanting but it can help explain the system and how they work. One can adapt these basics to their needs. It can be adapted to be a 24 volt system also with 24 volt rated panels. And the 12 volt batteries can be hooked up in series to make 24 volts. Pre-charge controller voltage reduction with the buck converters was only needed because of the repurposing of the 40 volt home panels we acquired and it was too much for a 12 volt charge controller, it would not have needed the buck converters for a 24 volt system and charge controller.
Originally posted in another forum:
“Finally got around to sharing the solar system we set up in my friends van. But it could be set up the same stationary also and as 24V as an option.
I’m not going to show the panels because they all look the same, just the heart of the charging works. And he has 6 deep cycle marine batteries in his bank hooked in parallel, and a 120 inverter. Standard common system so no need to show those either. But he is rated enough to run an electric chain saw and still only tax the inverter to about 50% of available power output. And run it for at least an hour from the batteries even without sun on the panels.
The panels are four old take offs from a large 36 panel home system that had to be replaced with new panels. They are rated as follows:
X4 - 40 Volts - 150 Watts - 5 Amps. ( So that is 0 - 40 Volts, 600 Watts total, and 20 amps total.)
(If these had been new panels we could have divided the two circuits which are set up in series in the panels. But because these were used and not putting out what they should they would only give us a little over 12V even in the best of sun when we divided them up to try as parallel. And a 12V battery system needs an input of at least 13.7 volts to charge efficiently).
So the 34V - 40V panel voltage rating was an issue for setting up as a 12V system rather than a 24V system. But we needed to have it 12V so that it would also integrate with the 12V vehicle system and be an addon system to work with the vehicle also aside from the 120 AC inverter output. This way not only could he pull from the solar system batteries for the vehicle DC system, but could also use the vehicle charging system to help recharge the battery bank while he is driving. And he can flip a switched solenoid to jump the starting system on the van from the solar system if the van battery went dead for some reason. Redundancy for both systems.
When setting up a system you have to work backwards, from what voltage and current rating your battery bank will be, and what voltage and current your charge controller will need to be. In this case we went overkill with the charge controller, that way if he ever needs to use it with more panels, or in a stationary application it is already rated for more. And now days they use very very little to keep the controller powered up anyhow. So the charge controller is rated as follows:
Max 36v input when used on a 24V system. And max 24 volt input when used on a 12V system at max 80 amp input. Now it might (risky) have handled up to 40V input, but it would have probably caused overheating issues and digital control issues. We did not want to overdrive it, better to be safe than sorry with electronics that could cause fires. So we ran the 40V panel output through buck convertors to step down the voltage going into the charge controller. The buck convertors are rated as follows:
X2 - 20A 300W CC CV Step Down Module Adjustable DC 6-40V to 1.2-36V Voltage Regulator Buck Converter Constant Current Power Supply. The efficiency can be up to 95%,(measured at 20A, converting 24V to 12V).
Because of the max 300 watt rating of each buck convertor we had to use two and divide the output of the four panels (600 watts) into two sets of two panels (300 watts each set) going into the buck convertors. All that was left to do after hooking it all up was to tune the adjustable buck convertors on a good day with max sun available on the panels. A tip for buck convertor efficiency is this... You want to try and set the convertor output just under the max input to the charge controller. The more you step the voltage down the harder the convertor has to work, they operate hotter, draw more themselves, and are less efficient. Max input of the charge controller is 24V so we set the output of the buck convertors at 22V output. The only 2V difference is easy on the buck convertors and the heat sinks get warm, but not hot.
Now if one wants to make a small system like this as backup for a home there only a few important additional technical changes. A grid main switch needs to be installed so that when you turn on the solar system, it simultaneously physically cuts the grid power. Last thing you want is for the grid power to come back on while you are on your solar inverter. And unless you foot the bill for a home system inverter, portable inverters can be used but will fault if the ground is hooked up on the receiving home system, so the male plugs into the inverter need the ground post deleted. And there is a bit of careful wiring in the AC power distribution box so that you can pipe in two separate 120V circuits from the inverter. And unless you have a big high rated portable inverter you will want to temporarily shut off and live without all your 240 appliances.”
He can run a couple skill saws or cut off saws or 120 screw guns through an all day project and still be almost topped off in the batteries if it is a sunny day. It will take quite a load and hang in there well. This would make an excellent smaller scale system for a home. You could run smaller appliances like microwaves and such with no problem.
You have a nice tagline..
Those cheapo panels are due to arrive tomorrow... I hope
they are good, will buy more if they are.
Correct. The panel give load & no load voltage and optimum operating voltage are on the panel.
I must admit, the ChiComs are known for playing around with ratings of solar panels. They way over rate the ability of the panels to produce power in watts.
Bought my first 3 panels in the early 1980’s, still have them. Only time I use them is for field day for a QRP rig. They were new surplus from Los Alamos Labs. rated at 1/2 amp at 16 volts. The size of the battery is also important.
Recently I bought a small 5V solar panel to charge my cell phone away from commercial power. It is simple and does a good job of charging my smart phone (computers and computer phones are really stupid).
I ordered some 18650 Lithium ion batteries and charger a week ago, am waiting for delivery soon. Have mixed feelings about them. They are very low leakage (charge leakage) and physically very small for the current capacity, but I am not sure how safe they are when unattended. When you see the stupid electric cars burn up it is sobering.
TF
Sounds like you might be wanting to repurpose a larger panel you acquired into a trickle charger? If it is going to be perpetually hooked to the battery to “maintain” it should have a charge controller so it goes on “float” when full. A larger panel usually puts out 17-22 volts.
The batteries will take the voltage, but without being controlled to go on float and stop charging when the battery is full, 18-22 volts will evaporate all the electrolyte out of it in a short time. Even continuing to put 14-15 volts to a battery after it is at full charge will also evaporate the electrolyte prematurely. Car alternators have cutoff charge controllers for the same reason.
I have used 20 volt panels as car chargers many many times. We go camping out in the sticks sometimes 40 miles out. We always take a 100w panel with us just in case we need to charge the battery for some reason. But when using it to charge a car battery back up I don’t use a charge controller because I will be physically monitoring it and unhooking it right away after it is charged. I am physically the charge controller.
But if you are looking to leave something hooked to it perpetually to maintain a car battery, that $20 charger is a DEAL! :)
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