New to solar

[dragonpilot]

I'm very new to solar and electricity in general so please bear with me. I recently purchased a portable two-100W panel Renogy kit. I comes with a 40A Renogy MPPT controller, cables and MC4 connectors. I have wired the panels in series.

I took it for a test run recently on a bright cloudless sunny spring day, about 70 degrees F. The panels were set at 39 degrees for my latitude facing south. I used a 5-6 year old 12 volt marine deep cycle battery that was already charged to about 80% or more...about 12.6v.

After connecting the battery to the controller I checked the settings and the only thing I had to do was select the correct battery type...typical lead acid battery in this case. I then connected the panels to the controller. Also note that I was not connected to any appliances at the time so there was no draw on the battery.

As I was fussing around getting the panels secured on a slippery surface I finally had time to toggle through the controller readouts. Everything seemed normal, except I noticed that the charge current was reading about 1.25-1.31 amps give or take. I was expecting something more like 5 amps or so. I also noticed that the readout switched from "boost" to "float".

Was the lower current reading that low because the battery I was using was nearly fully charged? Does the controller step down the charge current when it senses that the battery is reaching full charge? I suppose at some point during the charging process the controller is going to have to stop charging the battery to prevent boil over.

[Elorrum]

You’ve wired your panels in series. An mppt can handle the increased voltage, but I gather shade has a greater impact on the system: “The downside to series systems is shading problems. When panels are wired in series, they all in a sense depend on each other. If one panel is shaded it will affect the whole string. This will not happen in a parallel connection.”

does your controller need adjustment or selection for the greater voltage input, more than a parallel 12v setup? Is your goal to charge a 24v battery bank?

[Molotov]

I mounted my Renogy 100 Watt 12 Volt portable folding solar panel with waterproof 20A charger controller (solar suitcase) flat on cedar fence boards laid across my truck bed, aimed straight to the sky (no shade) and held down against wind lift with straps and a security chain. It provided all the power I needed for my little travel trailer for my 3 weeks on playa. My electrical load consisted of a temperature controlled ceiling exhaust fan, LP-gas refrigerator control board, interior LED lighting, and occasional water pump for showers and turlet flushing.

I carry the solar panel folded in its suitcase cover strapped down to one of my beds during transit. I have a Honda 3000 generator to run my air conditioner, but due to the mild temperatures last year, combined with the solar power, I never started it once.

[Popeye]

As Ellorum suggests, if you wire your panels in parallel you will get much better charging.
Connecting in parallel increases the current available.
Connecting in series increases the voltage.
Power=Volts X Current

[FlyingMonkey]

By default I think that charger will be in boost for 2 hours but that is an adjustable setting. The reason you are not seeing a lot of charge current is most likely because you are charging a charged battery. With MPPT charger controllers Power = V x A, so 24v x 5A = 120 = 12v x 10A. So series or parallel panels don't differ that much. PWM charger controllers (the old kind) are a different beast.

Lots of good info on this site https://explorist.life/time-to-charge-a ... alculator/

I don't want to be that guy but you should read the charge controller manual. It may be this one but I don't know what you have specifically https://www.renogy.com/template/files/M ... 20V2.0.pdf

Good luck.

[dragonpilot]

Good answers all! Thank you!

I'm using this system to just maintain my two 6v batteries. I've gone as long as 14 days on just these two batteries alone. In 2023 I ran two small Titan fans in the back of my propane fridge to aid in keeping fridge temp cool (worked great!), occasional water pump for flushing and a quick daily shower, the propane detector and my Figjam swamp cooler (about 5-6 hours a day). I borrowed a friend's 100W panel and that just kept my batteries topped off nicely. Figuring the 200W setup will be more than sufficient for my needs.

I read up on the pros and cons of series and parallel connections and for now I think series is the best choice. Shade on the playa won't be an issue anyway.

I did read the controller manual, and still digesting this new medium. As simple as it might be for some folks, my ignorance is on full display...but I'll get better the more I dive into this, but I am agreeing with the contention that I was trying to charge an already charged battery.

[some seeing eye]

I used a 5-6 year old 12 volt marine deep cycle battery that was already charged to about 80% or more...about 12.6v.

...I noticed that the charge current was reading about 1.25-1.31 amps give or take. I was expecting something more like 5 amps or so. I also noticed that the readout switched from "boost" to "float".

Was the lower current reading that low because the battery I was using was nearly fully charged? Does the controller step down the charge current when it senses that the battery is reaching full charge?

Yes, yes. That's normal. The charger senses the state ~ percent of charge by measuring the voltage of the battery terminals. Batteries have a charge curve with typical stages. In the first stage, bulk, the charger acts as a constant current source and pushes a lot of current. In the end stage, float, the charger becomes a voltage source and delivers a small amount of current. The battery type selector on the charger/solar charger knows the voltages for each battery type.

[motskyroonmatick]

You are on the right path Dragonpilot. The charge controller was reading the battery voltage as nearly fully charged so it throttled down the charging amps. This is normal operation for a properly set MPPT charge controller. You are also on the right path with having the panels series connected for higher voltage. The series connection gives you proper operating voltage for the MPPT charge controller earlier in the day when the sun hits the panels. This gives you more total time charging in the morning and evening by having the charge controller wake up and start charging earlier and stay charging later. You may want to change your battery chemistry to LiFePo4 for roughly double the capacity and half the weight of lead acid but you could explore that when your lead acid batteries stop working well.